JP2011017849A - Rear attachment lens and photographing optical system having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear attachment lens that is mounted to an image side of a main lens system, wherein fluctuation of many aberrations is small when the focal length of the whole system is changed to the longer side, especially the fluctuation of chromatic aberration is small, and that keeps high optical performance as the whole system.SOLUTION: The rear attachment lens is removably mounted to the image side of the main lens system, and changes the focal length to the longer side than the focal length of the main lens system. The rear attachment lens includes one or more diffractive optical elements, and one or more negative lenses GMn. It is required to appropriately set Abbe number and partial variance ratio differences νand Δθof material of the negative lenses GMn, the focal length fof the negative lenses GMn, the distance d from the lens surface of the attachment lens on the side closest to an object to the lens surface thereof on the side closest to the image, the distance tfrom the lens surface of the attachment lens on the side closest to the object to the lens surface of the negative lenses GMn on the object side, and the focal length f of the attachment lens.

Description

  The present invention is detachably mounted on the image side of a photographing lens (main lens system) used for a digital still camera, a video camera, a broadcast camera, etc., and the focal length of the entire system is compared with the focal length of the main lens system. It is related with the rear attachment lens which changes to the long one.

  Conventionally, there is a rear attachment lens that is detachably attached to the image plane side of the main lens system, which is a photographic lens (shooting optical system), and changes the focal length of the entire system to be longer than the focal length of the main lens system. It is known (patent documents 1 to 3).

  The rear attachment lens is characterized in that the entire optical system is smaller than a front attachment lens that is attached to the object side of the main lens system and changes the focal length of the entire system in the longer direction. For this reason, the rear attachment lens is widely used in a photographing optical system of an imaging apparatus such as a digital camera or a video camera, which has been downsized as a whole in recent years.

JP-A 63-106715 JP-A-11-183800 JP 2004-226648 A

  In general, even if the rear attachment lens is designed to be free from aberrations, the residual aberration of the main lens system increases as the magnification (focal length enlargement factor) increases when the rear attachment lens is attached. , Tend to degrade image quality.

  For example, when the magnification of the rear attachment lens is two times, the lateral aberration such as coma aberration and chromatic aberration of magnification of the main lens system is simply doubled, and the image quality deteriorates. Longitudinal aberrations such as spherical aberration, field curvature, and longitudinal chromatic aberration are magnified to the square of magnification, that is, four times. In a telephoto lens that often uses a rear attachment lens, chromatic aberration tends to deteriorate among various aberrations as the focal length increases. For this reason, when the rear attachment lens is attached to the main lens system, the enlarged chromatic aberration of magnification is the main cause of degrading the image quality.

  In recent digital cameras, the number of pixels and image quality have increased, and when a rear attachment lens is attached to the main lens system used in a digital camera, there is little overall chromatic aberration and high-quality images can be obtained. Is strongly demanded.

  The present invention is mounted on the image side of the main lens system, and there are few fluctuations of various aberrations when the focal length of the whole system is shifted to a longer side, and especially the fluctuation of chromatic aberration is small, and the high optical performance is maintained as the whole system. An object of the present invention is to provide a rear attachment lens that can be used.

The rear attachment lens of the present invention is detachably attached to the image plane side of the main lens system, and the rear attachment lens changes the focal length in a longer direction than the focal length of the main lens system. One or more diffractive optical elements and one or more negative lenses GMn, the Abbe number and partial dispersion ratio difference of the material of the negative lens GMn being ν _{d_Mn} , Δθ _{gF_Mn} , and the focal length of the negative lens GMn being f _Mn , The distance from the most object side lens surface of the rear attachment lens to the most image side lens surface is d, and the distance from the most object side lens surface of the rear attachment lens to the object side lens surface of the negative lens GMn. t _Mn , where f is the focal length of the rear attachment lens,
0.03 <f / (ν _{d_Mn} × f _Mn ) <0.20
0.015 <Δθ _{gF_Mn} <0.060
0.40 <t _{Mn /} d <0.72
It satisfies the following conditional expression.

  According to the present invention, when mounted on the image side of the main lens system, the variation of various aberrations when the focal length of the entire system is increased is small, especially the variation of chromatic aberration is small, and high optical performance is maintained as the entire system. A rear attachment lens that can be used is obtained.

Lens cross section and aberration diagram of main lens system Lens cross-sectional view of the rear attachment lens of Example 1, and lens cross-sectional view and aberration diagram when attached to the main lens system Lens sectional view and aberration diagram of rear attachment lens of Example 2 Lens sectional view and aberration diagram of rear attachment lens of Example 3 Lens sectional view and aberration diagram of the rear attachment lens of Example 4 Lens cross-sectional view and aberration diagram of the rear attachment lens of Example 5 Lens sectional view and aberration diagram of the rear attachment lens of Example 6 Illustration of diffractive optical element Illustration of diffractive optical element

  Hereinafter, the rear attachment lens of the present invention and the photographing optical system when the lens is attached to the main lens system (main lens) will be described.

  The rear attachment lens of the present invention is detachably mounted on the image plane side of the main lens system, and changes the focal length of the photographing optical system to be longer than the focal length of the main lens system.

The rear attachment lens of the present invention has at least one diffractive optical element and at least one negative lens GMn that satisfies a conditional expression described later. Besides, rear attachment lens of the present invention, most of the lens L _O in the object side to the lens surface on the image side of the lens L _OF that toward the image side sign of focal length between the lens L _O and first opposite sign This lens is a front lens unit LF. However, when the lens L _OF is further cemented with the image side lens, the lens up to the image side lens surface of the cemented lens is defined as the front lens group LF.

Also the lens closest from the lens L _I located on the image side to the object side sign of the focal length to the lens L _I and the lens surface of the first becomes the opposite sign lens L _IR of the object side rear lens group LR. However, when the lens _LIR is further cemented with the object side lens, the lens up to the object side lens surface of the cemented lens is defined as the rear lens group LR. At this time, a negative lens GMn that satisfies a conditional expression described later is disposed between the front lens group LF and the rear lens group LR.

  As the main lens system, for example, a telephoto lens, a telephoto zoom lens, and the like can be applied. The telephoto lens is the point where the maximum height from the optical axis where the light beam on the paraxial axis passes through the lens surface in front of the point where the optical axis and the pupil paraxial ray intersect, and the optical axis and the pupil paraxial ray intersect. This is an optical system that is larger than the maximum value of the height from the optical axis through which the paraxial light beam passes through the lens surface. The telephoto zoom lens is an optical system that satisfies the conditions of the telephoto lens described above at the zoom position at the wide-angle end.

  FIGS. 1A and 1B are a lens cross-sectional view and an aberration diagram of a main lens system (telephoto lens) selected as an example to which the rear attachment lens of the present invention is attached.

  FIG. 2A is a lens cross-sectional view of the rear attachment lens of Example 1 of the present invention. 2B and 2C are a lens cross-sectional view and aberration diagrams when the rear attachment lens of Example 1 is mounted on the image side of the main lens system of FIG. 1A.

  3A and 3B are a sectional view of a rear attachment lens of Example 2 of the present invention and a case where the rear attachment lens of Example 2 is mounted on the image side of the main lens system of FIG. It is an aberration diagram.

  4A and 4B show a lens cross-sectional view of the rear attachment lens of Example 3 of the present invention and a case where the rear attachment lens of Example 3 is mounted on the image side of the main lens system of FIG. It is an aberration diagram.

  FIGS. 5A and 5B are a sectional view of a rear attachment lens of Example 4 of the present invention and a rear attachment lens of Example 4 when mounted on the image side of the main lens system of FIG. It is an aberration diagram.

  6A and 6B show a lens cross-sectional view of the rear attachment lens of Example 5 of the present invention and a case where the rear attachment lens of Example 5 is mounted on the image side of the main lens system of FIG. It is an aberration diagram.

  FIGS. 7A and 7B are a sectional view of a rear attachment lens of Example 6 of the present invention and a rear attachment lens of Example 6 when mounted on the image side of the main lens system of FIG. It is an aberration diagram.

  In the lens cross-sectional view, the left side is the object side, and the right side is the image side. LA is a rear attachment lens, and M is a main lens system (master lens). The main lens system M is a telephoto lens having a single focal length. The main lens system M shown in FIGS. 1A and 2B is composed of a first lens unit L1 having a plurality of lenses and having a positive refractive power in order from the object side to the image side, and a positive lens and a negative lens. The second lens unit L2 has a negative refractive power and the third lens unit L3 has a plurality of lenses and has a positive refractive power. Further, focusing from an object at infinity to a near object is performed by moving the second lens unit L2 to the image plane side on the optical axis.

  SP is an aperture stop. G is an optical block corresponding to protective glass or the like. IP is an image plane, and when used as a photographing optical system for a video camera or a digital camera, an imaging surface of a solid-state imaging device (photoelectric conversion device) such as a CCD sensor or a CMOS sensor that receives an image is used for a silver salt film. When used as an imaging optical system of a camera, it corresponds to a film surface.

In the rear attachment lens LA shown in FIGS. 2A to 7A, LF is a front lens group, and LR is a rear lens group. DOE is a diffractive optical element. D is a diffractive optical part (diffractive optical surface) of the diffractive optical element DOE, and is disposed on the cemented surface (joined lens surface) of the two lenses. Of the diffracted light generated from the diffractive optical part D, the diffraction order m of the diffracted light used in this embodiment is 1, and the design wavelength λ ₀ is the wavelength of the d-line (587.56 nm). Note that the number of diffractive optical surfaces is not limited to one, and additional diffractive optical surfaces may be added, whereby even better optical performance can be obtained. The diffractive optical surface may be aspherical, and the base material may be plastic instead of glass as long as it transmits light.

Gn / L _O and L _OF are lenses in the front lens group LF. Gn / L _I and L _IR are lenses in the rear lens group LR. GMn is a negative lens. GMp is a positive lens. Gn is a negative lens.

  In the aberration diagram, d and g are d line and g line in this order. M and S are the meridional image surface, the sagittal image surface, and the lateral chromatic aberration is represented by the g-line. Fno is the F number, and ω is the half angle of view. In all aberration diagrams, the spherical aberration is drawn on a scale of 0.4 mm, the astigmatism is 0.4 mm, the distortion is 2%, and the lateral chromatic aberration is drawn on a scale of 0.05 mm. When a rear attachment lens is attached to the main lens system and the focal length of the entire system is increased, a large amount of lateral chromatic aberration occurs. Conventionally, correcting the lateral chromatic aberration at this time satisfactorily has been a major problem when using a rear attachment lens.

  It is known that it is effective to use a diffractive optical element to correct lateral chromatic aberration of an optical system. Therefore, in each embodiment, a diffractive optical element is used to satisfactorily correct lateral chromatic aberration, and a lens made of a glass material having an anomalous partial dispersion is disposed at an appropriate position in the optical path with an appropriate power. As a result, an imaging optical system having a high imaging performance by properly correcting the lateral chromatic aberration is obtained.

  Next, features of the configuration of the rear attachment lens of each example will be described. The rear attachment lens, which is mounted between the main lens and the camera (camera body) and increases the focal length of the entire system, has the highest incident ray height on the object side lens group. As the lens group is closer to the image side, the off-axis principal ray tends to pass through a position away from the optical axis. Therefore, as correction of chromatic aberration, in general, axial chromatic aberration is corrected by the lens group closest to the object side, and lateral chromatic aberration is corrected by the lens group of image side. However, since the correction direction of axial chromatic aberration and the correction direction of lateral chromatic aberration are contradictory, it is necessary to perform correction in a balanced manner.

  In general, in order to satisfactorily correct the lateral chromatic aberration at this time, a negative lens made of an anomalous dispersion material such as fluorite should be used at a position where the off-axis principal ray passes away from the optical axis. . However, since anomalous dispersive materials such as fluorite are mostly low in refractive index and low in dispersion, a large amount of power must be applied to effectively correct lateral chromatic aberration. If it does so, the curvature radius of a lens surface will become small, various aberrations other than chromatic aberration will increase, and it will become difficult to correct these. Further, since the Petzval sum is deteriorated due to the low refractive index, it is particularly difficult to correct curvature of field.

Therefore, the rear attachment lens of each embodiment is provided with the diffractive optical portion D at a position (lens surface) where the height of the incident position of the on-axis light beam is low and the height of the incident position of the off-axis principal ray is high. . As a result, the lateral chromatic aberration is corrected particularly well. In each embodiment, since the chromatic aberration of magnification is corrected by the diffractive optical part D, it is not necessary to use an anomalous dispersion material such as fluorite for the negative lens GMn. Therefore, the color variation of the spherical aberration is corrected by using a highly dispersive material for the negative lens GMn. Further, by using a material having a large partial dispersion ratio difference Δθ _gF in the positive direction for the negative lens GMn, the lateral chromatic aberration is corrected well.

Thus, by using a high dispersion material for the negative lens GMn, the color variation of the spherical aberration can be corrected, and the correction of the chromatic aberration of magnification is facilitated by increasing the partial dispersion Δθ _gF . In each embodiment, as described above, in addition to the diffractive optical portion D, the correction of chromatic aberration of magnification is also shared by the negative lens GMn, so that the chromatic aberration of the entire lens system is corrected well. Further, the correction of chromatic aberration in the rear attachment lens needs to be balanced because the correction direction of axial chromatic aberration and the correction direction of lateral chromatic aberration are contradictory.

Therefore, in the rear attachment lens of each embodiment, the front lens unit LF is moved from the lens L _O closest to the object side toward the image side, and the image side of the lens L _OF whose first focal length sign is opposite to that of the lens L _O is the image side. It consists of up to the lens surface. (However, when the lens L _OF is further cemented with the image side lens, the front lens group LF is defined up to the image side surface of the cemented lens.)
Then, it constituted by toward the lens L _I in the rear lens group LR closest to the image side to the object side is the sign of the focal length to the lens L _I and the lens surface of the first becomes the opposite sign lens L _IR of the object side Yes. However, when the lens L _IR is further cemented with the lens on the object side, the lens surface is up to the lens surface on the object side of the cemented lens. The negative lens GMn is positioned between the front lens group LF and the rear lens group LR.

  By adopting such a lens configuration, axial chromatic aberration is corrected by the front lens group LF including the positive lens and the negative lens, and various aberrations such as curvature of field are corrected by the rear lens group LR. The lateral chromatic aberration is corrected by the negative lens GMn and the diffractive optical part D. As a result, various aberrations including chromatic aberration can be corrected in a balanced manner. Of course, in each embodiment, some optical element may be arranged on the object side of the front lens group or on the image side of the rear lens group.

Next, features of the rear attachment lens of each embodiment will be described. In the rear attachment lens LA of each embodiment, among the one or more negative lenses GMn, the Abbe number and the partial dispersion ratio difference of at least one negative lens GMn are ν _{d_Mn} and Δθ _{gF_Mn} . The focal length of the negative lens GMn is assumed to be f _Mn . The distance from the most object-side lens surface of the rear attachment lens LA to the most image-side lens surface is d. The distance from the most object-side lens surface of the rear attachment lens LA to the object-side lens surface of the negative lens GMn is defined as t _Mn . Let f be the focal length of the entire rear attachment lens system. At this time,
0.03 <f / (ν _{d_Mn} × f _Mn ) <0.20 (1)
0.015 <Δθ _{gF_Mn} <0.060 (2)
0.40 <t _Mn / d <0.72 (3)
The following conditional expression is satisfied.

In addition, in the rear attachment lens of each embodiment, the focal lengths of the lens L _O and the lens L _OF are f _LO and f _LOF , respectively. Lens _{L I} and the lens _L, respectively the focal length of the _{IR f LI,} and _{f LIR.} At this time,
0.03 <f / (ν _{d_Mn} × f _Mn ) <0.20 (1)
0.015 <Δθ _{gF_Mn} <0.060 (2)
│f _LO /f│<7.0 (4)
│f _LOF /f│<7.0 (5)
│f _LI /f│<7.0 (6)
│f _LIR /f│<7.0 (7)
The following conditional expression is satisfied.

The Abbe number and refractive index of each lens material are based on the d-line. The Abbe number ν _{d_Mn} , partial dispersion ratio θ _{gF_Mn} , and partial dispersion ratio difference (abnormal partial dispersion ratio) Δθ _{gF_Mn} of the material of the negative lens GMn are as follows. The refractive index at the d-line of the negative lens GMn material N _{D_Mn,} the refractive index N _{G_Mn} in g-line, the refractive index N _{C_Mn} at C-line, the refractive index at the F-line and N _{F_Mn.} At this time, ν _{d_Mn} = (N _{d_Mn} −1) / ( _{NF_Mn−} N _{C_Mn} )
θ _{gF_Mn} = (N _{g_Mn} −N _{F_Mn} ) / (N _{F_Mn} −N _{C_Mn} )
Δθ _{gF_Mn} = θ _{gF_Mn} -(-1.625 × 10 ^-3 × ν _{d_Mn} + 0.642)
It is.

  Next, the technical meaning of each conditional expression described above will be described. Conditional expression (1) relates to the correction power of chromatic aberration of at least one negative lens GMn constituting the rear attachment lens LA. If the upper limit value of conditional expression (1) is exceeded, the power of the negative lens GMn becomes too large, so that the radius of curvature of the lens surface becomes small and it becomes difficult to correct chromatic aberration at a position where the image height is high.

  On the other hand, when the lower limit value of conditional expression (1) is exceeded, the correction power of chromatic aberration at the negative lens GMn becomes small, so that particularly large chromatic aberration of magnification remains, and the amount of correction of chromatic aberration at the diffractive optical element DOE increases. End up. In this case, the power of the diffractive optical element D must be increased, and the pitch of the diffraction grating needs to be reduced. If the pitch of the diffraction grating is made fine, manufacturing becomes difficult, which is not preferable. Conditional expression (1) is more preferably set as follows.

0.035 <f / (ν _{d_Mn} × f _Mn ) <0.150 (1a)
Conditional expression (2) relates to a partial dispersion ratio difference of materials of at least one negative lens GMn constituting the rear attachment lens LA. If the upper limit value of conditional expression (2) is exceeded, the partial dispersion ratio difference of the material of the negative lens GMn becomes too large, and the lateral chromatic aberration is overcorrected.

  On the other hand, if the lower limit of conditional expression (2) is exceeded, the anomalous dispersion characteristics of the material of the negative lens GMn will be weakened, so the secondary spectrum of lateral chromatic aberration will remain, and the chromatic aberration correction share in the diffractive optical part D The amount will increase. In this case, the power of the diffractive optical part D must be increased, and the pitch of the diffraction grating needs to be reduced. If the pitch of the diffraction grating is made fine, manufacturing becomes difficult, which is not preferable. Conditional expression (2) is more preferably set as follows.

0.015 <Δθ _{gF_Mn} <0.055 (2a)
Conditional expression (3) relates to an arrangement position in the optical path of at least one negative lens GMn constituting the rear attachment lens LA. When the upper limit value of conditional expression (3) is exceeded, the negative lens GMn is disposed in the image plane side direction of the rear attachment lens LA. Then, as shown in the conditional expression (1), a strong power negative lens GMn is disposed on the image plane side, which is advantageous for correcting chromatic aberration of magnification, but it is difficult to correct aberrations such as field curvature. Therefore, it is not preferable. On the other hand, when the lower limit value of conditional expression (3) is exceeded, the negative lens GMn is disposed closer to the object side, which affects the axial chromatic aberration and deteriorates the axial chromatic aberration. It is not preferable. Conditional expression (3) is more preferably set as follows.

0.45 <t _Mn /d<0.71 (3a)
Conditional expressions (4) to (7) relate to the focal lengths of some lenses constituting the rear attachment lens LA. If the upper limit of conditional expressions (4) to (7) is exceeded for each lens, the power of the lenses in each lens group will be greatly biased to either positive or negative, or the power of all lenses will be weak. For this reason, the correction of the basic aberration is insufficient, which is not preferable.

│f _LO /f│<5.0 (4a)
│f _LOF /f│<5.0 (5a)
│f _LI /f│<5.0 (6a)
│f _LIR /f│<5.0 (7a)
The conditional expressions (4a) to (7a) are more preferably set as follows.

│f _LO /f│<1.0 (4b)
│f _LOF /f│<1.0 (5b)
│f _LI /f│<4.6 (6b)
│f _LIR /f│<1.0 (7b)
According to each embodiment, by specifying each constituent element as described above, when the rear attachment lens LA is attached to the main lens system M, correction of various aberrations including chromatic aberration over the entire screen is improved. And a high-quality image can be easily obtained. In particular, by using the diffractive optical element DOE and the negative lens GMn having the above-described configuration, it is possible to maintain good optical performance, and to effectively correct lateral chromatic aberration that tends to occur near the periphery of the screen.

  Although the object of the present invention can be achieved by adopting the above-described configuration, it is more preferable that at least one of the following conditions is satisfied. According to this, further high optical performance can be easily achieved. can get.

Let N _{d_Mn be} the refractive index of the material of the negative lens GMn. The focal length of only the diffractive component (diffractive optical part D) of the diffractive optical element DOE is defined as f _D. A lens having at least one or more power is disposed on the image side from the diffractive optical element, and the combined magnification of all the lenses on the image side from the diffractive optical element is β _REAR . The rear attachment lens has one or more positive lenses GMp, and the refractive index and partial dispersion ratio difference of the material of the positive lens GMp are respectively N _Mp and Δθ _{gF_Mp} . The distance from the most object-side lens surface of the rear attachment lens to the object-side lens surface of the positive lens GMp is defined as t _Mp . Let f _{Mp be} the focal length of the positive lens GMp. Rear attachment lens has one or more negative lens Gn on the nonnegative lens GMn, the refractive index of the material of the negative lens Gn and N _n.

At this time,
1.75 <N _{d_Mn} <2.50 (8)
25 <f _D / f <130 (9)
0.9 <β _REAR <5.0 (10)
-2.5 <f / (N _Mp × f _Mp ) <-0.3 (11)
-0.020 <Δθ _{gF_Mp} <-0.001 (12)
0.6 <t _Mp / t _Mn <1.4 (13)
1.65 <N _n <2.50 (14)
It is preferable to satisfy one or more of the following conditional expressions.

  Conditional expression (8) relates to the refractive index of the material of the negative lens GMn. In the rear attachment lens, as described above, it is not necessary to use a low refractive index material having anomalous dispersion characteristics such as fluorite as the material of the negative lens GMn, so a high refractive index material that works favorably for Petzval sum is used. be able to. This makes it easy to correct the curvature of field favorably. If the upper limit of conditional expression (8) is exceeded, the material will be in a range that is difficult to use as an optical material. On the other hand, if the lower limit of conditional expression (8) is exceeded, the curvature of field becomes insufficiently corrected, which is not preferable. Conditional expression (8) is more preferably set as follows.

1.80 <N _{d_Mn} <2.30 (8a)
Conditional expression (9) is for appropriately maintaining the power of only the diffraction component of the diffractive optical element DOE while satisfactorily correcting various aberrations. Here, the focal length f _{D of} only the diffraction component is, as will be described later, when the power of only the diffraction component is φ _D ,
1 / f _D = φ _D = -2 · C ₂
It is. C ₂ is a phase coefficient in the equation (a) described later.

  If the upper limit of conditional expression (9) is exceeded, the power due to only the diffractive component in the diffractive optical element DOE becomes weak, so that it is difficult to effectively correct lateral chromatic aberration, which is not preferable. On the other hand, if the lower limit of conditional expression (9) is exceeded, the power due to only the diffraction component in the diffractive optical element DOE will increase. Then, it is necessary to make the pitch of the diffraction grating fine. If the pitch of the diffraction grating is made fine, manufacturing becomes difficult. Conditional expression (9) is more preferably set as follows.

25 <f _D / f <110 (9a)
Conditional expression (10) relates to the combined magnification of each lens arranged on the image side from the diffractive optical element DOE. The diffracted light generated from the diffractive optical element DOE is all unnecessary light except for the diffracted light in the design order. The imaging position of this unnecessary light is determined by the combined magnification of each lens on the image side with respect to the diffractive optical element DOE. When the upper limit of conditional expression (10) is exceeded, the negative power of the lens on the image side from the diffractive optical element DOE becomes too large. For this reason, in order to maintain power balance, it is necessary to place a lens (or a positive lens group) having a strong positive power closer to the object side than the diffractive optical element DOE. However, it is not preferable to place such a strong lens (group) because it becomes difficult to correct various aberrations.

  On the other hand, if the lower limit value of conditional expression (10) is exceeded, the image formation position of unnecessary light becomes close to the image plane, and the spot diameter of unnecessary light on the image plane becomes small. Conditional expression (10) is more preferably set as follows.

0.9 <β _REAR <3.0 (10a)
Conditional expression (11) relates to the relationship between the refractive index of the material of at least one positive lens GMp constituting the rear attachment lens and the focal length. If the upper limit value or lower limit value of conditional expression (11) is exceeded, it is difficult to achieve power balance with the negative lens GMn shown in conditional expression (1), and it becomes difficult to correct various aberrations. Conditional expression (11) is more preferably set as follows.

-2.2 <f / (N _Mp × f _Mp ) <-0.5 (11a)
Conditional expression (12) relates to the partial dispersion ratio difference of the material of the positive lens GMp disposed in the optical path of the rear attachment lens LA. When the upper limit value of conditional expression (12) is exceeded, the partial dispersion ratio difference is small, so that the correction power of the secondary spectrum of lateral chromatic aberration becomes small. On the other hand, when the lower limit of conditional expression (12) is exceeded, it becomes a material in a range that is difficult to use as an optical material. Conditional expression (12) is more preferably set as follows.

-0.010 <Δθ _{gF_Mp} <-0.001 (12a)
Conditional expression (13) relates to the arrangement position of the positive lens GMp arranged in the optical path of the rear attachment lens LA. When the upper limit value or lower limit value of conditional expression (13) is exceeded, it is too far from the negative lens GMn shown in conditional expression (1), making it difficult to balance the power with the negative lens GMn as a pair. In such a case, it is difficult to obtain a good effect of correcting the lateral chromatic aberration of the positive lens GMp disposed in the optical path, which is not preferable. Conditional expression (13) is more preferably set as follows.

0.7 <t _Mp / t _Mn <1.3 (13a)
Conditional expression (14) relates to the refractive index of the material of at least one negative lens Gn other than the negative lens GMn constituting the rear attachment lens. If the upper limit of conditional expression (14) is exceeded, the material will be in a range that is difficult to use as an optical material. On the other hand, if the lower limit value of conditional expression (14) is exceeded, the curvature of field becomes insufficiently corrected, which is not preferable. Conditional expression (14) is more preferably set as follows.

1.70 <N _n <2.30 (14a)
As described above, according to each embodiment, an imaging optical system having high optical performance can be obtained by correcting chromatic aberration over the entire screen.

The diffractive surface (diffractive optical surface) D of the diffractive optical element DOE provided in the optical path is preferably disposed on the cemented lens surface in order to improve dust resistance, assembly workability, and mechanical strength. The diffractive optical part D constituting the diffractive optical element DOE has a diffraction order m, a reference wavelength (d-line) λ, a vertical distance from the optical axis H, a 2i-order term phase coefficient C _2i , and a phase φ (H). Then, the phase φ (H) is changed to φ (H) = (2π · m / λ ₀ ) · (C ₂ · H ² + C ₄ · H ⁴ + ·· C _2i · H ²ⁱ )
This is expressed as follows. Phase factor in this case higher order not next zero phase counting C _2, it is better to be a phase counting C ₂ and opposite sign.

For example the phase coefficient _{C 2} is finite, the phase coefficient _{C 2} and _{C 4,} if the phase coefficients _{C 4} also finite becomes opposite sign. In phase coefficient _{C 2} is finite, the phase coefficients _{C 4} zero, and if the phase coefficient _{C 6} finite, the phase coefficient _{C 2} and _{C 6} are opposite sign. Order phase coefficient not next zero of the phase coefficient C _2, it is desirable that the phase coefficient C ₂ and opposite sign.

If all the phase coefficients from the phase coefficients C _{2 to} C _2i have the same sign, the pitch of the diffraction grating monotonously decreases as it goes to the peripheral part of the lens. However, it has a function to mitigate the if the phase coefficients of opposite sign and phase coefficient C _2, the pitch in the vicinity of the peripheral may be the same paraxial power is getting monotonously decreased. The pitch relaxation effect in the vicinity of the periphery increases as the distance H increases to the power of 2i, so that the coefficient having a smaller order i of the phase coefficient C _2i has a larger effect. Thus it is possible to sign the closest finite order coefficients to the phase coefficient C ₂ is effectively reduced pitch in the peripheral portion by the code and the opposite sign phase coefficient C _2. Thus, it is preferable to relax the pitch of the diffraction grating of the diffractive optical part D because the pitch is wide even if the power is the same, so that it is easy to manufacture.

In each embodiment, the diffractive optical unit refers to one or more diffraction gratings provided on a substrate (a flat plate or a lens). The diffractive optical element DOE is an element in which a diffractive optical part composed of one or more diffraction gratings is provided on a substrate (flat plate or lens). In the lens cross-sectional view, D provided on the cemented lens corresponds to the diffractive optical part. The refractive power (power = inverse number of focal length) phi _D of the diffractive optical portion D can be calculated as follows.
The shape of the diffraction grating of the diffractive optical part D is m, the diffraction order is m, the reference wavelength (d-line) is λ ₀ , the distance from the optical axis is H, and the phase is φ (H).
φ (H) = (2π · m / λ ₀ ) · (C ₂ · H ² + C ₄ · H ⁴ + ·· C _2i · H ²ⁱ )
... (a)
From the phase coefficient C ₂ of the second-order term, the refractive power φ _D is
φ _D = -2 · C ₂
It becomes. That is, the focal length _{f D} of the diffractive optical portion D is _{f D = -1 / (2 ·} C 2)
It is represented by

In each embodiment, in order to obtain better optical characteristics, it is desirable to dispose the positive lens GMp in the vicinity of the negative lens GMn (front and rear lenses). The positive lens GMp may be disposed as a single lens near the negative lens GMn or may be joined to the negative lens GMn. Further good optical characteristics can be obtained by balancing the power between the positive lens GMp and the negative lens GMn. Further, it is more preferable that the sign of the partial dispersion ratio difference Δθ _gF of the material of the positive lens GMp is negative because correction of the secondary spectrum of lateral chromatic aberration is facilitated.

Next, the features of the lens configuration of each example will be described. The reference numeral attached to each lens corresponds to the reference numeral attached to each lens described above. The rear attachment lens LA of Example 1 in FIG. 2A includes a front lens group LF that is composed of a negative lens L _O and a positive lens L _OF in order from the most object side. And it has a negative lens L _I and the rear lens group LR which is a positive lens L _IR in order from the most image side to the object side. The rear attachment lens LA has a negative refractive power as a whole. A negative lens GMn and a positive lens GMp are provided between the front lens group LF and the rear lens group LR. The negative lens GMn and the positive lens GMp are cemented, and the diffractive optical part D is provided on the cemented surface.

In Example 1, all negative lenses other than the negative lens GMn correspond to the negative lens Gn. The rear attachment lens LA of Example 2 in FIG. 3A includes a front lens group LF including a negative lens L _O and a positive lens L _OF in order from the most object side. And it has a negative lens L _I and the rear lens group LR which is a positive lens L _IR in order from the most image side to the object side. The rear attachment lens LA has a negative refractive power as a whole. A negative lens GMn and a positive lens GMp are provided between the front lens group LF and the rear lens group LR. The negative lens GMn and the positive lens GMp are cemented, and the diffractive optical part D is provided on the cemented surface.

In Example 2, all negative lenses other than the negative lens GMn correspond to the negative lens Gn. The rear attachment lens LA of Example 3 in FIG. 4A includes a front group LF that includes a negative lens L _O and a positive lens L _OF in order from the most object side. And has the most negative lens L _I in order from the image side to the object side, a rear lens group LR which is a positive lens L _IR and the negative lens of the cemented lens. The rear attachment lens LA has a negative refractive power as a whole. A negative lens GMn and a positive lens GMp are provided between the front lens group LF and the rear lens group LR. A diffractive optical part D is provided on the cemented surface of the cemented lens in the rear lens group LR.

In Example 3, the negative lens in the cemented lens constituting the diffractive optical element DOE and all the negative lenses other than the negative lens GMn correspond to the negative lens Gn. The rear attachment lens LA of Example 4 in FIG. 5A includes a front lens group LF including a negative lens L _O and a positive lens L _OF in order from the most object side. And it has a negative lens L _I and the rear lens group LR which is a positive lens L _IR in order from the most image side to the object side. The rear attachment lens LA has a negative refractive power as a whole. A negative lens GMn and a positive lens GMp are provided between the front lens group LF and the rear lens group LR. The negative lens GMn and the positive lens GMp are cemented, and the diffractive optical part D is provided on the cemented surface.

In Example 4, all negative lenses other than the negative lens GMn correspond to the negative lens Gn. The rear attachment lens LA of Example 5 in FIG. 6A includes a front lens group LF including a negative lens L _O and a positive lens L _OF in order from the most object side. And it has a negative lens L _I and the rear lens group LR which is a positive lens L _IR in order from the most image side to the object side. The rear attachment lens LA has a negative refractive power as a whole.
A negative lens GMn and a positive lens GMp are provided between the front group LF and the rear group LR. The negative lens GMn and the positive lens GMp are cemented, and the diffractive optical element D is provided on the cemented surface.

In Example 5, all negative lenses other than the negative lens GMn correspond to the negative lens Gn. The rear attachment lens LA of Example 6 in FIG. 7A includes a negative lens L _O and a front lens group LF including a cemented lens of a positive lens L _OF and a negative lens in order from the most object side. Yes. And has the most negative lens L _I in order from the image side to the object side, a rear lens group LR which is a positive lens L _IR and the negative lens of the cemented lens. The rear attachment lens LA has a negative refractive power as a whole. A negative lens GMn and a positive lens GMp are provided between the front group LF and the rear group LR. The negative lens GMn and the positive lens GMp are cemented, and the diffractive optical part D is provided on the cemented surface.

In Example 6, all negative lenses other than the negative lens GMn correspond to the negative lens Gn. In each embodiment, the diffractive optical element DOE itself may have an aspherical effect (optical action). The effect of the aspherical surface, in formula (a) of the diffractive optical element DOE of the diffractive optical portion of the phase of each example, fourth power coefficient C ₄ and later higher-order terms of the value of the term of the distance H from the optical axis It is made by having Thereby, in addition to the aspheric effect other than the chromatic aberration described above, the aspheric effect by the diffraction grating varies depending on the wavelength. For this reason, it becomes easy to correct the variation of the color difference of the spherical aberration by the effect of the aspherical surface.

  Here, the configuration of the diffractive optical element DOE used in the rear attachment lens LA of each example will be described. The diffractive optical part D constituting the diffractive optical element DOE disposed in the rear attachment lens is composed of a diffraction grating that is rotationally symmetric with respect to the optical axis.

  FIG. 8A is an enlarged cross-sectional view of a part of the diffractive optical part of the diffractive optical element 1. In FIG. 8A, a diffraction grating (diffractive optical part) 3 composed of one layer is provided on a substrate (transparent substrate) 2. FIG. 8B is an explanatory diagram showing the characteristics of the diffraction efficiency of the diffractive optical element 1. In FIG. 8B, the horizontal axis represents wavelength and the vertical axis represents diffraction efficiency. Note that the diffraction efficiency is the ratio of the amount of diffracted light to the total transmitted light beam, and the reflected light at the boundary surface of the grating portion 3a is not considered here because the explanation is complicated.

The optical material of the diffraction grating 3 is an ultraviolet curable resin (refractive index n _d = 1.513, Abbe number ν _d = 51.0). The grating thickness d ₁ of the grating part 3a is set to 1.03 μm so that the diffraction efficiency of + 1st order diffracted light is the highest at a wavelength of 530 nm. That is, the design order is + 1st order and the design wavelength is 530 nm.
In FIG. 8B, the diffraction efficiency of the + 1st order diffracted light is indicated by a solid line. Further, in FIG. 8B, the diffraction efficiencies of the diffraction orders in the vicinity of the design order (the 0th order and the + 2nd order which are + 1st order ± 1st order) are also shown. As can be seen from the figure, the diffraction efficiency at the design order is highest near the design wavelength, and gradually decreases at other wavelengths. The decrease in diffraction efficiency at this design order becomes diffracted light of other orders (unnecessary light), which causes flare. Further, when the diffractive optical element is used at a plurality of locations in the optical system, a decrease in diffraction efficiency at a wavelength other than the design wavelength leads to a decrease in transmittance.

Next, a laminated diffractive optical element in which a plurality of diffraction gratings made of different materials are laminated will be described. 9A is a partially enlarged cross-sectional view of the laminated diffractive optical element 1, and FIG. 9B is the wavelength of the diffraction efficiency of the + 1st order diffracted light of the diffractive optical element 1 shown in FIG. 9A. It is a figure showing dependence. In the diffractive optical element 1 shown in FIG. 9A, a first diffraction grating 104 made of an ultraviolet curable resin (refractive index n _d = 1.499, Abbe number ν _d = 54) is formed on a substrate 102. Further, a second diffraction grating 105 (refractive index n _d = 1.598, Abbe number ν _d = 28) is formed thereon. In this combination of materials, the grating thickness d ₁ of the grating portion 104 a of the first diffraction grating 104 is d ₁ = 13.8 μm, and the grating thickness d ₂ of the grating portion 105 a of the second diffraction grating 105 is d ₂ = 10. 5 μm.

  As can be seen from FIG. 9B, by using the diffractive optical element 1 including the diffraction gratings 104 and 105 having a stacked structure, 95% or more in the entire wavelength range (in this case, the visible region) in the diffracted light of the designed order. High diffraction efficiency is obtained. Note that in the diffractive optical element 1 having a laminated structure, the grating thicknesses of the two layers 104 and 105 may be equal depending on the combination of materials as shown in FIG. 9C. In this case, two diffraction grating layers may be arranged with an air layer therebetween.

The diffractive optical part is provided on the optical surface, but its base may be spherical, flat or aspheric. Further, the diffractive optical part may be made of a so-called replica aspherical surface, which is a method of attaching a film such as a plastic as a diffractive optical part (diffractive surface) to these optical surfaces. Since the diffractive optical part has a large anomalous dispersion, by providing the diffractive optical part at a position where the off-axis principal ray in the rear attachment lens passes through a position away from the optical axis in this way, particularly in the periphery of the screen, The lateral chromatic aberration can be corrected effectively. As for the shape of the diffraction grating, the phase φ (H) at the distance H from the optical axis is expressed by the following equation (a) as described above, where C _2i is the phase coefficient of the 2i-order term. Where m is the diffraction order and λ ₀ is the reference wavelength.

In general, the Abbe number (dispersion value) ν _d of a refractive optical material such as a lens or a prism is expressed by the following equation when the refractive power at each wavelength of d, C, and F lines is N _d , N _C , and N _F. Is done.

ν _d = (N _d −1) / (N _F −N _C )> 0 (b)
On the other hand, the Abbe number ν _d of the diffractive optical unit is ν _d = λ _d / (λ _F −λ _C ) (c) when the wavelengths of the d, C, and F lines are λ _d , λ _C , and λ _F. )
And ν _d = −3.45.

Thereby, the dispersibility at an arbitrary wavelength has an adverse effect on the refractive optical element. Further, the refractive power φ of the paraxial temporary diffracted light (m = 1) at the reference wavelength of the diffractive optical part is obtained when the coefficient of the second order term is C ₂ from the previous formula (a) representing the phase of the diffractive optical part. , Φ _D = −2 · C ₂ . Further, when the arbitrary wavelength is λ and the reference wavelength is λ ₀ , the refractive power change with respect to the reference wavelength of the arbitrary wavelength is expressed by the following equation.

_{φ D '= (λ / λ} 0) × (-2 · C 2) ··· (d)
Thus, as a feature of the diffractive optical part, by varying the phase coefficients C ₂ of Equation (a), large dispersion can be obtained by a weak paraxial refractive power change. This means that chromatic aberration is corrected without greatly affecting various aberrations other than chromatic aberration. With respect to the higher order coefficients of the phase coefficient C ₄ and later, the refractive power changes to the light incident height variation of the diffractive optical part can be obtained an effect similar to aspheric. At the same time, it is possible to change the refractive power at an arbitrary wavelength with respect to the reference wavelength according to the change in the incident light height. Therefore, it is effective for correcting lateral chromatic aberration.

Numerical examples 1 to 6 corresponding to the main lens and Examples 1 to 6 of the present invention are shown below. In each numerical example, i indicates the order of the surfaces from the object side, r _i is the radius of curvature of the i-th surface from the object side, d _i is the i-th and i + 1-th distance from the object side, nd _i and νd _i are the refractive index and Abbe number of the i-th optical member. f, fno, and 2ω represent the focal length, F-number, and angle of view (degree) of the entire system when focusing on an object at infinity, respectively. The diffractive optical element (diffractive surface) is expressed by giving a phase coefficient of the phase function of the above-described equation (a). Table 1 shows the relationship between the conditional expressions described above and the numerical values in the numerical examples. In Numerical Examples 1 to 6, the axial air distance from the final surface of the main lens M to the first R1 surface of the rear attachment lens is 20.56 mm.

(Main lens)
f = 299.99mm fno = 2.99 2ω = 8.26
Surface number rd nd νd Effective diameter
1 149.921 17.00 1.49700 81.5 103.45
2 -287.346 0.15 102.70
3 107.686 12.00 1.43387 95.1 93.56
4 948.608 4.36 92.17
5 -337.599 4.50 1.77250 49.6 92.08
6 151.227 14.92 86.70
7 82.152 15.00 1.49700 81.5 82.44
8 9334.428 0.15 80.48
9 52.124 5.00 1.58144 40.8 69.95
10 43.474 34.00 63.53
11 214.749 3.99 1.84666 23.8 49.49
12 -590.486 0.58 48.60
13 -509.687 2.20 1.88300 40.8 48.12
14 70.924 22.54 45.28
15 (Aperture) ∞ 7.64 40.64
16 139.913 1.80 1.84666 23.8 38.44
17 40.306 8.70 37.14
18 -250.105 1.00 36.28
19 63.188 5.00 1.84666 23.8 33.98
20 -180.442 1.70 1.75500 52.3 32.90
21 35.743 5.52 29.88
22 -93.126 1.65 30.02
23 83.876 2.50 30.99
24 106.897 4.70 1.72916 54.7 32.76
25 -211.862 2.64 33.64
26 54.058 6.00 1.48749 70.2 36.38
27 259.048 10.00 36.46
28 ∞ 2.00 1.51633 64.1 37.29
29 ∞ 37.29

(Numerical example 1)
f = -100.36mm Magnification = 2.00
Surface number rd nd νd Effective diameter
1 84.753 1.60 1.79952 42.2 24.12
2 24.264 0.52 23.53
3 28.169 5.50 1.65412 39.7 23.58
4 -71.204 2.10 23.59
5 -324.314 1.20 1.77250 49.6 22.98
6 16.475 10.70 1.62004 36.3 22.46
7 -22.559 1.20 1.88300 40.8 22.71
8 83.922 5.75 23.87
9 54.821 4.81 1.65412 39.7 28.88 (θgF = 0.5737)
10 (Diffraction) -130.000 2.00 1.84666 23.9 29.24 (θgF = 0.6218)
11 130.000 5.04 29.95
12 -52.511 5.20 1.65412 39.7 31.00
13 -24.446 0.30 31.94
14 -54.233 1.55 1.84666 23.9 31.64
15 -110.598 32.29

10th surface (diffractive surface)
C ₂ = 1.50597 × 10 ^-4 C ₄ = -3.34020 × 10 ^-7 C ₆ = 1.46058 × 10 ^-10

(Numerical example 2)
f = -95.09mm Magnification factor = 2.00
Surface number rd nd νd Effective diameter
1 91.740 1.60 1.88300 40.8 21.31
2 26.177 1.58 20.97
3 32.743 5.20 1.65412 39.7 22.14
4 -62.927 4.40 22.25
5 -83.044 1.20 1.74100 52.6 21.68
6 20.183 7.00 1.59270 35.3 21.81
7 -28.501 0.52 22.06
8 -27.028 1.20 1.88300 40.8 21.96
9 6861.465 2.41 22.95
10 68.994 4.10 1.65412 39.7 24.28 (θgF = 0.5737)
11 (Diffraction) -150.000 2.00 1.84666 23.9 24.65 (θgF = 0.6218)
12 152.241 6.86 25.13
13 -70.436 5.52 1.65412 39.7 28.25
14 -26.599 0.95 29.33
15 -62.879 1.55 1.84666 23.9 29.05
16 -244.453 29.53

11th surface (diffractive surface)
C ₂ = 1.36063 × 10 ⁻⁴ C ₄ = −2.555075 × 10 ⁻⁷ C ₆ = −2.31053 × 10 ⁻¹⁰

(Numerical Example 3)
f = -91.87mm Magnification factor = 2.00
Surface number rd nd νd Effective diameter
1 77.233 1.60 1.80610 40.9 21.36
2 24.999 2.54 20.99
3 31.371 5.68 1.65412 39.7 21.75
4 -54.942 1.70 21.89
5 -84.665 1.20 1.80400 46.6 21.47
6 19.477 8.40 1.66680 33.0 21.45
7 -22.136 1.20 1.88300 40.8 21.68
8 215.868 2.16 22.54
9 -201.876 1.50 1.92286 18.9 25.09 (θgF = 0.6495)
10 215.028 0.50 25.63
11 143.495 5.28 1.65412 39.7 26.06 (θgF = 0.5737)
12 -53.915 8.92 26.61
13 -51.934 2.50 1.48749 70.2 28.99
14 (Diffraction) -66.265 4.00 1.65412 39.7 29.73
15 -28.067 0.50 30.22
16 -56.710 1.55 1.84666 23.9 29.66
17 -568.091 30.20

14th surface (diffractive surface)
C ₂ = 1.72094 × 10 ⁻⁴ C ₄ = −6.19375 × 10 ⁻⁷ C ₆ = 6.96476 × 10 ⁻¹⁰

(Numerical example 4)
f = -118.19mm Magnification = 2.00
Surface number rd nd νd Effective diameter
1 80.347 1.60 1.88300 40.8 21.04
2 25.840 1.96 20.69
3 42.569 5.20 1.64769 33.8 21.41
4 -76.977 3.40 21.84
5 548.744 1.20 1.72916 54.7 22.04
6 20.432 7.00 1.64769 33.8 22.08
7 -28.828 0.52 22.20
8 -28.281 1.20 1.88300 40.8 21.96
9 74.461 1.00 22.70
10 48.707 4.10 1.72047 34.7 23.72 (θgF = 0.5834)
11 (Diffraction) -154.380 2.00 2.14352 17.8 24.00 (θgF = 0.659)
12 121.497 13.00 24.42
13 -45.389 5.52 1.65412 39.7 29.84
14 -24.834 0.95 31.34
15 -73.695 1.55 1.84666 23.8 31.39
16 -143.814 31.88

11th surface (diffractive surface)
C ₂ = 4.50457 × 10 ^-5 C ₄ = -4.26172 × 10 ^-8 C ₆ = 2.60284 × 10 ^-10

(Numerical example 5)
f = -127.67mm Magnification = 2.00
Surface number rd nd νd Effective diameter
1 70.575 1.60 1.88300 40.8 21.05
2 25.007 1.81 20.64
3 39.330 5.20 1.66680 33.0 21.28
4 -85.736 3.82 21.63
5 -538.480 1.20 1.72916 54.7 21.71
6 19.744 7.00 1.64769 33.8 21.83
7 -29.109 0.52 21.98
8 -29.121 1.20 1.88300 40.8 21.76
9 56.793 1.00 22.55
10 45.296 4.10 1.61340 44.3 23.66 (θgF = 0.5633)
11 (Diffraction) -69.724 2.00 1.94595 18.0 23.99 (θgF = 0.6544)
12 372.204 13.00 24.79
13 -45.562 5.52 1.65412 39.7 30.45
14 -25.527 0.95 31.99
15 -86.601 1.55 1.84666 23.8 32.20
16 -152.203 32.62

11th surface (diffractive surface)
C ₂ = 7.86284 × 10 ⁻⁵ C ₄ = −3.51592 × 10 ⁻⁸ C ₆ = −4.330296 × 10 ⁻¹⁰

(Numerical example 6)
f = -114.44mm Magnification = 2.00
Surface number rd nd νd Effective diameter
1 57.666 1.60 1.88300 40.8 21.01
2 25.706 2.75 20.55
3 37.881 5.20 1.62588 35.7 21.63
4 -64.992 1.50 1.71300 53.9 21.78
5 -111.456 6.59 21.91
6 -412.938 1.20 1.72916 54.7 21.75
7 19.123 7.00 1.64769 33.8 21.79
8 -30.335 0.52 21.92
9 -30.950 1.20 1.88300 40.8 21.68
10 46.714 2.75 22.36
11 40.169 4.64 1.61340 44.3 25.14 (θgF = 0.5633)
12 (Diffraction) -51.298 2.00 1.94595 18.0 25.34 (θgF = 0.6544)
13 -6280.367 6.15 26.22
14 -44.037 1.50 1.83481 42.7 28.09
15 111.173 8.25 1.65412 39.7 30.87
16 -25.243 0.95 31.88
17 -114.118 1.55 1.84666 23.8 32.61
18 -157.402 32.99

12th surface (diffractive surface)
C ₂ = 5.13153e × 10 ^-5 C ₄ = -2.77008 × 10 ^-8 C ₆ = 2.05002 × 10 ^-10

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

M is a main lens system LA is a rear attachment lens LF is a front lens group LR is a rear lens group GMn is a negative lens GMp is a positive lens DOE is a diffractive optical element

Claims (11)

In a rear attachment lens that is detachably mounted on the image plane side of the main lens system and changes the focal length to be longer than the focal length of the main lens system, the rear attachment lens includes one or more diffractive optical elements; It has one or more negative lenses GMn, the Abbe number and partial dispersion ratio difference of the material of the negative lens GMn is ν _{d_Mn} , Δθ _{gF_Mn} , the focal length of the negative lens GMn is f _Mn , the most object side of the rear attachment lens The distance from the lens surface to the most image-side lens surface is d, the distance from the most object-side lens surface of the rear attachment lens to the object-side lens surface of the negative lens GMn is t _Mn , and the rear attachment lens When the focal length is f,
0.03 <f / (ν _{d_Mn} × f _Mn ) <0.20
0.015 <Δθ _{gF_Mn} <0.060
0.40 <t _{Mn /} d <0.72
A rear attachment lens that satisfies the following conditional expression: In a rear attachment lens that is detachably mounted on the image plane side of the main lens system and changes the focal length to be longer than the focal length of the main lens system, the rear attachment lens includes one or more diffractive optical elements; most of the lens L _O in the object side toward the image side sign of the focal length to the lens surface on the image side of the lens L _O and the first opposite sign lens L _oF, or the lens L _oF is the image side When the lens is cemented, the lens up to the lens surface closest to the image side of the cemented lens is a front lens group, and the sign of the focal length from the lens L _I closest to the image side toward the object side is the lens L to the lens surface _I and the object side of the first becomes the opposite sign lens L _IR, or when the lens L _IR is joined to the object side lens, the most object side lens of the cemented lens When the lens to the surface and the rear lens group has one or more negative lenses GMn between the front lens group and the rear lens group, the Abbe number and the partial dispersion ratio difference between the negative lens GMn material [nu _{D_Mn} , [Delta] [theta] _{GF_Mn,} the negative lens GMn focal length f _Mn of the lens _{L O} and the lens _{L oF} respective _f LO the focal length _{of f LOF,} the lens _{L I} and the lens _{L IR} focal length of each f _LI , f _LIR , where f is the focal length of the rear attachment lens,
0.03 <f / (ν _{d_Mn} × f _Mn ) <0.20
0.015 <Δθ _{gF_Mn} <0.060
│f _LO /f│<7.0
│f _LOF /f│<7.0
│f _LI /f│<7.0
│f _LIR /f│<7.0
A rear attachment lens that satisfies the following conditional expression: When the refractive index of the material of the negative lens GMn is N _{d_Mn} ,
1.75 <N _{d_Mn} <2.50
The rear attachment lens according to claim 1, wherein the following conditional expression is satisfied. When the focal length by only the diffraction component of the diffractive optical element is f _D ,
25 <f _D / f <130
The rear attachment lens according to claim 1, wherein the following conditional expression is satisfied. When a lens having at least one or more power is disposed on the image side from the diffractive optical element, and the combined magnification of all the lenses on the image side from the diffractive optical element is β _REAR ,
0.9 <β _REAR <5.0
The rear attachment lens according to claim 1, wherein the following conditional expression is satisfied.   6. The rear attachment lens according to claim 1, wherein the diffractive optical part of the diffractive optical element is disposed on a joint surface of two lenses. 7. The diffractive optical unit constituting the diffractive optical element has a diffraction order m, a reference wavelength (d-line) λ, a vertical distance from the optical axis H, a 2i-order phase coefficient C _2i , and a phase φ ( H),
φ (H) = (2π · m / λ ₀ ) · (C ₂ · H ² + C ₄ · H ⁴ + ·· C _2i · H ²ⁱ )
The high-order phase coefficient which is not zero next to the phase count C ₂ is represented by the following formula, and has a sign opposite to that of the phase count C _2. Rear attachment lens. The rear attachment lens has one or more positive lenses GMp, and the refractive index and partial dispersion ratio difference of the material of the positive lens GMp are respectively N _Mp , Δθ _{gF_Mp} , and the lens surface closest to the object side of the rear attachment lens, When the distance to the object-side lens surface of the positive lens GMp is t _Mp and the focal length of the positive lens GMp is f _Mp ,
-2.5 <f / (N _Mp × f _Mp ) <-0.3
-0.020 <Δθ _{gF_Mp} <-0.001
0.6 <t _Mp / t _Mn <1.4
The rear attachment lens according to claim 1, wherein the following conditional expression is satisfied. The rear attachment lens has one or more negative lens Gn in addition to the negative lens GMn, the refractive index of the negative lens Gn material when the N _n,
1.65 <N _n <2.50
The rear attachment lens according to claim 1, wherein the following conditional expression is satisfied.   An imaging optical system comprising: a main lens system; and the rear attachment lens according to claim 1 detachably attached to an image side of the main lens system.   An imaging apparatus comprising: the imaging optical system according to claim 10; and a solid-state imaging device that receives an image formed by the imaging optical system.