CN106054361B

CN106054361B - Optical system and image pickup apparatus

Info

Publication number: CN106054361B
Application number: CN201610203890.5A
Authority: CN
Inventors: 野田隆行; 田口博规; 濑川敏也; 平川纯; 太幡浩文; 河合祥子; 山根宏大; 森勇辉
Original assignee: Tamron Co Ltd
Current assignee: Tamron Co Ltd
Priority date: 2015-04-03
Filing date: 2016-04-01
Publication date: 2020-05-19
Anticipated expiration: 2036-04-01
Also published as: JP6544971B2; CN106054361A; JP2016197169A

Abstract

The invention provides an optical system and an imaging device which can realize high chromatic aberration correction by miniaturization and light weight, and can maintain good imaging performance even when the environmental temperature changes. In order to achieve the above object, the present invention provides an optical system including at least one group of lens groups including a diffraction surface, wherein at least any one group of the lens groups includes a first lens having the same sign as the whole group of the lens groups and the largest refractive power and satisfying a predetermined conditional expression, and an i-th lens other than the first lens having the same sign as the whole group of the lens groups and satisfying the predetermined conditional expression, and an image pickup apparatus including the optical system.

Description

Optical system and image pickup apparatus

Technical Field

The present invention relates to an optical system and an imaging apparatus, and more particularly, to an optical system applied to an imaging apparatus using a solid-state imaging device such as a digital camera or a digital video camera, and an imaging apparatus including the optical system.

Background

At present, imaging devices using solid-state imaging elements, such as digital cameras and digital video cameras, have become widespread. As an imaging optical system used in such an imaging apparatus, a zoom optical system that can change a focal length is widely used. In a zoom optical system, a focal length is changed by changing an interval of lens groups. At this time, since the height of light incident on each lens group or the incident angle changes, each aberration such as axial chromatic aberration or magnification chromatic aberration also changes. In order to suppress the variation of these aberrations, high imaging performance is achieved over the entire zoom range, and a high-level optical design is required in order to suppress the size and weight of the zoom optical system.

Therefore, in recent years, a zoom optical system has been proposed which realizes high chromatic aberration correction using a diffractive optical element having optical characteristics different from those of a refractive optical system. For example, in the zoom optical system described in patent document 1, in addition to an anomalous low dispersion material conventionally used for chromatic aberration correction, the use of a diffractive optical element realizes high imaging performance that can cope with a solid-state imaging element capable of performing high-resolution imaging of 100 ten thousand pixels or more while suppressing an increase in the number of lenses necessary for aberration correction.

Such a zoom optical system is widely used as an imaging optical system of a fixed-mount type imaging device that is fixed to a vehicle body, a building, or the like and is used for a specific purpose, such as an in-vehicle imaging device, a monitoring imaging device, or the like, in addition to an imaging device that can be carried by a user, such as a single lens reflex camera, a mirror-less interchangeable lens camera, or a digital camera. Further, in any of the applications, a demand has been made for a brighter zoom optical system having high imaging performance, a small size and a light weight, and a small F-number.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-134304

Disclosure of Invention

Problems to be solved by the invention

However, refractive powers of the respective optical components constituting the imaging optical system vary due to temperature changes. Therefore, when the difference between the ambient temperature and the temperature at the time of design (normal temperature) is large, the optical performance of the imaging optical system also changes. In the above-described anomalous low-dispersion material, the change in refractive power tends to be more severe with a change in ambient temperature than in other glass materials, and therefore, in the case of an optical system including an anomalous low-dispersion material, particularly in the case of a zoom optical system including an anomalous low-dispersion material, the possibility that the change in the ambient temperature causes a change in the focal position or the amount of post-focusing is increased. Further, when the lens having a strong refractive power is made of an anomalous low dispersion material, the variation of spherical aberration and the like tends to be increased with the change of the environmental temperature. In the zoom optical system, particularly when the focal point position or the back focus amount in the zoom lens changes due to a change in the ambient temperature, the imaging performance is significantly degraded.

In many of the above-described fixed-mount image pickup apparatuses, a zoom lens is used as an image pickup optical system, and is often used in an environment where environmental temperature changes greatly. There are also many machines that have a fixed image capturing device, an adjustable image capturing angle when the device is placed, and do not have an auto-focus function. Therefore, the contour of the subject image becomes unclear due to the ambient temperature, and the application of the image pickup apparatus of the fixed attachment type such as monitoring cannot be realized in some cases.

The invention provides an optical system and an imaging device which are small and light, realize high chromatic aberration correction and maintain good imaging performance even when the environmental temperature changes.

Means for solving the problems

In order to achieve the above object, the present invention provides an optical system including at least one lens group including a diffraction surface, wherein at least any one of the lens groups including the diffraction surface is a predetermined lens group including the diffraction surface, wherein a lens having a maximum refractive power among lenses having a refractive power of the same sign as a refractive power exhibited by the entire predetermined lens group is a first lens among the predetermined lens groups including the diffraction surface, and wherein the first lens satisfies the following conditional expression (1) when any one other than the first lens among lenses having a refractive power of the same sign as a refractive power exhibited by the entire predetermined lens group is an i-th lens among the predetermined lens groups, and wherein the i-th lens satisfies the following conditional expression (2).

dndtP1×10⁶>-5…(1)

Ndi≥-0.014×νdi+2.5…(2)

Wherein "dndt" is a temperature coefficient of absolute refractive index of the lens in vacuum (absolute dn/dT) for a light ray having a wavelength of 632.8nm in a temperature range of 20 ℃ to 40 ℃, and "dndtP 1" is the dndt of the first lens, "Ndi" is the refractive index of the i-th lens for the d-line, "ν di" is the abbe number of the i-th lens for the d-line, and "d-line" is a light ray having a wavelength of 587.56 nm.

The present invention also provides an imaging device including the optical system of the present invention and an imaging element for converting an optical image formed by the optical system on an image plane side of the optical system into an electric signal.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide an optical system and an imaging apparatus that are compact and lightweight, realize high chromatic aberration correction, and maintain good imaging performance even when the ambient temperature changes.

Drawings

Fig. 1 is a sectional view showing an example of a lens configuration at a wide-angle end of an optical system according to embodiment 1 of the present invention.

Fig. 2 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 1.

Fig. 3 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 1.

Fig. 4 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 1.

Fig. 5 is a sectional view showing an example of a lens configuration at the wide-angle end of an optical system according to embodiment 2 of the present invention.

Fig. 6 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 2.

Fig. 7 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 2.

Fig. 8 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 2.

Fig. 9 is a sectional view showing an example of a lens configuration at the wide-angle end of an optical system according to embodiment 3 of the present invention.

Fig. 10 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 3.

Fig. 11 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 3.

Fig. 12 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 3.

Fig. 13 is a sectional view showing an example of a lens configuration at the wide-angle end of an optical system according to embodiment 4 of the present invention.

Fig. 14 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 4.

Fig. 15 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 4.

Fig. 16 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 4.

Fig. 17 is a sectional view showing an example of a lens configuration at the wide-angle end of an optical system according to embodiment 5 of the present invention.

Fig. 18 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 5.

Fig. 19 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 5.

Fig. 20 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 5.

Fig. 21 is a sectional view showing an example of a lens configuration at the wide-angle end of an optical system according to example 6 of the present invention.

Fig. 22 is a sectional view showing an example of a lens configuration at the telephoto end of the optical system according to embodiment 6.

Fig. 23 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram in infinity focusing at the telephoto end of the optical system of example 6.

Fig. 24 is a spherical aberration diagram, an astigmatism diagram, and a distortion aberration diagram at the time of infinity focusing at the wide-angle end of the optical system of example 6.

Fig. 25 is a diagram showing temperature variations of the back focus of the optical system of example 1.

Fig. 26 is a diagram showing temperature variations of the back focus of the optical system of the comparative example.

Description of the symbols

1G … first lens group, 2G … second lens group, 3G … third lens group, 4G … fourth lens group, CG … shield glass, IMG … image surface

Detailed Description

Embodiments of the optical system and the imaging apparatus according to the present invention will be described below.

1. Optical system

1-1, basic constitution of optical System

The optical system of the present embodiment is characterized by having at least one lens group including a diffraction surface, at least any one of the lens groups including the diffraction surface being a specified lens group, the specified lens group having a lens having a maximum refractive power among lenses having a refractive power of the same sign as the refractive power exhibited by the specified lens group as a whole as a first lens, and the specified lens group having any one of lenses having a refractive power of the same sign as the refractive power exhibited by the specified lens group as a whole except for the first lens being an i-th lens, the first lens satisfying a conditional expression (1) described later, and the i-th lens satisfying a conditional expression (2) described later. The structure of the optical system will be described below.

1-1-1, lens group comprising diffractive surface

First, a lens group including a diffraction surface will be described. The lens group including the diffraction surface means that, among optical components constituting the lens group, at least one optical component has an optical surface that is a diffraction surface.

Here, the diffraction surface has a diffraction grating structure defined by a phase difference function expressed by the following formula. For example, a diffractive optical element can be obtained by forming a diffraction grating structure on an optical surface of an optical element such as a glass lens or a plastic lens by a cutting method, a photolithography method, a molding method, or the like. Further, a multilayer diffractive optical element having a diffraction grating structure on an optical surface may be formed by providing one or more resin layers having a diffraction grating structure on the optical surface (spherical surface/aspherical surface) of the optical module. In the present invention, a lens group including such a diffractive optical element is referred to as a lens group including a diffraction surface.

In the above-mentioned formula, the compound of formula,

(h) is a phase difference function, "m" is the diffraction order, "λ" is the normalized wavelength. Further, "C1", "C2", "C3" and "C4" are diffraction surface coefficients, and "h" is a length from the optical axis in the same radial direction. Further, the standardized wavelength is a wavelength in a wavelength range in which the optical system is used, for example, a wavelength in a visible light wavelength range is preferable.

As the diffractive optical element, a single-layer diffractive optical element having the above-described diffractive surface only on the surface in contact with the air layer can be used. In addition to the multilayer diffractive optical element of the above-described embodiment, for example, a multilayer diffractive optical element of a laminated type in which a diffraction surface is formed between one glass material layer and another glass material layer, such as a case where a bonding surface of a bonding lens is the diffraction surface, may be used. When the multilayer diffractive optical element is used, chromatic aberration and the like can be corrected favorably in a wider wavelength range than in the case of the single-layer diffractive optical element. In the multilayer diffractive optical element, the glass material layer is not limited to a layer made of an optical glass material, and may be a layer made of an optical element forming material other than optical glass such as optical plastic.

Also, the diffraction surface may be spherical or aspherical. When the diffraction surface is an aspherical surface, each aberration such as chromatic aberration can be corrected more favorably with a smaller number of optical elements.

The optical system of the present embodiment has the above-described specified lens group, and thus can perform favorable correction of chromatic aberration and the like with less optical components than a normal refractive optical system having no diffraction surface. Therefore, the optical system can be reduced in size and weight, and can realize high-level chromatic aberration correction.

Further, when the optical system is configured to include a diffraction surface, the temperature characteristics of the entire optical system can be improved. Specifically, when the optical system has a diffraction surface, chromatic aberration and the like can be corrected satisfactorily with a small number of optical components as described above. Therefore, the number of optical components made of a glass material that is effective in correction of chromatic aberration or the like but has poor temperature characteristics, for example, made of an abnormally low dispersion material or the like can be reduced. Further, when the predetermined lens group is formed to include the first lens satisfying the conditional expression (1) and the i-th lens satisfying the conditional expression (2) described below, the entire optical system can maintain good imaging performance even when the ambient temperature changes.

Here, the refractive power exhibited by the entire predetermined lens group may be positive or negative, and is not particularly limited, but positive refractive power is preferable from the viewpoint that correction based on chromatic aberration and the like becomes better.

In addition, when the optical system includes a plurality of diffraction surfaces, it is preferable that the diffraction surfaces are arranged in different lens groups. For example, a zoom optical system has a plurality of lens groups, and a focal length is changed by changing an interval of each lens group. Therefore, the most effective arrangement for correcting various aberrations such as chromatic aberration by the diffraction surface differs depending on the position of each lens group. Therefore, when a plurality of diffraction surfaces are included in the zoom optical system, it is preferable to arrange the diffraction surfaces in different lens groups from the viewpoint of obtaining an optical system with higher imaging performance.

In addition, the optical system may have at least one lens group including at least one diffraction surface, one or more lens groups including one diffraction surface, or one or more lens groups including a plurality of diffraction surfaces. When there are a plurality of lens groups including a diffraction surface, any one of the lens groups may be the designated lens group, and two or more lens groups may be the designated lens group, or all lens groups including a diffraction surface may be the designated lens group.

1) Lens structure

Next, a lens structure of the above-described specified lens group will be described. The predetermined lens group includes at least two lenses, i.e., the first lens and the i-th lens, and the configuration of the other specific lenses is not particularly limited as long as the predetermined lens group includes a diffraction surface. The optical surface of the first lens and/or the i-th lens may be the diffraction surface, and the optical surface of a lens other than the first lens and the i-th lens constituting the predetermined lens group may be the diffraction surface. The specific lens configuration of the predetermined lens group is not particularly limited, but the number of lenses constituting the predetermined lens group is preferably 3 to 6 from the viewpoint of performing more favorable chromatic aberration correction and preventing the increase in size and weight of the optical system.

Further, the prescribed lens group preferably includes a lens having a refractive power of a sign different from that of the entire prescribed lens group. The first lens group and the i-th lens have refractive powers of the same sign as the refractive power exhibited by the specified lens group as a whole. Therefore, when the configuration including the lens having the refractive power of a different sign from those of the first lens and the i-th lens is adopted, it is possible to correct each aberration such as chromatic aberration more favorably, and also, when the ambient temperature changes, it is possible to suppress aberration variation by canceling the aberration with the lens having the refractive power of a different sign from those of the first lens and the i-th lens.

2) First lens

Next, the first lens will be described. The first lens is one of optical components constituting the above-specified lens group, and is a lens having the largest refractive power among lenses having refractive powers of the same sign as that of the specified lens group. That is, when the refractive power displayed by the designated lens group as a whole is positive, the refractive power of the first lens is positive, and the refractive power displayed by the designated lens group as a whole is negative, the refractive power of the first lens is negative. The first lens is a lens having the same sign as the entire specified lens group and the largest refractive power, and therefore, if the optical characteristics of the first lens change when the ambient temperature changes, there is a high possibility that the optical characteristics of the specified lens group change accordingly. As a result, the optical characteristics of the optical system change, and particularly, when the focal length, the focal position, the back focus amount, and the like change, the image plane cannot accurately form a subject image, which results in a problem of significantly reduced imaging performance. Therefore, in the optical system of the present embodiment, by using the first lens as the lens satisfying the conditional expression (1), as described later, it is possible to suppress a change in the optical characteristics of the first lens due to a change in the ambient temperature, and to suppress a change in the focal length, focal position, back focus amount, and the like of the optical system, thereby maintaining high imaging performance. The conditional expression (1) will be described in detail below.

3) I lens

The i-th lens is one of optical components constituting the above-described specified lens group, as well as the first lens, and has a refractive power of the same sign as the refractive power exhibited by the specified lens group as a whole. The refractive power of the i-th lens is smaller than that of the first lens, so that even if the optical characteristics of the i-th lens change upon a change in the ambient temperature, the change in the optical characteristics of the above-specified lens group can be suppressed. Further, by using the lens of the i-th lens satisfying the conditional expression (2), as described later, chromatic aberration and the like can be corrected more favorably, and an optical system with high imaging performance can be obtained. Further, conditional expression (2) will be described in detail below.

The i-th lens is not particularly limited as long as it has a refractive power of the same sign as the refractive power exhibited by the entire predetermined lens group and is a lens other than the first lens. However, from the viewpoint of more favorably correcting chromatic aberration and the like, resulting in an optical system with higher imaging performance while suppressing an increase in the number of lenses constituting the lens group, and from the viewpoint of improving chromatic aberration, the i-th lens is more preferably a lens having a refractive power next to that of the first lens among lenses having the same sign as described above. That is, the i-th lens is preferably a lens having the second largest refractive power among lenses having the same sign of the refractive power exhibited by the entire specified lens group (hereinafter, referred to as "second lens")

In the present embodiment, the focal length and refractive power of a lens are values obtained by assuming that the lens is a single lens in which both surfaces of the lens are in contact with an air layer even when the lens constitutes a part of a cemented lens, and the radius of curvature (R) of each surface of the lens is used₁、R₂) The refractive index (n) of the lens material itself and the center thickness (tc) of the lens. Specifically, the value obtained by the following formula is used. Further, the refractive power is the reciprocal (1/f) of the focal length (f).

1-1-2, other lens group

The optical system may include a lens group not including a diffraction surface, in addition to the above-described lens group. The sign of the refractive power, the lens configuration, and the like of the lens group not including the diffraction surface are not particularly limited, and an appropriate form can be adopted based on the optical characteristics required for the optical system.

1-2 examples of lens group configurations

Next, an example of the lens group configuration of the optical system will be described. The optical system may be a fixed focal length single focus optical system or a variable focal length zoom optical system. In both cases, the specific lens group configuration and the like are not particularly limited, but the following configuration examples can be cited. The zoom optical system of the present invention includes a zoom lens, and the like.

1-2-1, monofocal optical system

When the optical system is a monofocal optical system, various lens group configurations such as a two-group configuration and a three-group configuration can be adopted, and the number of lens groups, the focal power arrangement, the specific lens configuration of each lens group, and the like are not particularly limited. For example, there may be mentioned two groups of a lens group having negative refractive power and a lens group having positive refractive power in order from the object side, three groups of a lens group having positive refractive power, a lens group having positive refractive power and a lens group having negative refractive power in order from the object side, three groups of a lens group having positive refractive power, a lens group having negative refractive power and a lens group having positive refractive power in order from the object side, three groups of a lens group having negative refractive power, a lens group having positive refractive power and a lens group having positive refractive power in order from the object side, and the like.

In the case of any of the above configurations, it is preferable that at least any one of the lens groups having positive refractive power is the above specified lens group from the above viewpoint. In the case where the single focus optical system includes a plurality of groups of lens groups having positive refractive power, and any one of the groups is the predetermined lens group, the lens group having positive refractive power through which axial light passes with a maximum beam diameter is preferably the predetermined lens group. By providing the diffraction surface in the lens group having positive refractive power through which the axial light passes with the largest beam diameter, each aberration can be most effectively corrected for each wavelength range of light, respectively.

1-2-2 zoom optical system

When the optical system is a zoom optical system, the number of lens groups or the arrangement of focal strength, the specific lens configuration of each lens group, the operation of each lens group during zooming, and the like are not particularly limited as long as the plurality of lens groups are arranged and the focal length can be changed by changing the interval between each lens group when zooming from the wide-angle end to the telephoto end.

Further, with this zoom optical system, it is also preferable that at least any one of the lens groups having positive refractive power is the above-specified lens group from the same viewpoint as in the case of the single focus optical system. In the zoom optical system, when any one of the plurality of groups is the predetermined group, it is preferable that the group having the positive refractive power through which the axial light passes with the largest beam diameter is the predetermined group. By providing the diffraction surface in the lens group having positive refractive power through which the axial light passes with the largest beam diameter, each aberration can be most effectively corrected for each wavelength range of light, respectively. However, in the zoom optical system, generally, lens groups through which light passes with the largest beam diameter in the axial direction at the wide-angle end and the telephoto end are different. Therefore, from the viewpoint of making the correction of each aberration such as chromatic aberration good over the entire zoom range, it is more preferable that the lens group having positive refractive power through which the axial light beam passes with the largest beam diameter at the wide-angle end and the lens group having positive refractive power through which the axial light beam passes with the largest beam diameter at the telephoto end each include a diffraction surface. In addition, when there are a plurality of lens groups including a diffraction surface, at least any one of the lens groups may be the predetermined lens group and may include the first lens and the i-th lens. However, the object image is larger at the telephoto end than at the wide-angle end, and thus, the deviation of the focal position or the like and the variation of each aberration have a large influence on the imaging performance as compared with the wide-angle end. Therefore, it is preferable to set a lens group having positive refractive power through which axial light passes with the largest beam diameter at the telephoto end as the above-specified lens group. Hereinafter, specific configuration examples of the zoom optical system will be described.

1) Two groups are formed

In the case where the optical system is a zoom optical system including two groups, it is preferable that at least one group of lens groups having positive refractive power including the diffraction surface is provided, and the respective lens groups are relatively moved so that the distance between the first lens group and the second lens group can be changed in zooming from the wide-angle end to the telephoto end. For example, two negative/positive groups may be formed, which include, in order from the object side, a first lens group having negative refractive power and a second lens group having positive refractive power and including a diffraction surface. In this case, upon zooming from the wide-angle end to the telephoto end, it is preferable that the first lens group and/or the second lens group are moved so that the interval between the first lens group and the second lens group is reduced.

2) Three groups of

In the case where the optical system is a zoom optical system having three groups, it is preferable to include at least one group of lens groups having positive refractive power including the diffraction surface, and to relatively move the lens groups so that the interval between the lens groups can be changed in zooming from the wide-angle end to the telephoto end.

Specifically, a negative/positive/negative three-group configuration having, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive or negative refractive power may be formed. In this case, it is preferable that the third lens group is fixed and the first lens group and/or the second lens group is moved when zooming from the wide angle end to the telephoto end. Further, the refractive power of the third lens group may be infinitely small. In the zoom optical system having such a configuration, the second lens group is preferably the above-mentioned predetermined lens group from the viewpoint that fluctuation of each aberration can be more favorably suppressed even when the so-called ambient temperature changes.

Further, a positive/negative/positive three-group configuration including a first lens group having positive refractive power including a diffraction surface, a second lens group having negative refractive power, and a third lens group having positive refractive power may be formed. In this case, it is preferable that the first lens group is fixed and the second lens group and/or the third lens group is moved when zooming from the wide angle end to the telephoto end. In the zoom optical system having such a configuration, it is preferable that the first lens group and/or the second lens group is the above-mentioned specified lens group from the viewpoint that even when the so-called ambient temperature changes, the variation of each aberration can be more favorably suppressed over the entire zoom range, and favorable imaging performance can be maintained.

3) Is composed of four groups

In the case where the optical system is a zoom optical system having four groups, it is preferable to include at least one group of lens groups having positive refractive power including the diffraction surface, and to relatively move the lens groups so that the interval between the lens groups can be changed in zooming from the wide-angle end to the telephoto end.

Specifically, a positive/negative/positive four-group configuration having, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power may be formed. In this case, it is preferable that the first lens group is fixed and at least one of the other lens groups is moved when zooming from the wide angle end to the telephoto end. In the zoom optical system having such a configuration, it is preferable that any one of the first lens group, the third lens group and the fourth lens group is the above-mentioned specified lens group from the viewpoint of more favorably correcting various aberrations and the like in a wide wavelength range from a visible light wavelength range to a near infrared wavelength range.

Further, a negative/positive four-group configuration including, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power may be formed. In this case, it is preferable that the fourth lens group is fixed and at least one of the other lens groups is moved when zooming from the wide angle end to the telephoto end. In the zoom optical system having such a configuration, the second lens group is preferably the above-mentioned predetermined lens group, from the viewpoint that even when the so-called ambient temperature changes, the variation of each aberration can be suppressed more favorably, and favorable imaging performance can be maintained.

4) Five groups of components

In the case where the optical system is a zoom optical system having five groups, it is preferable to include at least one group of lens groups having positive refractive power including the diffraction surface, and to relatively move the lens groups so that the interval between the lens groups can be changed in zooming from the wide-angle end to the telephoto end.

Specifically, a positive/negative/positive/negative five-group configuration may be formed having, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having positive refractive power, and a fifth lens group having negative refractive power. In this case, it is preferable that the first lens group is fixed and at least one of the other lens groups is moved during zooming from the wide-angle end to the telephoto end. In the zoom optical system having such a configuration, it is preferable that any one of the first lens group, the third lens group, and the fourth lens group is the above-mentioned predetermined lens group, from the viewpoint that fluctuation of each aberration can be suppressed more favorably even when the so-called ambient temperature changes, and that favorable imaging performance can be maintained.

Further, a positive/negative/positive five-group configuration may be formed which includes, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having positive refractive power, and a fifth lens group having positive refractive power. At this time, it is preferable to move at least one of the lens groups in zooming from the wide-angle end to the telephoto end. In the zoom optical system having such a configuration, the fourth lens group is preferably the above-mentioned predetermined lens group, from the viewpoint that fluctuation of each aberration can be more favorably suppressed when the so-called ambient temperature changes, and favorable imaging performance can be maintained.

In this optical system, it is preferable that the single-focus optical system and the zoom optical system each include a first lens group having positive refractive power and disposed closest to the object side, and the first lens group includes at least one cemented lens composed of two lenses. When the first lens group includes at least one cemented lens, the axial chromatic aberration can be corrected satisfactorily at the maximum focal length displayed by the optical system. Further, the chromatic aberration of magnification can be corrected satisfactorily at the minimum focal length displayed by the optical system. In this case, when any one of the lenses constituting the cemented lens is made of an abnormally low dispersion material, the chromatic aberration can be corrected more favorably.

In addition to the cemented lens, the first lens group preferably includes a single lens having both surfaces in contact with the air layer. In this case, the field curvature can be corrected satisfactorily at the minimum focal length displayed by the optical system.

1-3 shockproof tremble group

In addition, when the optical system of the present invention is either a single focus optical system or a zoom optical system, all or a part of any one of the lens groups included in the optical system may be moved in a direction perpendicular to the optical axis, and used as a vibration prevention group for correcting image blur or the like caused by vibration or the like during image capturing.

1-4, conditional formula

Next, in the optical system of the present invention, as described above, the first lens satisfies the following conditional expression (1), and the i-th lens satisfies the following conditional expression (2). The optical system preferably satisfies conditional expressions (3) to (19) described later. Hereinafter, each conditional expression will be described in order.

dndtP1×10⁶>-5…(1)

Ndi≥-0.014×νdi+2.5…(2)

Wherein "dndt" is a temperature coefficient of absolute refractive index of the lens in vacuum (absolute dn/dT) in a temperature range of 20 ℃ to 40 ℃ inclusive for light having a wavelength of 632.8nm, "dndtP 1" is a dndt of the first lens, "Ndi" is a refractive index of the i-th lens for d-line, "ν di" is an abbe number of the i-th lens for d-line, and "d-line" is light having a wavelength of 587.56 nm.

1-4-1, conditional expression (1)

The conditional expression (1) is a condition that the first lens needs to satisfy. When the conditional expression (1) is satisfied, the amount of change in refractive index of the first lens per unit temperature when the ambient temperature changes is small. As described above, the first lens is a lens having the largest refractive power among lenses having refractive powers of the same sign as that of the specified lens group. The lens satisfying the conditional expression (1) has a small change in refractive index when the ambient temperature changes. Therefore, by setting the first lens to a lens satisfying the conditional expression (1), it is possible to suppress a change in the optical characteristics of the first lens when the ambient temperature changes. As a result, changes in the optical characteristics of the optical system, particularly in the focal length, focal position, amount of post-focusing, and the like, can be suppressed. Further, variation in spherical aberration can be suppressed satisfactorily. This enables the imaging performance of the optical system to be maintained even when the ambient temperature changes.

When the first lens does not satisfy the conditional expression (1), the amount of change in the refractive index of the first lens when the ambient temperature changes is larger than when the conditional expression (1) is satisfied. The first lens is a lens having the largest refractive power among the specified lens groups, and thus when a change in ambient temperature causes a change in refractive index of the first lens, the focal length or the like of the specified lens group changes, and as a result, the possibility that the focal length, focal position, back focus amount, or the like of the optical system changes becomes high. Further, the variation of the spherical aberration also becomes large. Therefore, depending on the ambient temperature, the subject image cannot be accurately imaged on the image plane, and the imaging performance is significantly degraded, which is not preferable.

From the viewpoint of obtaining these effects, the first lens more preferably satisfies the following conditional expression (1-a).

dndtP1×10⁶>-4.5…(1-a)

1-4-2, conditional expression (2)

The conditional expression (2) is an expression relating to the optical characteristics of the material (glass material) constituting the i-th lens. When the i-th lens is made of a glass material having a refractive index on a straight line represented by "-0.014 × ν di + 2.5" or a refractive index larger than the straight line, which is shown in a glass material characteristic diagram in which the abbe number of the glass material is the abscissa and the refractive index of the glass material with respect to the d-line is the ordinate, chromatic aberration and the like can be corrected more favorably, and an optical system having high imaging performance can be obtained. Here, the glass material satisfying the conditional expression (2) is a so-called anomalous low dispersion material in many cases, and the optical characteristics are likely to change when the ambient temperature changes. However, as described above, the refractive power of the i-th lens is smaller than that of the first lens. Therefore, even if the optical characteristics of the i-th lens change when the ambient temperature changes, it is possible to maintain high imaging performance while suppressing the change in the optical characteristics of the optical system.

On the other hand, if the i-th lens satisfying the conditional expression (2) is not included, there arises a problem that correction of chromatic aberration and the like is insufficient, and it is difficult to obtain an optical system having high imaging performance.

1-4-3, conditional expression (3)

In this optical system, the first lens preferably satisfies the following conditional expression (3).

Nd1<-0.02×νd1+2.95…(3)

Wherein "Nd 1" is a refractive index of the first lens for a d-line, and "vd 1" is an abbe number of the first lens for the d-line.

The conditional expression (3) is also a formula for the glass material of the first lens, as in the conditional expression (2). When the first lens satisfies the conditional expression (3), the optical characteristics of the first lens are less likely to change when the ambient temperature changes, and the imaging performance of the optical system can be maintained more favorably.

When the first lens does not satisfy the condition (3), since the refractive power of the first lens is large, there arises a problem that an object image cannot be accurately imaged on the image plane depending on the environmental temperature for the same reason as that stated in the condition (1), and the imaging performance is lowered, which is not preferable.

From the viewpoint of obtaining these effects, the first lens more preferably satisfies the following conditional expression (3-a).

Nd1<-0.014×νd1+2.5…(3-a)

1-4-4, conditional expression (4)

In this optical system, the first lens preferably satisfies the following conditional expression (4).

αP1×10⁷<120…(4)

here, the specified temperature range is, for example, a temperature range of-30 ℃ to 70 ℃ inclusive, and when the value of the average linear expansion coefficient α (-30/70) in a temperature range of-30 ℃ to 70 ℃ inclusive is not clear, it is preferable that the average linear expansion coefficient α in any temperature range of-50 ℃ to 90 ℃ inclusive including a temperature range of-0 ℃ to 40 ℃ inclusive is used.

The conditional expression (4) is also an expression concerning the glass material of the first lens. When the first lens satisfies the conditional expression (4), the linear expansion coefficient is small in a temperature range of-30 ℃ to 70 ℃, and changes in the thickness of the first lens and the like can be suppressed even when the ambient temperature changes. Therefore, even when the ambient temperature changes, it is possible to suppress variations in the refractive power of the first lens and variations in the lens interval between the first lens and another lens, and to suppress variations in the focal position and the amount of post-focusing caused by these variations. As a result, the imaging performance of the optical system can be maintained even more favorably even when the ambient temperature changes.

From the viewpoint of obtaining these effects, the first lens more preferably satisfies the following conditional expression (4-a), and still more preferably satisfies conditional expression (4-b).

αP1×10⁷<100…(4-a)

αP1×10⁷<90…(4-b)

1-4-5, conditional expression (5)

In the optical system, the predetermined lens group preferably satisfies the following conditional expression (5).

-65<dndtP1×Fg×Pw1×10⁷<65…(5)

Where "Fg" is the focal length of the above-specified lens group, and "Pw 1" is the refractive power of the first lens.

Conditional expression (5) is an expression regarding the amount of change in refractive index per unit temperature of the first lens, the focal length of the above-specified lens group, and the refractive power of the first lens. When the conditional expression (5) is satisfied, the refractive index change per unit temperature and the value of the refractive power of the first lens are within appropriate ranges with respect to the focal length of the specified lens group, and thus, it is possible to more favorably suppress variations in the focal position or the back focus amount of the optical system due to changes in the ambient temperature, and also possible to more favorably suppress variations in spherical aberration. As a result, the imaging performance of the optical system can be maintained even more favorably even when the ambient temperature changes.

From the viewpoint of obtaining these effects, the above-specified lens group more preferably satisfies the following conditional expression (5-a).

-60<dndtP1×Fg×Pw1×10⁷<60…(5-a)

In the case where the optical system is a zoom optical system having two groups, the predetermined lens group more preferably satisfies the following conditional expression (5-b) from the viewpoint of obtaining the above-described effects.

-25<dndtP1×Fg×Pw1×10⁷<25…(5-b)

1-4-6, conditional expression (6)

In this optical system, the predetermined lens group preferably has at least one lens in addition to the first lens and the i-th lens, and satisfies the following conditional expression (6).

0<Σdoe{νdx/fx}×ft≤150…(6)

Where, "Σ doe { ν dx/fx }" is the sum of { ν d/fi } values of the respective lenses constituting the above-mentioned specified lens group, "ν dx" is the abbe number of the respective lenses constituting the above-mentioned specified lens group with respect to the d-line, "fx" is the focal length of the respective lenses constituting the above-mentioned specified lens group, and "ft" is the maximum focal length that can be displayed by the optical system.

The predetermined lens group includes at least one lens in addition to the first lens and the i-th lens, and when the abbe number, the focal length, and the like of each lens constituting the predetermined lens group with respect to the d-line satisfy the conditional expression (6), the variation of each aberration when the ambient temperature changes tends to be smaller, and the imaging performance can be maintained more favorably.

From the viewpoint of obtaining these effects, the optical system more preferably satisfies the following conditional expression (6-a) (except for the case of a zoom optical system composed of two groups).

20<Σdoe{νdx/fx}×ft≤130…(6-a)

In the case where the optical system is a zoom optical system having two sets, from the viewpoint of obtaining the above-described effects, the optical system preferably satisfies the following conditional expression (6-b), and more preferably satisfies the following conditional expression (6-c).

0<Σdoe{νdx/fx}×ft≤130…(6-b)

0<Σdoe{νdx/fx}×ft≤35…(6-c)

1-4-7, conditional expression (7)

Using the above-mentioned phase difference function formula

(h) When the diffraction surface is represented, the optical system preferably satisfies the following conditional expression (7).

-25<C01×Fg×1000<5…(7)

Where "C01" is the diffraction surface coefficient as described above, "h" is the length from the optical axis in the same radial direction, and "Fg" is the focal length of the above-specified lens group.

Conditional expression (7) is an expression concerning the shape of the diffraction surface and the focal length of the specified lens group. When conditional expression (7) is satisfied, chromatic aberration and the like can be corrected more favorably, and an optical system with higher imaging performance can be obtained. Further, the focal length (fD) of paraxial first-order diffracted light by the diffraction surface can be represented by-1/(2 × C01).

1-4-8, conditional expression (8)

And, using the above-mentioned phase difference function formula

(h) When the diffraction surface is represented, the optical system preferably satisfies the following conditional expression (8).

-1.5<C01×tan(ωw)×fw×1000<0…(8)

Where "C01" is the diffraction surface coefficient as described above, "ω w" is the half angle of view at the minimum focal length that the optical system can display, and "fw" is the minimum focal length that the optical system can display. In addition, the minimum focal length that the optical system can display means the focal length of the optical system when the optical system is a single-focus optical system, and the focal length of the optical system when the optical system is a zoom optical system is the focal length of the optical system at the wide-angle end.

The conditional expression (8) is an expression concerning the shape of the diffraction surface, the minimum focal length that the optical system can display, and the current angle of view. When conditional expression (8) is satisfied, chromatic aberration correction of the minimum focal length that can be displayed by the optical system can be performed more favorably.

1-4-9, conditional expression (9)

The optical system preferably satisfies the following conditional expression (9).

-0.05≤Δ(d-s)/f≤0.05…(9)

Where "f" is an arbitrary focal length that can be displayed by the entire optical system, "Δ (d-s)" is a paraxial image position of the s-line with respect to the d-line at the arbitrary focal length that can be displayed by the entire optical system, and the "s-line" is a ray of light having a wavelength of 852.11 nm.

Here, when the optical system is a zoom optical system, it is more preferable that the following conditional expression (9-a) and/or conditional expression (9-b) be satisfied.

-0.05≤WΔ(d-s)/fW≤0.05…(9-a)

-0.01≤TΔ(d-s)/fT≤0.01…(9-b)

Where "fW" is a focal length of the entire optical system at the wide-angle end, "fT" is a focal length of the entire optical system at the telephoto end, "W Δ (d-s)" is a paraxial image formation position of the s-line with respect to the d-line at the wide-angle end, "T Δ (d-s)" is a paraxial image formation position of the s-line with respect to the d-line at the telephoto end, "d-line" is a ray of light having a wavelength of 852.11nm as described above.

When the conditional expression (9) is satisfied, since the difference (focus shift) between the paraxial imaging position of the d-line of the light in the visible wavelength range and the paraxial imaging position of the s-line of the light in the near infrared wavelength range is small, the focus position does not change even when the wavelength of the light used in the optical system changes between the visible wavelength range and the near infrared wavelength range, and the fluctuation of each aberration can be suppressed. Therefore, even when the wavelength of the light used in the optical system changes between the visible light wavelength range and the near infrared wavelength range in the case where the ambient temperature changes, the optical system satisfying the conditional expression (9) can maintain high imaging performance while suppressing variations in the focal position, the amount of post-focusing, and the like.

When the optical system is a zoom optical system, by further satisfying conditional expression (9-a) and/or conditional expression (9-b) in addition to conditional expression (9), it is possible to prevent a change in focal position and suppress a change in each aberration even when the wavelength of light used changes between the visible wavelength range and the near infrared wavelength range at an arbitrary focal length displayed by the zoom optical system. That is, even when the wavelength of light used in the zoom optical system changes between the visible light wavelength range and the near infrared wavelength range in the case where the ambient temperature changes, the optical system can maintain high imaging performance while suppressing variations in the focal position, the amount of post-focusing, and the like.

From the viewpoint of obtaining these effects, when the optical system is a zoom optical system, it is more preferable that the following conditional expression (9-a) 'and conditional expression (9-b)' are satisfied.

-0.02≤WΔ(d-s)/fW≤0.02…(9-a)’

-0.005≤TΔ(d-s)/fT≤0.005…(9-b)’

In the conditional expression (9), "f" in the conditional expression (9-a) is a focal length at the wide-angle end, and the conditional expression is the same as the conditional expression (9). Further, since the object image at the telephoto end is larger than that at the wide-angle end, the difference in the paraxial imaging position has a larger influence on the imaging performance than that at the wide-angle end. Therefore, even when the wavelength range of the light used changes by satisfying the conditional expression (9-b) at the telephoto end, the contour of the subject image can be prevented from becoming unclear, and good imaging performance can be obtained over the entire zoom range.

1-4-10, conditional expression (10)

In this optical system, the lens having a diffraction surface preferably satisfies the following conditional expression.

-3.0≤Σ{θCs/(fd×νd)}/Σ{1/(fd×νd)}≤3.0…(10)

Where θ Cs is (nC-ns)/(nF-nC), "nC" is the refractive index of the lens having a diffraction surface for the C line (656.27nm), "ns" is the refractive index of the lens having a diffraction surface for the s line, "nF" is the refractive index of the lens having a diffraction surface for the F line (486.13nm), "fd" is the focal length of the lens having a diffraction surface for the d line, and "vd" is the abbe number of the lens having a diffraction surface for the d line.

Here, the "lens having a diffraction surface" refers to the above-mentioned diffractive optical element, and the diffractive optical element may be either a single-layer diffractive optical element or a multi-layer diffractive optical element. The "refractive index of the lens having the diffraction surface" refers to the refractive index of the diffractive optical element, and the multilayer diffractive optical element refers to the refractive index of a layer (lens) disposed closer to the image plane side than the diffraction surface.

Conditional expression (10) is an expression relating to the rate of change of the refractive power of the diffractive optical element in the visible light wavelength range and the rate of change of the refractive power at C-s line. When the conditional expression (10) is satisfied, the diffraction optical element has low anomalous dispersion in the wavelength range of C-line to s-line, and can perform chromatic aberration correction satisfactorily in the wavelength range of the secondary spectrum in addition to the primary spectrum. Therefore, an optical system having high imaging performance over a wide wavelength range can be obtained.

On the other hand, if the conditional expression (10) is not satisfied, the diffraction optical element exhibits anomalous dispersion in the C-line to s-line wavelength range, and therefore, in the C-line to s-line wavelength range, correction of the secondary spectrum becomes difficult, and it may be difficult to obtain good imaging performance in the wavelength range.

From the viewpoint of obtaining the above-described effects, it is more preferable to satisfy the following conditional expression (10-a).

-1.0≤Σ{θCs/(fd×νd)}/Σ{1/(fd×νd)}≤2.0…(10-a)

In addition, when the optical system includes a plurality of diffraction planes, at least one of the diffraction planes preferably satisfies the conditional expression (10), and more preferably all the diffraction planes satisfy the conditional expression (10). The same applies to conditional expression (10-a). The same applies to the diffraction surface arranged in the lens group including the diffraction surface, which does not include the first lens and the i-th lens.

1-4-1, conditional expression (11)

In this optical system, the lens having a diffraction surface preferably satisfies the following conditional expression. The lens having a diffraction surface is the same as that in the case of conditional expression (11).

-15≤Σ{1/(fd×νd)}/Σ{θgF/(fd×νd)}≤15…(11)

Where θ gF is (ng-nF)/(nF-nC), "ng" is the refractive index of the above-described lens having a diffraction surface for g line (435.84nm), "nF" is as described above, "nC" is the refractive index of the above-described lens having a diffraction surface for C line (656.27nm), "ns," "fd," and "vd" are as described above, respectively.

Conditional expression (11) is an expression relating to the difference between the refractive power change rate of the diffractive optical element in the visible light wavelength range and the refractive power change rate at F-g line. When the conditional expression (3) is satisfied, the diffraction optical element has low anomalous dispersion in the wavelength range from the F line to the g line, and can satisfactorily correct chromatic aberration in the wavelength range for the secondary spectrum in addition to the primary spectrum. Therefore, an optical system having high imaging performance over a wide wavelength range can be obtained.

On the other hand, if the conditional expression (11) is not satisfied, the diffraction optical element exhibits anomalous dispersion in the wavelength range from F-line to g-line, and therefore, correction of the secondary spectrum in the wavelength range from F-line to g-line becomes difficult, and it may be difficult to obtain good imaging performance in the wavelength range.

From the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (11-a).

-13≤Σ{1/(fd×νd)}/Σ{θgF/(fd×νd)}≤7.0…(11-a)

In addition, when the optical system includes a plurality of diffraction planes, at least one of the diffraction planes preferably satisfies the conditional expression (11), and more preferably all the diffraction planes satisfy the conditional expression (11). The same applies to conditional expression (11-a).

1-4-12, conditional expression (12)

In this optical system, the predetermined lens group preferably satisfies the following conditional expression (12).

0<νd1/Pw1/fw<1300…(12)

Wherein "vd 1" is the abbe number of the first lens for d-line, "Pw 1" is the refractive power of the first lens, and "fw" is the minimum focal length that the optical system can display.

When the conditional expression (12) is satisfied, it is possible to suppress variations in the focus position and the amount of post-focusing even when the ambient temperature changes. When the conditional expression (12) is satisfied, chromatic aberration correction of the optical system can be performed more favorably. Therefore, an optical system with higher imaging performance can be obtained, and high imaging performance can be maintained even when the ambient temperature changes.

1-4-13, conditional expression (13)

In this optical system, the following conditional expression (13) is preferably satisfied.

-100<dndtP1×Pw1×fw×10⁷<40…(13)

Here, "Pw 1" and "fw" are as described above.

The conditional expression (13) is an expression regarding the amount of change in refractive index per unit temperature of the first lens, the refractive power of the first lens, and the minimum focal length that the optical system can display. When the conditional expression (13) is satisfied, the refractive index change and the refractive power value per unit temperature of the first lens are within appropriate ranges with respect to the minimum focal length that can be displayed by the optical system, and the imaging performance of the optical system can be maintained more favorably even when the minimum focal length and the ambient temperature change.

From the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (13-a).

-100<dndtP1×Pw1×fw×10⁷<40…(13-a)

In the case where the optical system is a zoom optical system having two sets, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (13-b).

-6<dndtP1×Pw1×fw×10⁷<6…(13-b)

In the case where the optical system is a zoom optical system having three groups, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (13-c).

-50<dndtP1×Pw1×fw×10⁷<50…(13-c)

In the case where the optical system is a zoom optical system having four groups, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (13-d).

-12<dndtP1×Pw1×fw×10⁷<12…(13-d)

1-4-14, conditional expression (14)

In this optical system, the following conditional expression (14) is preferably satisfied.

-130<dndtP1×Pw1×ft×10⁷<260…(14)

Here, "Pw 1" and "ft" are as described above.

Conditional expression (14) is an expression regarding the amount of change in the refractive index per unit temperature of the first lens, the refractive power of the first lens, and the maximum focal length that the optical system can display. When the conditional expression (14) is satisfied, the refractive index change and the refractive power value per unit temperature of the first lens are within appropriate ranges with respect to the maximum focal length that can be displayed by the optical system, and the imaging performance of the optical system can be maintained more favorably even when the maximum focal length and the ambient temperature change.

From the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (14-a).

-130<dndtP1×Pw1×ft×10⁷<60…(14-a)

In the case where the optical system is a zoom optical system configured by two groups, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (14-b).

-12<dndtP1×Pw1×ft×10⁷<13…(14-b)

In the case where the optical system is a zoom optical system having three groups, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (14-c).

-120<dndtP1×Pw1×ft×10⁷<120…(14-c)

In the case where the optical system is a zoom optical system having four groups, from the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (14-d).

-80<dndtP1×Pw1×ft×10⁷<80…(14-d)

1-4-15, conditional expression (15)

In the optical system, in the predetermined lens group, when a second lens having the second largest refractive power is used as the second lens among lenses having the same refractive power as the refractive power displayed in the entire predetermined lens group, the first lens and the second lens preferably satisfy the following conditional expression (15).

-130<dndtP1×Pw1×fw+dndtP2×Pw2×fw×10⁷<0…(15)

Where "dndtP 2" is the above-mentioned "dndt" of the second lens, "Pw 2" is the refractive power of the second lens, and "dndtP 1," "Pw 1," and "fw" are as described above.

In the above-described specified lens group, when the first lens having the largest refractive power and the second lens having the second largest refractive power among the lenses having the refractive powers having the same sign as the refractive power displayed by the specified lens group as a whole satisfy the relationship of the above-described conditional expression (15), the amount of change in refractive index per unit temperature and the refractive power of the second lens are within appropriate ranges, and the imaging performance can be maintained more favorably even when the ambient temperature changes.

From the viewpoint of obtaining the above-described effects, the optical system more preferably satisfies the following conditional expression (15-a).

-120<dndtP1×Pw1×fw+dndtP2×Pw2×fw<0…(15-a)

1-4-16, conditional expression (16)

In the optical system, the first lens and the second lens preferably satisfy the following conditional expression (16).

When the conditional expression (16) is satisfied, even when the optical system is a zoom optical system, the imaging performance can be maintained well over the entire zoom range even when the ambient temperature changes.

From the viewpoint of obtaining the above-described effects, the first lens and the second lens more preferably satisfy the following conditional expression (16-a).

In the case where the optical system is a zoom optical system including two groups, the first lens and the second lens preferably satisfy the following conditional expression (16-b) from the viewpoint of obtaining the above-described effects.

In the case where the optical system is a zoom optical system having three groups, the first lens and the second lens preferably satisfy the following conditional expression (16-c) from the viewpoint of obtaining the above-described effects.

In the case where the optical system is a zoom optical system having four groups, the first lens and the second lens preferably satisfy the following conditional expression (16-d) from the viewpoint of obtaining the above-described effects.

1-4-17, conditional expression (17)

In the optical system, the second lens preferably satisfies the following conditional expression (17).

Nd2≥-0.014×νd2+2.5…(17)

Where "Nd 2" is a refractive index of the second lens for the d-line, and "ν d 2" is an abbe number of the second lens for the d-line.

When the second lens satisfies the conditional expression (17), the i-th lens may be the second lens. As a result, as described above, while chromatic aberration and the like are corrected more favorably to obtain an optical system with higher imaging performance, the number of lenses constituting the lens group can be suppressed from increasing, and the optical system can be configured compactly.

1-4-18, conditional expression (18)

Preferably, the optical system includes a first lens group having a positive refractive power and disposed closest to the object side, the first lens group including at least one cemented lens composed of two lenses, and the cemented lens disposed closest to the object side in the first lens group satisfies the following conditional expression (18).

30<|νa1-νa2|<50…(18)

Wherein "ν a 1" is an abbe number of an object side lens constituting the cemented lens for d-line, and "ν a 2" is an abbe number of an image plane side lens constituting the cemented lens for d-line.

When this optical system satisfies the conditional expression (18), chromatic aberration can be corrected more favorably by abbe number difference of the cemented lens in addition to chromatic aberration correction by the diffractive optical element, and an optical system with high imaging performance can be obtained.

When the numerical value of conditional expression (18) is equal to or less than the lower limit value, the abbe number difference between the positive and negative lenses constituting the cemented lens is small, and particularly when the optical system is a zoom optical system having a high zoom ratio, the axial chromatic aberration on the telephoto end side may not be sufficiently formed, which is not preferable. On the other hand, when the numerical value of the above formula (18) is not less than the upper limit, the abbe number difference between the two lenses constituting the cemented lens becomes excessively large. In this case, the i-th lens constituting the cemented lens is not preferable because it is an anomalous dispersion material and axial chromatic aberration cannot be sufficiently formed.

The chromatic aberration includes a linear component and a nonlinear component (anomalous dispersion) which are linear depending on the wavelength. When the first lens is a lens constituting a cemented lens, it is more advantageous to correct chromatic aberration of a nonlinear component than when the first lens is in the form of a single lens. When the second lens is a lens constituting a cemented lens, chromatic aberration of the nonlinear component can be further corrected by another lens in combination with the second lens among the cemented lenses. Further, when the cemented lens includes a diffraction surface, chromatic aberration of the line component can be ensured more favorably.

1-4-19, conditional expression (19)

When the optical system includes the first lens group, the first lens group preferably includes a single lens having both surfaces in contact with an air layer in addition to the cemented lens, and satisfies the following conditional expression (19).

0.21<|NPa1-NP1|<6…(19)

Wherein "NPa 1" is a refractive index of a lens having a positive refractive power constituting a cemented lens arranged closest to the object side in the first lens group with respect to d-line, and "NP 1" is a refractive index of the single lens arranged closest to the object side in the first lens group with respect to d-line.

When the conditional expression (19) is satisfied, the first lens group can condense the beam diameter, and the optical system can be configured compactly. When the optical system is a zoom optical system, the field curvature on the wide-angle end side can be corrected particularly favorably by satisfying the conditional expression (19).

When the optical system is a zoom optical system having four or more lens groups and a high zoom ratio, the first lens group is composed of, in order from the object side, a cemented lens in which a lens having a negative refractive power and a lens having a positive refractive power are cemented, and a single lens having a positive refractive power both surfaces of which are in contact with an air layer, and the single lens having a positive refractive power satisfies the above conditional expression (19), it is possible to correct the axial chromatic aberration on the telephoto end side more favorably.

2. Image pickup apparatus

Next, an imaging device of the present invention will be explained. The imaging device of the present invention includes the optical system of the present invention and an imaging element provided on the image plane side of the optical system and converting an optical image formed by the optical system into an electric signal. Here, the image pickup device and the like are not particularly limited, and a solid-state image pickup device such as a CCD sensor or a CMOS sensor can be used. The imaging device of the present invention is suitable for use as an imaging device using a solid-state imaging element such as a digital camera or a digital video camera. The imaging device may be a lens-fixed type in which a lens is fixed to a housing, or may be a lens-interchangeable type such as a single lens reflex camera or a mirror-less interchangeable lens camera.

Next, the present invention will be described in detail by way of examples and comparative examples. However, the present invention is not limited to the following examples. The optical system of each of the following examples is an imaging optical system used in an imaging apparatus (optical apparatus) such as a digital camera, a digital video camera, a silver salt film camera, or the like. In each lens cross-sectional view, the left side of the drawing is the object side, and the right side is the image plane side.

Example 1

1) Construction of optical system

Fig. 1 is a lens cross-sectional view showing a lens configuration at infinity focusing at a wide-angle end of an optical system according to example 1 of the present invention, and fig. 2 is a lens cross-sectional view showing a lens configuration at infinity focusing at a telephoto end. The optical system is a variable focal length zoom optical system, and is composed of, in order from the object side, a first lens group 1G having negative refractive power, and a second lens group 2G having positive refractive power.

The first lens group 1G is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface facing the object side, a biconcave lens having a negative refractive power, and a meniscus lens having a positive refractive power with a convex surface facing the object side. The second lens group 2G is composed of, in order from the object side, a meniscus lens having a positive refractive power with a convex surface facing the object side, a biconvex lens having a positive refractive power, a meniscus lens having a negative refractive power with a convex surface facing the object side, a cemented lens to which the biconvex lens having a positive refractive power and the biconcave lens having a negative refractive power are cemented, and a biconvex lens having a positive refractive power. Here, the second lens group 2G is the above-mentioned specified lens group mentioned in the present invention, and the object side surface of the above-mentioned concave lens, which is in contact with the air layer, constituting the above-mentioned cemented lens included in the second lens group 2G is a diffraction surface DOE. The biconvex lens constituting the cemented lens is the first lens mentioned in the present invention. Also, the biconvex lens disposed at the closest image side of the second lens group 2G is the second lens mentioned in the present invention. These have refractive powers of the same sign as the second lens group as a whole. An aperture stop is disposed between the first lens group 1G and the second lens group 2G.

In the optical system according to example 1, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G moves toward the image plane side, and the second lens group 2G moves toward the object side.

"CG" shown on the image plane side of the second lens group 2G is cover glass or cover glass, and indicates a low-pass filter, an infrared cut filter, or the like. The term "IMG" is an image plane, and specifically indicates an image pickup plane of a solid-state image pickup device such as a CCD sensor or a CMOS sensor, an adhesive surface of a silver salt film, or the like. These symbols and the like are also the same in the cross-sectional views of the lenses shown in examples 2 to 16.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Lens data of the optical system are shown in table 1. In table 1, "surface No." indicates the order of lens surfaces (surface numbers) from the object side, "r" indicates the radius of curvature of the lens surfaces, "d" indicates the interval on the optical axis of the lens surfaces, "Nd" indicates the refractive index for the d-line (wavelength λ is 587.6nm), and "vd" indicates the abbe number for the d-line. When the lens surface is an aspherical surface, the surface is numbered and denoted by "+ (asterisk)". When the lens surface is a diffraction surface, the surface is numbered with "# (well character)". When the lens surface is aspherical and/or diffractive, the column of the radius of curvature "r" shows the radius of curvature.

Further, the aspherical surface coefficients of the aspherical surfaces shown in table 1 are defined by the following equations in table 2 (2-1). In Table 2(2-1), "E-a" represents ". times.10^-a”。

In the above formula, "R" represents a curvature, "h" represents a height from the optical axis, "k" represents a conical coefficient, and "a 4", "a 6", "A8", and "a 10" … represent aspheric coefficients of respective powers.

Table 2(2-2) shows the focal length (F), F value (Fno), half field angle (ω), and variable interval shown in table 1 of the entire optical system. In table 2(2-2), "6", "7" and "19" respectively mean the variable intervals "d 6", "d 7" and "d 19" shown in table 1, and the reference numeral of "d" is omitted in table 2 (2-2). The focal lengths of the lens groups included in the optical system are shown in table (2-3), where f1 denotes the focal length of the first lens group 1G, and f2 denotes the focal length of the second lens group 2G.

The diffraction surfaces are shown in table 3 as the surface number (surface No), the diffraction order (m), the normalized wavelength (λ), and the diffraction surface coefficients (C01, C02, C03, and C04). Wherein, C01, C02, C03 and C04 correspond to C1, C2, C3 and C4 of the phase difference function, respectively. Table 19 shows the numerical values of conditional expressions (1) to (19). In each table, the length unit is "mm", and the unit of the viewing angle is "°". The same applies to the tables shown in examples 2 to 9, and therefore, the description thereof will be omitted below.

Fig. 3 shows a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 4 shows a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system. Each longitudinal aberration diagram is represented by spherical aberration, astigmatism, and distortion aberration in order from the left side of the drawing. In the graph showing the spherical aberration, the ordinate represents a ratio to the open F value, the abscissa represents defocus, the solid line represents spherical aberration of the d-line (wavelength λ: 587.5618nm), the broken line represents spherical aberration of the s-line (wavelength λ: 852.1100nm), and the dashed-dotted line represents spherical aberration of the g-line (wavelength λ: 435.8343 nm). In the graph showing astigmatism, the vertical axis represents image height, the horizontal axis represents defocus, the solid line represents astigmatism in the sagittal plane, and the broken line represents astigmatism in the meridional plane. In the figure showing the distortion aberration, the vertical axis represents the image height, and the horizontal axis represents the distortion aberration. The same applies to the items related to the longitudinal aberration diagrams described in examples 2 to 9, and therefore, the description thereof will be omitted below.

TABLE 1

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	41.300	0.900	1.9108	35.25	0.716	0.673	TAFD35	1.911	2.0065
2	9.610	4.992
										3	-34.750	0.700	1.7725	49.60	0.773	0.641	S-LAH66	1.773	1.8056
4	12.900	1.344
										5	16.910	3.000	1.9459	17.98	0.646	0.749	FDS18	1.946	2.2483
6	92.810	d6
										7	INF	d7
8*	16.456	1.500	1.5920	67.02	0.807	0.624	M-PCD51	1.592	1.5617
										9*	35.020	0.100
10	13.210	5.050	1.4970	81.54	0.796	0.626	S-FPL51	1.497	1.3584
										11	-13.972	0.100
12#	65.844	0.700	1.5814	40.89	0.737	0.667	E-FL5	1.581	1.9275
										13	7.118	4.450	1.5168	64.20	0.821	0.623	BSC7	1.517	1.6012
14	-14.676	0.600	1.6034	38.03	0.728	0.674	S-TIM5	1.603	1.9676
										15	8.572	0.444
16	12.916	2.400	1.7725	49.60	0.773	0.641	S-LAH66	1.773	1.8056
										17	-35.345	3.000
18	INF	1.500	1.5168	64.20	0.821	0.623	S-BSL7	1.517
										19	INF	d19

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 2

(2-1)

Face NO.	k	A4	A6	A8	A10
						8	1.06	-6.3963E-05	-1.8163E-06	-1.8715E-07	2.3779E-09
9	0.50	1.9628E-04	-1.3522E-06	-1.7275E-07	2.6750E-09

2-2)

F	2.884	4.783	7.265
				Fno	1.260	9.000	9.000
ω	79.932	42.042	27.017
				6	23.121	10.696	6.958
7	7.200	4.634	0.595
				19	3.872	6.395	9.768

(2-3)

f1	-8.975
		f2	12.133

TABLE 3

Noodle No	12
		Diffraction order	1
Normalized wavelength	500nm
		C01	-3.015E-04
C02	-1.000E-06
		C03	6.338E-08
C04	0.000E+00

Example 2

1) Construction of optical system

Fig. 5 is a lens cross-sectional view showing a lens configuration at infinity focusing at the wide-angle end of the optical system according to example 2 of the present invention, and fig. 6 is a lens cross-sectional view showing a lens configuration at infinity focusing at the telephoto end. The optical system is a zoom optical system with a variable focal length, and is composed of a first lens group 1G with positive refractive power, a second lens group 2G with negative refractive power and a third lens group 3G with positive refractive power in sequence from the object side.

The first lens group 1G is composed of a cemented lens in which a meniscus lens having a convex surface directed to the object side and a double convex lens having a positive refractive power are cemented in this order from the object side. The second lens group 2G is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface facing the object side, and a cemented lens to which a biconcave lens having a negative refractive power and a meniscus lens having a positive refractive power with a convex surface facing the object side are cemented. The third lens group 3G is composed of, in order from the object side, a biconvex lens having a positive refractive power, a cemented lens to which the biconvex lens having a positive refractive power and a biconcave lens having a negative refractive power are cemented, and a biconvex lens having a positive refractive power. Here, the third lens group 3G is the above-specified lens group mentioned in the present invention, and further, the biconvex lens arranged closest to the object side of the third lens group 3G is the first lens mentioned in the present invention, and the biconvex lens arranged at the second position from the object side of the third lens group 3G, that is, the positive lens constituting the cemented lens is the second lens mentioned in the present invention.

In the optical system according to example 2, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G is fixed, the second lens group 2G is moved to the image plane side, and the third lens group is moved to the object side.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Table 4 shows lens data of the optical system, table 5(5-1) shows aspheric coefficients of the aspheric surfaces shown in table 4, and table 5(5-2) shows the wide angle end, the intermediate focal length, the focal length (F) and F-number (Fno) at the telephoto end, the half angle of view (ω), and the variable intervals on the optical axis of the optical system. Table 5(5-3) shows the focal length of each lens group of the optical system, f1 is the focal length of the first lens group 1G, f2 is the focal length of the second lens group 2G, and f3 is the focal length of the third lens group 3G. Regarding the diffraction surfaces included in the third lens group 3G, the surface number (surface No), the number of diffraction orders (m), the normalized wavelength (λ), and the diffraction surface coefficients (C01, C02, C03, C04) thereof are shown in table 6. Table 19 shows the numerical values of conditional expressions (1) to (19).

Fig. 7 shows a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 8 shows a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system.

TABLE 4

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	21.867	0.982	1.8467	23.78	0.672	0.711	FDS90	1.847	2.1671
2	16.222	2.742	1.5935	67.00	0.809	0.625	PCD51	1.593	1.5620
										3	-240.757	d3
4	68.680	0.700	1.4875	70.44	0.836	0.619	FC5	1.487	1.5138
										5	14.147	1.811
6	-14.388	0.700	1.7433	49.22	0.770	0.638	NBF1	1.743	1.8109
										7	15.847	1.715	1.9459	17.98	0.646	0.749	FDS18	1.946	2.2483
8	47.188	d8
										9*	7.966	4.700	1.6188	63.85	0.788	0.630	M-PCD4	1.619	1.6061
10*	-15.177	0.150
										11	6.873	2.500	1.4970	81.61	0.798	0.627	FCD1	1.497	1.3575
12#	-14.878	0.600	1.7408	27.76	0.690	0.700	E-FD13	1.741	2.1114
										13	4.111	2.112
14*	38.286	2.427	2.0018	19.32	0.650	0.739	M-FDS2	2.002	2.2295
										15*	-28.394	d15
16	INF	1.200	1.5168	64.20	0.821	0.623	S-BSL7	1.517	1.6012
										17	INF	0.500

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 5

(5-1)

Face NO.	k	A4	A6	A8	A10
						9	-0.69	-2.5930E-05	1.5126E-06	-3.7540E-08	-3.0122E-10
10	0.00	4.0013E-04	-3.7357E-06	-2.5573E-08	3.4067E-11
						14	0.00	8.6459E-04	-2.6059E-05	3.2865E-07	9.6568E-08
15	0.00	2.5423E-04	-3.2446E-05	-9.6602E-08	3.6215E-08

(5-2)

F	8.968	14.326	22.880
				Fno	2.000	2.000	2.000
ω	20.093	11.993	7.384
				3	1.430	6.414	10.778
8	16.468	10.300	4.777
				15	5.471	6.655	7.814

(5-3)

f1	38.837
		f2	-11.074
g3	10.922

TABLE 6

Noodle No	12
		Diffraction order	1
Normalized wavelength	500nm
		C01	-3.712E-04
C02	2.200E-05
		C03	-2.000E-06
C04	9.701E-08

Example 3

1) Construction of optical system

Fig. 9 is a lens cross-sectional view showing a lens configuration at infinity focusing at the wide-angle end of an optical system according to example 3 of the present invention, and fig. 10 is a lens cross-sectional view showing a lens configuration at infinity focusing at the telephoto end. The optical system is a zoom optical system with a variable focal length, and is composed of a first lens group 1G with positive refractive power, a second lens group 2G with negative refractive power and a third lens group 3G with positive refractive power in sequence from the object side.

The first lens group 1G is composed of a cemented lens in which a meniscus lens having a convex surface directed to the object side and a double convex lens having a positive refractive power are cemented in this order from the object side. The second lens group 2G is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface facing the object side, and a cemented lens to which a biconcave lens having a negative refractive power and a meniscus lens having a positive refractive power with a convex surface facing the object side are cemented. The third lens group 3G is composed of, in order from the object side, a biconvex lens having a positive refractive power, a cemented lens to which the biconvex lens having a positive refractive power and a biconcave lens having a negative refractive power are cemented, and a biconvex lens having a positive refractive power. Here, the third lens group 3G is the above-mentioned specified lens group mentioned in the present invention. Further, the biconvex lens disposed closest to the object side of the third lens group 3G is the first lens mentioned in the present invention, and the biconvex lens constituting the above-described cemented lens is the second lens mentioned in the present invention. The joint surface of the cemented lens constituting the first lens group 1G and the joint surface of the cemented lens included in the third lens group 3G are diffraction surfaces DOE, respectively. An aperture stop is disposed between the second lens group 2G and the third lens group 3G.

In the optical system according to example 3, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G is fixed, the second lens group 2G is moved to the image plane side, and the third lens group 3G is moved to the object side.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Table 7 shows lens data of the optical system, table 8(8-1) shows aspheric coefficients of the aspheric surfaces shown in table 7, and table 8(8-2) shows the wide angle end, the intermediate focal length, the focal length (F) at the telephoto end, the F value (Fno), the half field angle (ω), and the variable intervals on the optical axis of the optical system. The focal length of each lens group included in the optical system is shown in table 8(8-3), f1 is the focal length of the first lens group 1G, f2 is the focal length of the second lens group 2G, and f3 is the focal length of the third lens group 3G. Table 9 shows the diffraction surfaces included in the first lens group 1G and the third lens group 3G, their surface numbers (surface nos), diffraction orders (m), normalized wavelengths (λ), and diffraction surface coefficients (C01, C02, C03, C04). Table 19 shows the numerical values of conditional expressions (1) to (19).

Fig. 11 is a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 12 is a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system.

TABLE 7

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	18.580	0.982	1.8467	23.78	0.672	0.711	FDS90	1.847	2.1671
2#	13.598	2.742	1.5182	58.96	0.787	0.633	E-C3	1.518	1.6746
										3	-130.565	d3
4	68.680	0.700	1.4875	70.44	0.836	0.619	FC5	1.487	1.5138
										5	14.147	1.811
6	-14.388	0.700	1.7433	49.22	0.770	0.638	NBF1	1.743	1.8109
										7	15.847	1.715	1.9459	17.98	0.646	0.749	FDS18	1.946	2.2483
8	47.188	d8
										9	INF	d9
10*	8.047	4.700	1.6188	63.85	0.788	0.630	M-PCD4	1.619	1.6061
										11*	-14.895	0.150
12	7.402	2.500	1.4875	70.44	0.836	0.619	FC5	1.487	1.5138
										13#	-15.908	0.600	1.7408	27.76	0.690	0.700	E-FD13	1.741	2.1114
14	4.451	2.112
										15*	39.241	2.427	2.0018	19.32	0.650	0.739	M-FDS2	2.002	2.2295
16*	-32.865	d16
										17	INF	1.200	1.5168	64.20	0.821	0.623	S-BSL7	1.517	1.6012
18	INF	d18

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 8

(8-1)

Face NO.	k	A4	A6	A8	A10
						10	-0.69	-5.1513E-05	2.6048E-06	-3.2572E-08	0.0000E+00
11	0.00	4.0036E-04	-2.5713E-06	-4.2274E-09	0.0000E+00
						15	0.00	8.8324E-04	-3.4974E-05	1.5655E-06	0.0000E+00
16	0.00	3.8533E-04	-3.5592E-05	6.8577E-07	0.0000E+00

(8-2)

F	9.168	14.686	23.473
				Fn0	1.714	1.859	2.093
ω	19.556	11.671	7.183
				3	1.430	6.414	10.778
8	12.117	7.133	2.769
				9	4.352	3.167	2.008
16	5.471	6.655	7.814
				18	0.731	-0.731	-0.019

(8-3)

f1	38.683
		f2	-11.074
f3	10.999

TABLE 9

(9-1)

Noodle No	2
		Diffraction order	1
Normalized wavelength	500nm
		C01	-5.900E-05
C02	-6.234E-08
		C03	1.171E-08
C04	-1.352E-10

(9-2)

Noodle No	13
		Diffraction order	1
Normalized wavelength	500nm
		C01	-5.309E-04
C02	-9.000E-06
		C03	1.000E-06
C04	-1.445E-08

Example 4

1) Construction of optical system

Fig. 13 is a lens cross-sectional view showing a lens configuration at infinity focusing at the wide-angle end of an optical system according to example 4 of the present invention, and fig. 14 is a lens cross-sectional view showing a lens configuration at infinity focusing at the telephoto end. The optical system is a zoom optical system with a variable focal length and sequentially comprises a first lens group 1G with negative refractive power, a second lens group 2G with positive refractive power, a third lens group 3G with negative refractive power and a fourth lens group 4G with positive refractive power from the object side.

The first lens group 1G is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface facing the object side, a biconcave lens having a negative refractive power, a biconvex lens having a positive refractive power, and a biconcave lens having a negative refractive power. The second lens group 2G is composed of, in order from the object side, a biconvex lens having a positive refractive power, an aperture stop, and a cemented lens in which a meniscus lens having a negative refractive power with a convex surface facing the object side and a biconvex lens having a positive refractive power are cemented. The third lens group 3G is composed of a biconcave lens having a negative refractive power and a biconvex lens having a positive refractive power in order from the object side. The fourth lens group 4G is composed of a biconvex lens having a positive refractive power. Here, the second lens group 2G is the above-mentioned specified lens group mentioned in the present invention. The joint surface of the joint lens constituting the second lens group 2G is a diffraction surface DOE. Further, the biconvex lens constituting the above-described cemented lens is the first lens mentioned in the present invention, and the biconvex lens disposed on the most object side of the second lens group 2G is the second lens mentioned in the present invention.

In the optical system according to example 4, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G moves toward the image plane side while drawing a convex locus, the second lens group 2G moves toward the object side, the third lens group 3G moves toward the object side while drawing a convex locus, and the fourth lens group 4G is fixed.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Table 10 shows lens data of the optical system, table 11(11-1) shows aspheric coefficients of the aspheric surfaces shown in table 10, and table 11(11-2) shows the wide angle end, the intermediate focal length, the focal length (F) and F-number (Fno) at the telephoto end, the half angle of view (ω), and the variable intervals on the optical axis of the optical system. Table 11(11-3) shows the focal lengths of the respective lens groups of the optical system, f1 is the focal length of the first lens group 1G, f2 is the focal length of the second lens group 2G, f3 is the focal length of the third lens group 3G, and f4 is the focal length of the fourth lens group 4G. As for the diffraction surfaces included in the second lens group 2G, the surface number (surface No), the number of diffraction orders (m), the normalized wavelength (λ), and the diffraction surface coefficients (C01, C02, C03, C04) are shown in table 12. Table 19 shows the numerical values of conditional expressions (1) to (19).

Fig. 15 is a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 16 is a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system.

Watch 10

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	20.859	0.500	1.8830	40.80	0.499	0.565	TAFD30	1.883	1.9288
2	9.500	7.875
										3	-73.957	0.500	1.6385	55.45	0.529	0.547	BACD18	1.639	1.7237
4	12.285	1.444
										5*	17.692	5.301	1.9027	31.00	0.478	0.594	L-LAH86	1.903	2.0660
6	-22.700	0.412
										7	-18.438	0.500	1.6188	63.85	0.537	0.542	M-PCD4	1.619	1.6061
8*	18.285	d8
										9*	11.239	4.168	1.5920	67.02	0.551	0.536	M-PCD51	1.592	1.5617
10*	-29.794	0.100
										11	INF	2.768
12	26.460	0.480	1.8000	29.84	0.472	0.602	S-NBH55	1.800	2.0822
										13#	7.247	3.323	1.4875	70.44	0.573	0.530	FC5	1.487	1.5138
14	-26.997	d14
										15	-12.976	0.500	1.6200	36.30	0.492	0.587	E-F2	1.620	1.9918
16	12.988	1.134
										17	19.171	1.586	1.8503	32.27	0.476	0.593	S-LAH71	1.850	2.0482
18	-57.591	d18
										19*	30.968	2.370	1.4971	81.56	0.545	0.538	M-FCD1	1.497	1.3582
20*	-13.411	0.100
										21	INF	1.500	1.5168	64.20	0.821	0.623	BSC7	1.517	1.6012
22	INF	5.400

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 11

(11-1)

Face NO.	k	A4	A6	A8	A10
						5	0.00	-9.6959E-05	1.8072E-06	-2.0709E-08	1.8215E-10
8	0.00	-2.5182E-04	2.5917E-06	-3.2052E-08	2.2086E-10
						9	0.00	-1.0146E-04	3.3365E-07	-9.5110E-09	3.7357E-11
10	0.00	6.6062E-05	2.7714E-07	-6.8887E-09	4.9588E-11
						19	0.00	5.3488E-04	1.7167E-05	3.8292E-07	2.4674E-08
20	0.00	6.5212E-04	4.0285E-05	-1.8054E-06	1.2744E-07

(11-2)

F	3.086	9.525	31.119
				Fno	1.239	2.771	6.258
ω	57.755	18.869	5.823
				8	38.343	9.721	0.162
14	2.625	10.381	32.167
				18	0.100	2.535	10.536

(11-3)

f1	-9.903
		f2	13.957
f3	-34.245
		f4	19.166

TABLE 12

Noodle No	13
		Diffraction order	1
Normalized wavelength	500nm
		C01	-2.343E-04
C02	4.019E-07
		C03	-6.624E-08
C04	5.796E-11

Example 5

1) Construction of optical system

Fig. 17 is a lens cross-sectional view showing a lens configuration at infinity focusing at the wide-angle end of the optical system according to example 5 of the present invention, and fig. 18 is a lens cross-sectional view showing a lens configuration at infinity focusing at the telephoto end. The optical system is a zoom optical system with a variable focal length and sequentially comprises a first lens group 1G with positive refractive power, a second lens group 2G with negative refractive power, a third lens group 3G with positive refractive power and a fourth lens group 4G with positive refractive power from the object side.

The first lens group 1G is composed of, in order from the object side, a cemented lens in which a meniscus lens having a convex surface facing the object side and having negative refractive power and a biconvex lens having positive refractive power are cemented, and a meniscus lens having a convex surface facing the object side and having positive refractive power. The second lens group 2G is composed of, in order from the object side, a biconcave lens having a negative refractive power, and a cemented lens in which the biconcave lens having a negative refractive power and a meniscus lens having a positive refractive power with a convex surface facing the object side are cemented. The third lens group 3G is composed of a meniscus lens having a positive refractive power with a convex surface toward the object side. The fourth lens group 4G is composed of a biconvex lens having a positive refractive power, and a cemented lens to which a biconcave lens having a negative refractive power and a biconvex lens having a positive refractive power are cemented. Here, the fourth lens group 4G is the above-mentioned specified lens group mentioned in the present invention. The joint surface of the joint lens constituting the fourth lens group 4G is a diffraction surface DOE. In addition, in the fourth lens group 4G, the biconvex lens constituting the cemented lens described above is the first lens mentioned in the present invention, and the biconvex lens disposed closest to the object side is the second lens mentioned in the present invention. An aperture stop is disposed on the object side of the third lens group 3G.

In the optical system according to example 5, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G is fixed, the second lens group 2G is moved to the image plane side, the third lens group 3G is fixed, the fourth lens groups are moved to the object side along different loci, and the fourth lens group 4G is moved to the object side along a locus of a convex.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Table 13 shows lens data of the optical system, table 14(14-1) shows aspheric coefficients of the aspheric surfaces shown in table 13, and table 14(14-2) shows the wide angle end, the intermediate focal length, the focal length (F) and F value (Fno) at the telephoto end, the half angle of view (ω), and the variable intervals on the optical axis of the optical system. The focal length of each lens group included in the optical system is shown in table 14(14-3), f1 is the focal length of the first lens group 1G, f2 is the focal length of the second lens group 2G, f3 is the focal length of the third lens group 3G, and f4 is the focal length of the fourth lens group 4G. Regarding the diffraction surfaces included in the fourth lens group 4G, the surface number (surface No), the number of diffraction orders (m), the normalized wavelength (λ), and the diffraction surface coefficients (C01, C02, C03, C04) are shown in table 15. Table 19 shows the numerical values of conditional expressions (1) to (19).

Fig. 19 is a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 20 is a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system.

Watch 13

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	58.529	1.000	1.8467	23.78	0.455	0.619	FDS90	1.847	2.1671
2	27.640	8.838	1.5928	68.62	0.528	0.544	FCD515	1.593	1.5393
										3	-184.699	0.150
4	23.729	3.757	1.6385	55.45	0.529	0.547	BACD18	1.639	1.7237
										5	92.840	d5
6	-143.393	0.700	1.8830	40.80	0.499	0.565	TAFD30	1.883	1.9288
										7	8.289	2.778
8	-18.129	1.000	1.6968	55.46	0.545	0.543	LAC14	1.697	1.7236
										9	9.418	2.075	1.9212	23.96	0.457	0.620	FDS24	1.921	2.1646
10	285.004	d10
										11	INF	0.827
12*	12.466	2.578	1.5533	71.68	0.536	0.540	M-FCD500	1.553	1.4965
										13*	84.596	d13
14*	12.942	2.592	1.5533	71.68	0.536	0.540	M-FCD500	1.553	1.4965
										15*	-28.489	0.300
16	-28.680	0.700	1.7174	29.50	0.471	0.603	E-FD1	1.717	2.0870
										17#	10.088	2.424	1.6204	60.34	0.546	0.539	BACD16	1.620	1.6552
18	-16.829	d18
										19	INF	1.500	1.5168	64.20	0.821	0.623	S-BSL7	1.517	1.6012
20	INF	2.670

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 14

(14-1)

Face NO.	k	A4	A6	A8	A10
						12	0.00	-1.5336E-04	-1.1094E-06	-1.9116E-08	-1.0995E-09
13	0.00	-1.1849E-04	3.4898E-07	-7.0104E-08	-2.6431E-10
						14	0.00	-1.4108E-04	2.5968E-06	-7.3425E-08	2.0901E-09
15	0.00	6.4100E-05	3.1479E-06	-6.0588E-08	1.6423E-09

(14-2)

F	5.146	15.737	48.922
				Fno	1.431	1.666	1.673
ω	34.342	11.341	3.597
				5	1.200	13.343	21.106
10	21.829	9.686	1.923
				13	9.040	6.710	9.697
18	7.413	9.743	6.755

(14-3)

f1	35.412
		f2	-7.414
f3	26.091
		f4	16.291

Watch 15

Noodle No	17
		Diffraction order	1
Normalized wavelength	500nm
		C01	-3.005E-04
C02	2.000E-05
		C03	-1.000E-06
C04	1.590E-08

Example 6

1) Construction of optical system

Fig. 21 is a lens cross-sectional view showing a lens configuration at infinity focusing at the wide-angle end of the optical system according to example 6 of the present invention, and fig. 22 is a lens cross-sectional view showing a lens configuration at infinity focusing at the telephoto end. The optical system is a zoom optical system with a variable focal length and sequentially comprises a first lens group 1G with positive refractive power, a second lens group 2G with negative refractive power, a third lens group 3G with positive refractive power and a fourth lens group 4G with positive refractive power from the object side.

The first lens group 1G is composed of, in order from the object side, a cemented lens in which a meniscus lens having a convex surface facing the object side and having negative refractive power and a biconvex lens having positive refractive power are cemented, and a meniscus lens having a convex surface facing the object side and having positive refractive power. The second lens group 2G is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface directed to the object side, and a cemented lens to which a biconcave lens having a negative refractive power and a biconvex lens having a positive refractive power are cemented. The third lens group 3G is composed of a meniscus lens having a positive refractive power with a convex surface toward the object side. The fourth lens group 4G is composed of a biconvex lens having a positive refractive power, and a cemented lens to which a biconcave lens having a negative refractive power and a biconvex lens having a positive refractive power are cemented. Here, the first lens group 1G is the above-mentioned specified lens group mentioned in the present invention. The cemented surface of the cemented lens constituting the first lens group 1G and the object side surface of the meniscus lens constituting the third lens group are diffraction surfaces DOE, respectively. In the first lens group 1G, the biconvex lens constituting the cemented lens described above is the first lens mentioned in the present invention, and the biconvex lens disposed closest to the object side is the second lens mentioned in the present invention. An aperture stop is disposed on the object side of the third lens group 3G.

In the optical system according to example 6, upon zooming from the wide-angle end to the telephoto end, the first lens group 1G is fixed, the second lens group 2G is moved to the image plane side, the third lens group 3G is fixed, the fourth lens groups are moved to the object side along different loci, and the fourth lens group 4G is moved to the object side along a locus of a convex.

2) Numerical example

Next, a numerical example of the optical system, in which specific numerical values are introduced, will be described. Table 16 shows lens data of the optical system, table 17(17-1) shows aspheric coefficients of the aspheric surfaces shown in table 16, and table 17(17-2) shows the wide angle end, the intermediate focal length, the focal length (F) and F-number (Fno) at the telephoto end, the half angle of view (ω), and the variable intervals on the optical axis of the optical system. Table 17(17-3) shows the focal length of each lens group included in the optical system, f1 is the focal length of the first lens group 1G, f2 is the focal length of the second lens group 2G, f3 is the focal length of the third lens group 3G, and f4 is the focal length of the fourth lens group 4G. The diffraction surfaces included in the first lens group 1G and the third lens group 3G are shown in table 18 by the surface number (surface No), the number of diffraction orders (m), the normalized wavelength (λ), and the diffraction surface coefficients (C01, C02, C03, C04). Table 19 shows the numerical values of conditional expressions (1) to (19).

Fig. 23 is a longitudinal aberration diagram in infinity focusing at the wide-angle end of the optical system, and fig. 24 is a longitudinal aberration diagram in infinity focusing at the telephoto end of the optical system.

TABLE 16

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	47.632	1.000	1.8467	23.78	0.455	0.619	FDS90	1.847	2.1671
2#	29.296	6.500	1.5935	67.00	0.552	0.537	PCD51	1.593	1.5620
										3	-309.939	0.150
4	27.817	3.945	1.6968	55.46	0.545	0.543	LAC14	1.697	1.7236
										5	81.483	d5
6	496.669	0.700	1.8830	40.80	0.499	0.565	TAFD30	1.883	1.9288
										7	8.853	3.235
8	-12.527	1.000	1.6968	55.46	0.545	0.543	LAC14	1.697	1.7236
										9	12.362	3.143	1.9212	23.96	0.457	0.620	FDS24	1.921	2.1646
10	-125.651	d10
										11	INF	1.905
12*#	12.897	2.485	1.5920	67.02	0.551	0.536	M-PCD51	1.592	1.5617
										13*	54.767	d13
14*	15.218	3.189	1.4971	81.56	0.545	0.538	M-FCD1	1.497	1.3582
										15*	-13.939	0.300
16	-19.701	0.700	1.5927	35.45	0.485	0.593	FF5	1.593	2.0037
										17	13.332	3.383	1.4970	81.61	0.544	0.539	FCD1	1.497	1.3575
18	-14.419	d18
										19	INF	1.500	1.5168	64.20	0.821	0.623	S-BSL7	1.517	1.6012
20	INF	2.670

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 17

(17-1)

Face NO.	k	A4	A6	A8	A10
						12	0.00	-2.3790E-04	-2.5175E-06	-5.2102E-09	-2.7683E-09
13	0.00	-2.4892E-04	8.9631E-08	-1.2327E-07	-2.0564E-10
						14	0.00	-1.8969E-04	1.3487E-06	-1.2750E-07	1.9589E-09
15	0.00	1.4077E-04	1.1812E-06	-1.2462E-07	2.0293E-09

(17-2)

F	5.140	15.721	48.583
				Fno	1.440	1.584	1.649
ω	35.066	11.203	3.598
				5	0.471	12.869	20.785
10	20.978	8.580	0.665
				13	8.012	5.670	8.381
18	7.316	9.659	6.947

(17-3)

f1	36.168
		f2	-7.504
f3	27.269
		f4	16.572

Watch 18

(18-1)

Noodle No	2
		Diffraction order	1
Normalized wavelength	500nm
		C01	-1.020E-04
C02	5.890E-08
		C03	-3.183E-10
C04	6.091E-13

(18-2)

Noodle No	12
		Diffraction order	1
Normalized wavelength	500nm
		C01	-3.359E-04
C02	6.167E-06
		C03	-1.756E-07
C04	1.603E-09

Comparative example

Next, a comparative example will be described. Here, as a comparative example, a zoom optical system having a negative/positive two-group configuration having almost the same lens configuration as in example 1 is exemplified. The lens configuration of the optical system of the comparative example is almost the same as that of example 2, and therefore, explanation and illustration thereof are omitted. Also, table 20 shows lens data of the optical system. Table 21(21-1) shows aspheric coefficients of the aspheric surfaces shown in table 20, and table 21(21-2) shows the respective focal lengths (F), F-number (Fno), half field angle (ω), and variable intervals on the optical axis at the wide angle end, the intermediate focal length, and the telephoto end of the optical system. Table 21(21-3) shows the focal lengths of the lens groups included in the optical system, where f1 is the focal length of the first lens group 1G, and f2 is the focal length of the second lens group 2G.

Watch 20

Face NO.	r	d	nd	vd	θCs	θgF	Glass material	Ndi	Value of formula (2)
										1	41.3	0.9	1.911	35.25	0.486	0.582	TAFD35_HOYA	1.911	2.0065
2	9.61	4.992
										3	-34.75	0.7	1.773	49.6	0.526	0.552	SLAH66_OHARA	1.773	1.8056
4	12.9	1.344
										5	16.91	3	1.946	17.98	0.438	0.654	FDS18_HOYA	1.946	2.2483
6	92.81	23.49
										7	0	7.2
8*	17.71	1.5	1.592	67.02	0.551	0.536	MPCD51_HOYA	1.592	1.5617
										9*	35.113	0.1
10#	13.65	5.05	1.497	81.54	0.543	0.537	SFPL51_OHARA	0.178	1.3584
										11	-13.65	0.1
12	42.7	0.7	1.581	40.75	0.502	0.577	STIL25_OHARA	-0.554	1.9295
										13*	7.4	4.45	1.497	81.54	0.543	0.537	SFPL51_OHARA	0.178	1.3584
14*	-15.18	0.6	1.603	38.03	0.495	0.583	STIM5_OHARA	-0.586	1.9676
										15	9.81	0.444
16	17.7	2.4	1.773	49.6	0.526	0.552	SLAH66_OHARA	-0.186	1.8056
										17	-26.14	6.35

The corresponding formula (2) ═ 0.014v di +2.5

TABLE 21

(21-1)

Face NO.	k	A4	A6	A8	A10
						8	2.0600E+00	-6.3963E-05	-1.8163E-06	-1.8715E-07	2.3779E-09
9	1.5000E+00	1.9628E-04	-1.3522E-06	-1.7275E-07	2.6750E-09

(21-2)

F	2.92	4.8	7.7
				Fno	1.261	1.535	2.037
ω	78.258	41.815	25.39
				8	23.492	11.149	6.466
9	7.2	4.634	0.595
				16	6.35	8.916	12.955

(21-3)

f1	-8.97472
		f2	12.385

Fig. 25 and 26 show temperature fluctuations of the back focus on the t-line, s-line, d-line, and F-line, respectively, in the optical system of example 2 and the optical system of the comparative example. It is understood that the back focal point of the optical systems of example 2 and comparative example changes with the change in the ambient temperature, but the amount of fluctuation of the optical system of example 2 is smaller than that of the optical system of comparative example. In the optical system of example 2, when the back focus at 20 ℃ is set as the reference, the amount of fluctuation of the back focus at 60 ℃ is 0.01mm or less (see fig. 25). In contrast, in the optical system of the comparative example, when the back focus at 20 ℃ is set as the reference, the variation of the back focus at 60 ℃ is 0.02mm or more (see fig. 26). In particular, in the optical system of the comparative example, the axial chromatic aberration amount greatly fluctuates due to the environmental temperature change, and the difference between the F-line and the d-line is 0.5 μm at the wide-angle end and 3 μm at the telephoto end, as compared with that at normal temperature. In the optical system of example 2, the difference was 0.2 μm at the wide-angle end and 0.6 μm at the telephoto end, and the range of variation of the optical system of the comparative example was 1 time or more as compared with the optical system of example 2. In addition, with respect to the optical system of the comparative example, the value of conditional expression (1) was-6.2, showing a large negative value.

In the optical system according to the other embodiment, the change in the back focus due to the change in the ambient temperature is small, and the variation in each aberration such as the axial chromatic aberration can be suppressed.

Industrial applicability

Claims

1. An optical system which is a zoom optical system having a plurality of groups of lens groups, at least any one of the groups of lens groups being a specified lens group, and zooming by changing an interval of each of the lens groups,

having at least one lens group comprising a diffractive surface,

when at least any one of the lens groups including the diffraction surface is set as the specified lens group,

of the specified lens groups, a lens having the largest refractive power among lenses having refractive powers of the same sign as the refractive power displayed by the specified lens group as a whole is taken as a first lens,

in the specified lens group, any one of lenses other than the first lens among lenses having a refractive power of the same sign as that of the refractive power exhibited by the specified lens group as a whole is set as an i-th lens,

the first lens satisfies the following conditional expression (1),

the i-th lens satisfies the following conditional expression (2),

dndtP1×10⁶>-5 …(1)

Ndi≥-0.014×νdi+2.5 …(2)

wherein "dndt" is a temperature coefficient of absolute refractive index of the lens in vacuum (absolute dn/dT) in a temperature range of 20 ℃ to 40 ℃ inclusive for a light ray having a wavelength of 632.8nm, "dndtP 1" is a dndt of the first lens, "Ndi" is a refractive index of the i-th lens for a d-line, "ν di" is an abbe number of the i-th lens for a d-line, and "d-line" is a light ray having a wavelength of 587.56 nm.

2. The optical system according to claim 1, wherein the first lens satisfies the following conditional expression (3),

Nd1<-0.02×νd1+2.95 …(3)

3. The optical system according to claim 1, wherein the first lens satisfies the following conditional expression (4),

αP1×10⁷>120 …(4)

wherein "α P1" is a value of the average linear expansion coefficient α (-30/70) of the first lens in a specified temperature range including a temperature range of 0 ℃ or higher and 40 ℃ or lower.

4. The optical system according to claim 1, wherein the specified lens group satisfies the following conditional expression (5),

-65<dndtP1×Fg×Pw1×10⁷<65 …(5)

where "Fg" is the focal length of the specified lens group, and "Pw 1" is the refractive power of the first lens.

5. The optical system according to claim 1, wherein the specified lens group has at least one lens in addition to the first lens and the i-th lens, and satisfies the following conditional expression (6),

0<Σdoe{νdx/fx}×ft≤150 …(6)

where, "Σ doe { ν dx/fx }" is a sum of { ν d/fi } values of the respective lenses constituting the specified lens group, "ν dx" is an abbe number of the respective lenses constituting the specified lens group with respect to a d-line, "fx" is a focal length of the respective lenses constituting the specified lens group, and "ft" is a maximum focal length that can be displayed by the optical system.

6. The optical system of claim 1, wherein the following phase difference function is used

When the diffraction surface is expressed, the optical system satisfies the following conditional expression (7),

-25<C01×Fg×1000<5 …(7)

where "m" is the diffraction order, "λ" is the normalized wavelength, "C01," "C02," "C03," and "C04" are the diffraction surface coefficients, "h" is the length from the optical axis in the same radial direction, and "Fg" is the focal length of the specified lens group.

7. The optical system of claim 1, wherein the following phase difference function is used

When the diffraction surface is expressed, the optical system satisfies the following conditional expression (8),

-1.5<C01×tan(ωw)×fw×1000<0 …(8)

where "m" is the diffraction order, "λ" is the normalized wavelength, "C01," "C02," "C03," and "C04" are the diffraction surface coefficients, "h" is the length from the optical axis in the same radial direction, "ω w" is the half angle of field at the minimum focal length that the optical system can display, and "fw" is the minimum focal length that the optical system can display.

8. The optical system of claim 1, wherein the optical system satisfies the following conditional expression (9),

-0.05≤Δ(d-s)/f≤0.05 …(9)

where "f" is an arbitrary focal length that can be displayed by the entire optical system, "Δ (d-s)" is a paraxial image position of the s-line for the d-line at the arbitrary focal length that can be displayed by the entire optical system, and the "s-line" is a ray of light having a wavelength of 852.11 nm.

9. The optical system according to claim 1, wherein the lens having a diffraction surface satisfies the following conditional expression (10),

-3.0≤Σ{θCs/(fd×νd)}/Σ{1/(fd×νd)}≤3.0 …(10)

where θ Cs is (nC-ns)/(nF-nC), "nC" is the refractive index of the lens with a diffraction surface for C line (656.27nm), "ns" is the refractive index of the lens with a diffraction surface for s line (852.11nm), "nF" is the refractive index of the lens with a diffraction surface for F line (486.13nm), "fd" is the focal length of the lens with a diffraction surface for d line, and "vd" is the abbe number of the lens with a diffraction surface for d line.

10. The optical system according to claim 1, wherein the lens having a diffraction surface satisfies the following conditional expression (11),

-15≤Σ{1/(fd×νd)}/Σ{θgF/(fd×νd)}≤15 …(11)

where θ gF is (ng-nF)/(nF-nC), "ng" is the refractive index of the lens with a diffraction surface for g-line (435.84nm), "nF" is the refractive index of the lens with a diffraction surface for F-line (486.13nm), "nC" is the refractive index of the lens with a diffraction surface for C-line (656.27nm), "ns" is the refractive index of the lens with a diffraction surface for s-line (852.11nm), "fd" is the focal length of the lens with a diffraction surface for d-line, and "vd" is the abbe number of the lens with a diffraction surface for d-line.

11. The optical system according to claim 1, wherein the specified lens group satisfies the following conditional expression (12),

0<νd1/Pw1/fw<1300 …(12)

12. The optical system of claim 1, wherein the optical system satisfies the following conditional expression (13),

-100<dndtP1×Pw1×fw×10⁷<40 …(13)

where "Pw 1" is the refractive power of the first lens, and "fw" is the minimum focal length that the optical system can display.

13. The optical system of claim 1, wherein the optical system satisfies the following conditional expression (14),

-130<dndtP1×Pw1×ft×10⁷<260 …(14)

where "Pw 1" is the refractive power of the first lens, and "ft" is the maximum focal length that the optical system can display.

14. The optical system according to claim 1, wherein, in the specified lens group, when a lens having a refractive power of the same sign as that of the refractive power exhibited by the specified lens group as a whole is set as a second lens, the first lens and the second lens satisfy a following conditional expression (15),

-130<dndtP1×Pw1×fw+dndtP2×Pw2×fw×10⁷<0 …(15)

wherein "dndtP 2" is the dndt of the second lens, "Pw 1" is the refractive power of the first lens, "Pw 2" is the refractive power of the second lens, and "fw" is the minimum focal length that the optical system can display.

15. The optical system according to claim 1, wherein, in the specified lens group, when a lens having a refractive power of the same sign as that of the refractive power exhibited by the specified lens group as a whole is set as a second lens, the first lens and the second lens satisfy a following conditional expression (16),

wherein "dndtP 2" is the dndt of the second lens, "Pw 1" is the refractive power of the first lens, "Pw 2" is the refractive power of the second lens, "fw" is the minimum focal length that the optical system can display, and "ft" is the maximum focal length that the optical system can display.

16. The optical system according to claim 1, wherein the first lens satisfies the following conditional expression (3-a),

Nd1<-0.014×νd1+2.5 …(3-a)

17. The optical system according to claim 1, wherein, in the specified lens group, when a lens having a refractive power of the same sign as that displayed by the specified lens group as a whole is set as a second lens, the second lens satisfies a following conditional expression (17),

Nd2≥-0.014×νd2+2.5 …(17)

wherein "Nd 2" is a refractive index of the second lens for a d-line, and "vd 2" is an abbe number of the second lens for the d-line.

18. The optical system according to claim 1, wherein the number of lenses constituting the specified lens group is 3 or more and 6 or less.

19. The optical system according to claim 1, wherein the specified lens group has a positive refractive power.

20. The optical system according to claim 1, wherein a first lens group having a positive refractive power and disposed closest to the object side is provided, the first lens group including at least one cemented lens composed of two lenses, in the first lens group, the cemented lens disposed closest to the object side satisfies a following conditional expression (18),

30<|νa1-νa2|<50 …(18)

21. An imaging apparatus comprising the optical system according to any one of claims 1 to 20, and an imaging element provided on an image plane side of the optical system and configured to convert an optical image formed by the optical system into an electric signal.