JP2002072082A - Optical system and optical equipment using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a telephoto type optical system having a large aperture ratio whose image forming performance is improved by combining a dioptric system and a diffraction optical device, and optical equipment using the optical system. SOLUTION: This optical system is provided with a 1st lens group having positive refractive power and a 2nd lens group having negative refractive power in order from an object side, and the 1st lens group is constituted of the diffraction optical device consisting of a diffraction grating having rotationally symmetric shape with respect to an optical axis and having positive refractive power, one or more positive lenses and one or more negative lenses. Assuming that ϕD means the refractive power of the diffraction optical device having the positive refractive power in the 1st lens group, ϕ1 means the refractive power of the 1st lens group, ϕ means the refractive power of an entire lens system and L means the entire length (a distance from the 1st lens group to an image surface) of the lens, the optical system satisfies conditional expressions 0.005<ϕD/ϕ1<0.05, 0.45<ϕ/ϕ1<0.5 and 0.50<ϕ×L<0.75.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system and an optical apparatus using the same, and more particularly, to a telephoto taking lens having a large aperture and improved imaging performance using a refractive optical system and a diffractive optical element. It is suitable for optical devices such as a silver halide photographic camera, a video camera, an electronic still camera, and a digital camera.

[0002]

2. Description of the Related Art Conventionally, as a lens type suitable for a photographing optical system having a long focal length, an optical system having a front lens group having a positive refractive power and a rear lens group having a negative refractive power in order from the object side. A so-called telephoto lens is known.

[0003] In general, a telephoto lens having a long focal length,
As the focal length increases, chromatic aberration such as axial chromatic aberration and chromatic aberration of magnification among various aberrations tends to be worsened. In order to satisfactorily correct these chromatic aberrations, there are various telephoto lenses that perform achromatization by combining a positive lens using a low dispersion material with anomalous partial dispersion such as fluorite and a negative lens using a high dispersion material. It has been proposed.

Anomalous partial dispersion glass such as fluorite is effective in correcting chromatic aberration, but has the drawback that it is difficult to process and is very expensive, and other low dispersion glass having no specific gravity and anomalous partial dispersion. It is relatively large, and there is also a drawback that using it makes the entire lens system heavier. (For example, fluorite has a specific gravity of 3.18 and FK01 has a specific gravity of 3.63.
On the other hand, FK5 having a small anomalous partial dispersibility has a specific gravity of 2.46, and BK7 has a specific gravity of 2.52. ) And, the surface of the abnormally dispersed glass is relatively easily damaged.
If K01 or the like has a large diameter, it also has a disadvantage that it is easily cracked by a rapid temperature change.

On the other hand, as a method of correcting chromatic aberration of an optical system, a diffraction grating having a diffractive action is provided on a lens surface or a part of the optical system, compared to a method of combining two glass materials (lenses) having different dispersions. A method of reducing chromatic aberration using a diffractive optical element is described in SPIE Vol. 1354 Inte
References such as rnational Lens Design Conference (1990), JP-A-4-213421, and Kokai Hei6-324.
262, JP-A-6-331887 and USP
No. 5,044,706.

Japanese Patent Application Laid-Open No. Hei 6-324262 discloses an F-number F2.F2 composed of at least one diffractive optical element having a positive refractive power and at least one refractive optical element having a negative refractive power. A telephoto lens in which chromatic aberration of about 8 has been corrected relatively well is disclosed. In addition, Japanese Unexamined Patent Publication
Similarly, Japanese Patent No. 331887 discloses a telephoto lens having an F-number of about F2.8 in which a chromatic aberration is corrected relatively well by combining a diffractive optical element and a refractive optical element.

Further, focusing in many photographing lenses (optical systems) is performed by moving the entire photographing lens or moving the photographing lens. When the photographing lens is a telephoto lens having a long focal length, the photographing lens becomes large and heavy, and it is mechanically difficult to move the entire photographing lens to perform focusing.

For this reason, many telephoto lenses focus by moving some lens groups. Of these, various types that use an inner focus type in which focusing is performed by moving a part of the central lens group in a relatively small and lightweight lens system other than the front lens group of the photographing lens have been proposed. ing.

For example, Japanese Patent Application Laid-Open No. Sho 55-147606 discloses an inner focus telephoto lens having a focal length of 300 mm and an F number of 2.8.
And Japanese Patent Application Laid-Open No. 59-65821, the focal length is 1
An inner-focus telephoto lens of about 35 mm and an F number of about 2.8 has been proposed.

In any of the inner focus type telephoto lenses proposed here, the first lens unit having a positive refractive power, the second lens unit having a negative refractive power, and the third lens unit having a positive refractive power are arranged in order from the object side. It has three lens groups, and focuses by moving the second group on the optical axis.

[0011]

Assuming that the photographing lens is used in a silver halide photographic camera, the focal length is converted to that for a silver halide photographic camera.
In the case of a super-telephoto lens from about to 4, even if aberration correction is performed using a diffractive optical element in the optical system, the telephoto ratio is limited to about 0.9, and in many cases, the overall length of the lens becomes longer. Therefore, there is a problem that the lens weight becomes heavy.

According to the present invention, by appropriately using a diffractive optical element in an optical system, a telephoto lens having a small tele ratio (short lens length) and high optical performance while favorably correcting various aberrations such as chromatic aberration. It is an object of the present invention to provide a mold type optical system and an optical device using the same.

[0013]

According to a first aspect of the present invention, an optical system having a diffractive optical element according to the present invention comprises, in order from the object side, a first lens group having a positive refractive power; A first lens group having a positive refractive power composed of a diffraction grating having a rotationally symmetrical shape with respect to the optical axis, one or more positive lenses, and one or more lenses; Φ: Refractive power of a diffractive optical element having a positive refractive power in the first lens group φ1: Refractive power of the first lens group φ: Refractive power of the entire lens system L: Overall length of the lens (first lens group) (Distance from the group to the image plane), satisfying the following condition: 0.005 <φD / φ1 <0.05 0.45 <φ / φ1 <0.5 0.50 <φ × L <0.75 It is characterized by doing.

According to a second aspect of the present invention, when focusing from an object at infinity to an object at a short distance in the first aspect,
The second lens group is moved toward the image side on the optical axis, and satisfies a conditional expression of -1 <φ / φ2 <−0.2, where φ2 is the refractive power of the second lens group. And

A third aspect of the present invention is characterized in that, in the first or second aspect of the present invention, a third lens unit having a positive refractive power is provided on the image plane side of the second lens unit.

The optical apparatus according to the fourth aspect of the present invention is the optical apparatus according to the first or second aspect.
Alternatively, it is characterized by having three optical systems.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1, FIG. 3, FIG. 5, FIG. 7, FIG.
1 is a lens cross-sectional view of Numerical Examples 1 to 6 of the optical system described later of the present invention. 2, 4, 6, 8, 10, and 12
FIG. 7 is an aberration diagram of Numerical Examples 1 to 6 of the optical system of the present invention.
In the lens cross-sectional view, L1 is a first group having positive refractive power (first lens group), L2 is a second group having negative refractive power (second lens group), and L3 is a third group having positive refractive power (second lens group). 3 lens group). SP is an aperture stop and IP is an image plane. G is a glass block such as an optical filter, a protective glass, and a face plate.

The DOE is a diffractive optical element (diffractive surface) having a positive refractive power and composed of a diffraction grating having a rotationally symmetric shape with respect to the optical axis.

Although the diffractive optical element DOE is provided on the cemented lens surface in the first lens group, it may be provided on another lens surface. The diffractive optical element of the present embodiment (diffractive surface) is a refracting surface and a diffractive surface when used in an optical system, utilizing the physical phenomenon that the appearance of chromatic aberration for a light beam of a certain reference wavelength appears in the opposite direction. I have.

Further, such a diffractive optical element has an effect of an aspheric lens by changing the period of the periodic structure, thereby reducing aberration.

In the present embodiment, the second lens unit L2 is moved to the image plane side as indicated by an arrow to focus from an object at infinity to a close object. In the present embodiment, at least one diffractive optical element DOE is provided in the first group in which the incident height of the light beam is high and the aberration correction is relatively easy, and the refractive power of the first group and the refractive power of the entire lens system are conditional expressions ( 1)-(3)
As a result, an optical system having high optical performance with little aberration fluctuation at the time of focusing over the entire screen and the entire object distance is achieved.

That is, the optical system of the present invention is arranged in order from the object side.
A first lens group having a positive refractive power and a second lens group having a negative refractive power, wherein the first lens group has a positive refractive power composed of a diffraction grating having a rotationally symmetric shape with respect to an optical axis. It consists of a diffractive optical element, one or more positive lenses, and one or more negative lenses. ΦD: Refractive power of a diffractive optical element having a positive refractive power in the first lens group φ1: Refractive power of the first lens group φ: 0.005 <φD / φ1 <0.05 0.45 <φ / φ1 <0.5 0, where L is the refractive power of the entire lens system and the total length of the lens (the distance from the first lens group to the image plane). 50 <φ × L <0.75 is satisfied.

Here, the refractive power φ is represented by φ = 1 / f where f is the focal length.

The technical meaning of conditional expressions (1), (2) and (3) will be described below.

Conditional expression (1) relates to the ratio of the refractive power of the diffractive surface DO to the refractive power of the first lens group, and exceeds the upper limit of conditional expression (1).
When the refractive power of the lens becomes strong, various aberrations such as spherical aberration and chromatic spherical aberration are deteriorated, and cannot be completely corrected by the aspherical effect (higher-order phase term) of the diffraction surface.

If the lower limit of condition (1) is exceeded, axial chromatic aberration cannot be canceled (corrected).

Conditional expression (2) relates to the ratio of the refractive power of the first lens unit to that of the entire system. If the refractive power of the first lens unit becomes smaller than the upper limit of conditional expression (2), it is difficult to shorten the total lens length. Becomes

If the lower limit of conditional expression (2) is exceeded, it is advantageous to shorten the total length of the lens, but it is not preferable because various aberrations such as spherical aberration generated in the first lens unit are increased and cannot be corrected. .

The conditional expression (3) relates to the telephoto ratio. If the total length of the optical system becomes longer than the upper limit of the conditional expression (3), the refractive power of the first lens group will be reduced, and the required refractive power of the diffractive optical element will also be reduced. Cannot correct axial chromatic aberration in the above wavelength range.

If the total length of the optical system is shorter than the lower limit of the conditional expression (3), various aberrations such as spherical aberration generated in the first lens unit are increased and cannot be corrected.

It is more preferable that conditional expressions (1) to (1) are satisfied.
It is preferable to set the numerical value range of (3) as follows.

0.008 <φD / φ1 <0.030.3 <φ / φ1 <0.40.4 <φ × L <0.73 Next, the diffraction provided in the first lens group in the present embodiment. The optical surface (diffraction surface) will be described.

In the embodiment of the present invention, the case where one diffractive surface having a positive refractive power is provided is shown. However, a further diffractive surface may be added. Can be The diffractive surface to be added may have either a positive refractive power or a negative refractive power. In particular, when a diffractive surface having a negative refractive power is added, a paraxial pupil ray near the image plane of the optical system is generated. It is preferable to arrange the position where the incidence height is relatively high and the incidence height of the paraxial on-axis ray is relatively low. Thereby, chromatic aberration of magnification can be corrected more favorably. Further, although each diffraction surface is arranged on a flat or spherical lens, it may be based on an aspheric surface or may be provided on both surfaces. Furthermore, it may be applied to the cemented surface of the cemented lens, and the material of the base is not particularly limited to glass as long as it allows light to pass through.

In particular, regarding the diffractive surface in the first lens group, a lens whose convex surface is directed to a plane or an object side so that rays from an on-axis object point and an off-axis object point enter the diffractive surface as perpendicularly as possible. It is preferable to provide a lens surface with a gentle concave surface facing a surface or an object, so that a decrease in diffraction efficiency can be mitigated. Desirably, the diffractive surface should be set on the lens surface such that light rays are incident at less than ± 15 ° with respect to the normal to the lens surface.

The diffraction grating shape of the diffractive optical surface represented by the above-described phase shape is actually realized in the form of a kinoform as shown in FIG.

In FIG. 13, reference numeral 1 denotes a base material on which a diffraction surface is provided, reference numeral 2 denotes an optical material (resin portion) for forming a diffraction grating, and reference numeral 3 denotes a diffraction grating (diffraction surface).

FIG. 14 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element shown in FIG. In the actual configuration of the diffraction element, a diffraction grating 3 having a grating thickness d such that the primary diffraction efficiency becomes 100% at a wavelength of 530 nm is formed on the resin portion 2 applied to the surface of the base material 1 described above.

As is apparent from FIG. 14, the diffraction efficiency at the design order decreases as the distance from the optimized wavelength of 530 nm increases. Due to the decrease, the order 0 order near the design order, 2
The next-order diffracted light increases, which becomes a flare and leads to a decrease in the resolution of the optical system.

Accordingly, in the present invention, a diffraction grating configuration in which diffraction gratings of different materials are arranged in a stacked manner as shown in FIG. 15 is employed as one of the embodiments. With such a configuration, it is possible to obtain a high first-order diffraction efficiency in a wider wavelength range. FIG. 16 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element having this configuration.

As can be seen from this figure, the diffraction efficiency of the design order has a high diffraction efficiency of 95% or more over the entire use wavelength range by using the diffraction grating having the laminated structure. Thereby, the imaging optical system of the present invention can obtain a high resolution, and the optical performance is further improved.

The material of the diffractive optical element here is not limited to resin, but depending on the substrate 1, the first
May be directly formed on the substrate 1.

Further, when the laminated structure has three or more layers as shown in FIG. 17, better optical performance can be obtained.

With such a configuration, it is possible to reduce the grating thickness of the portion of the diffraction grating that touches the air layer. As a result, flare due to scattered light generated at the wall portion of the edge of the diffraction grating is reduced, and a decrease in diffraction efficiency due to an increase in the incident angle of light incident on the diffraction grating can be reduced, thereby further improving optical performance. .

FIG. 18 shows one example of the diffractive optical element having this configuration.
The wavelength dependence of the second-order diffraction efficiency is shown.

Further, by making the diffraction grating a laminated structure as shown in the figure, the grating surface can be made hard to contact with the outside air, and unnecessary scattering light which deteriorates the image quality due to adhesion of dirt or dirt can be generated. Can be reduced.

Of course, the arrangement of the diffractive optical surface on the joint surface of the lens as in this embodiment is effective from such a viewpoint.

The optical system aimed at by the present invention can be achieved by the above-described configuration. To further improve the optical performance, it is preferable to satisfy at least one of the following conditions.

(A-1) Focusing from an object at infinity to an object at a short distance is performed by moving the second lens group toward the image side on the optical axis. When φ2: the refracting power of the second lens group, −1 <φ / φ2 <−0.2 (4)

Conditional expression (4) relates to the ratio of the refracting power of the entire system to the second lens group. The conditional expressions (1) to (3) are satisfied, and various aberrations including chromatic aberration are included, including fluctuations due to focusing. It is intended to provide an optical system which is more favorable and is lightweight and suitable for a camera having an autofocus function, and has a small extension amount of a lens group during focusing.

When the refractive power of the second lens group is increased beyond the upper limit value of the conditional expression (4), the refractive power of the first lens group is also increased, and the lens outer diameter of the second lens group and the extension at the time of focusing. Although the amount is reduced, both the aberration in the reference state and the aberration fluctuation due to focusing become worse, which is not preferable.

When the lower limit of conditional expression (4) is exceeded, the second condition
When the refractive power of the lens group weakens. Although it is advantageous for aberration correction, it is not preferable because the lens outer diameter and the extension amount increase.

It is more preferable to set the numerical range of conditional expression (4) as follows.

-0.7 <φ / φ2 <−0.25 (A-2) The second lens group is composed of one negative lens having a concave surface facing the image surface side or a cemented lens of a positive lens and a negative lens. That is.

According to this, it becomes easy to perform focusing at high speed.

(A-3) The diffractive surface is provided on the cemented surface of the cemented lens of the first lens unit, which is composed of a positive lens having a convex surface facing the object side and a positive lens having a convex surface facing the image surface side. . According to this, it becomes easy to effectively correct the chromatic aberration by the diffraction effect of the diffraction surface.

Next, a video camera using the optical system of the present invention.
An embodiment of (optical device) will be described with reference to FIG.

In FIG. 19, reference numeral 10 denotes a video camera body, 11 denotes an optical system of the present invention, 12 denotes an image pickup device such as a CCD for receiving a subject image by the optical system 11, and 13 denotes a subject image received by the image pickup device 12. The recording means 14 is a finder for recording a subject image displayed on a display element (not shown). The display element is constituted by a liquid crystal panel or the like, and displays a subject image formed on the imaging element 12.

As described above, by applying the optical system of the present invention to an optical device such as a video camera, an optical device having a small size and high optical performance is realized.

Next, numerical examples of the optical system according to the present invention will be described.
In the numerical examples, ri is the i-th radius of curvature in order from the object side, and di is the i-th surface and i + 1th in order from the object side.
The lens thickness or air gap of the i-th surface, ni and νi are the refractive index and Abbe number of the i-th optical member in order from the object side. Table 1 shows the relationship between the above-described conditional expressions and the numerical examples.

The phase shape の of the diffraction surface in the embodiment is represented by the following equation. ψ (h, m) = (2π / mλ0) (C2h2 + C4h4
+ C6h6) where h: height in the direction perpendicular to the optical axis m: diffraction order of diffracted light λ0: design wavelength Ci: phase coefficient (i = 1, 2, 3,...)

In each embodiment, the diffraction order m of the diffracted light is 1, and the design wavelength λ0 is the wavelength of the d-line (587.56n).
m).

The refractive power φD of the diffraction surface D for an arbitrary wavelength λ and an arbitrary diffraction order m is the lowest phase coefficient C
It can be expressed as follows using ₁ .

ΦD (λ, m) = − 2C1mλ / λ0

[0064]

[Outside 1]

[0065]

[Outside 2]

[0066]

[Outside 3]

[0067]

[Outside 4]

[0068]

[Outside 5]

[0069]

[Outside 6]

[0070]

[Table 1]

[0071]

According to the present invention, as described above, by appropriately using a diffractive optical element in an optical system, various aberrations such as chromatic aberration can be favorably corrected, and the tele ratio is small (the overall length of the lens is short). A telephoto optical system having high optical performance and an optical apparatus using the same can be achieved.

In addition, according to the present invention, an optical system having a light-weight, compact, and high-quality diffractive optical element can be achieved while satisfactorily correcting various aberrations including chromatic aberration while having a large aperture ratio. be able to.

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view of a numerical example 1 of an optical system according to the present invention.

FIG. 2 is an aberration diagram of a numerical example 1 of the optical system according to the present invention.

FIG. 3 is a lens cross-sectional view of Numerical Example 2 of the optical system of the present invention.

FIG. 4 is an aberration diagram of a numerical example 2 of the optical system according to the present invention.

FIG. 5 is a lens cross-sectional view of a numerical example 3 of the optical system according to the present invention.

FIG. 6 is an aberration diagram of a numerical example 3 of the optical system according to the present invention.

FIG. 7 is a lens sectional view of a numerical example 4 of the numerical optical system according to the present invention;

FIG. 8 is an aberration diagram of a numerical example 4 of the optical system according to the present invention.

FIG. 9 is a lens cross-sectional view of Numerical Example 5 of the optical system of the present invention.

FIG. 10 is an aberration diagram for a numerical example 5 of the optical system according to the present invention;

FIG. 11 is a lens sectional view of a numerical example 6 of the optical system according to the present invention;

FIG. 12 is an aberration diagram of a numerical example 6 of the optical system according to the present invention;

FIG. 13 is a schematic cross-sectional view of a single-layer diffraction grating according to the present invention.

FIG. 14 is a graph showing the diffraction efficiency of a single-layer diffraction grating according to the present invention.

FIG. 15 is a schematic cross-sectional view of a laminated diffraction grating according to the present invention.

FIG. 16 is a graph showing the diffraction efficiency of the laminated diffraction grating in the present invention.

FIG. 17 is a schematic cross-sectional view of a three-layer diffraction grating according to the present invention.

FIG. 18 is a graph showing the diffraction efficiency of a three-layer diffraction grating according to the present invention.

FIG. 19 is a schematic view of a main part of an optical apparatus according to the present invention.

[Explanation of symbols]

 L1 First lens group L2 Second lens group L3 Third lens group DOE Diffractive optical element SP Stop G Glass block ΔS Sagittal image plane ΔM Meridional image plane d d line g g line

Claims (4)

[Claims] 1. A first lens having a positive refractive power in order from the object side.
A second lens group having a negative refractive power, wherein the first lens group includes a diffractive optical element having a positive refractive power and a diffraction grating having a rotationally symmetric shape with respect to an optical axis; ΦD: refractive power of a diffractive optical element having a positive refractive power in the first lens group φ1: refractive power of the first lens group φ: refractive power of the entire lens system L: Assuming the total lens length (the distance from the first lens group to the image plane), 0.005 <φD / φ1 <0.05 0.45 <φ / φ1 <0.5 0.50 <φ × L <0. 75, wherein the following conditional expression is satisfied: 2. When focusing from an object at infinity to an object at a short distance, the second lens group is moved to the image side on the optical axis, and when φ2: refractive power of the second lens group, −1 < 2. The optical system according to claim 1, wherein a conditional expression of φ / φ2 <−0.2 is satisfied. 3. An optical system according to claim 1, further comprising a third lens group having a positive refractive power on the image plane side of said second lens group. 4. An optical apparatus comprising the optical system according to claim 1, 2 or 3.