JPH05142469A - Large-aperture intermediate telephoto lens - Google Patents

Abstract

PURPOSE:To obtain the large-aperture intermediate telephoto lens with high performance which is a photographic lens having an about 29 deg. photographic view angle and an about 1.4 F number and makes the defocusing in a defocusing area before or after an in-focus position excellent. CONSTITUTION:This intermediate telephoto lens consists of a front group including three positive lenses which have large-curvature convex surfaces on the object side and at least one negative lens which has a concave surface on the image side and a rear group consisting of a cemented lens, a negative lens which has a large-curvature concave surface on the object side, and at least two positive lens; and a gradient index lens is used in the rear group.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a high-performance large-scale image pickup apparatus which has a photographing screen of about 29 ° and an F-number of about 1.4, which is suitable for photographic lenses. It relates to a medium aperture telephoto lens.

[0002]

2. Description of the Related Art In recent years, it has become mainstream to mount variable magnification lenses that can easily change the angle of view in both single-lens reflex cameras and lens shutter cameras. However, it is difficult for a variable power lens to be a bright lens system with a small F number, and aberration correction cannot be said to be sufficient as compared with a single focus lens. Therefore, in response to the voice of the user who demands brightness and image quality, the monofocal lens has been improved in image quality by increasing the aperture ratio.

Conventionally, a so-called Gauss type lens is known as a lens system which can relatively easily obtain a high image quality with a large aperture ratio. This Gauss type lens is mainly used in a bright lens system from a standard angle of view to a region with a narrow angle of view called a medium telephoto. However, if the F number is reduced to about 1.4 and the lens system is brightened while maintaining a predetermined angle of view, back focus, etc., it is particularly difficult to simultaneously correct sagittal coma and spherical aberration. Become.

Further, in a bright mid-telephoto lens having a photographing field angle of about 29 ° and an F number of about 1.4, which is used for portraits, not only the performance at a focused position but also the defocus areas before and after that are taken. The way of blurring is a problem, and therefore astigmatism must be corrected particularly well.

The shooting angle of view is about 29 ° and the F number is 1.4.
As a conventional example of a bright medium telephoto lens of the order of magnitude, Japanese Patent Laid-Open No.
There are lens systems described in JP-A No. 9-48723, JP-A No. 62-244010, and JP-A No. 63-20562.

However, in these conventional examples, sagittal coma aberration is large in response to correction of spherical aberration, and it cannot be said that the entire system is sufficiently corrected.

The lens system disclosed in Japanese Patent Laid-Open No. 1-302311 has an imaging angle of view of 29 ° and an F number of about 1.2 by using an aspherical surface.

However, in such a lens system, even the smallest lens has an effective diameter of about 40 mm, and there is a problem that the cost becomes high in terms of accuracy of the aspherical surface. The applicant has developed the lens system disclosed in Japanese Patent Application Laid-Open No. 2-50116. This lens system is a system in which spherical aberration and sagittal coma aberration are simultaneously corrected by using a gradient index lens having a gradient index distribution in the optical axis direction, which is easier to increase the aperture than an aspherical surface. However, in this conventional example, although spherical aberration and sagittal coma aberration are well corrected, the curvature of the image plane is relatively large.

[0009]

SUMMARY OF THE INVENTION The present invention is a lens system having an image pickup angle of view of about 29 ° and an F number of about 1.4. In addition to correcting sagittal coma and spherical aberration, the curvature of the image plane is suppressed. It is an object of the present invention to provide a high-performance large-aperture medium-telephoto lens in which astigmatism is satisfactorily corrected to favorably blur in the defocus areas before and after the in-focus position.

[0010]

The lens system of the present invention comprises a front group on the object side and a rear group on the image side with a diaphragm interposed therebetween, and the front group has a strong convex surface directed in order from the object side to the object side. At least three positive lenses and at least one negative lens having a strong concave surface facing the image side arranged close to the diaphragm are included, and the front group as a whole is composed of five or more lenses. The group is composed of a cemented lens in which a biconcave lens and a biconvex lens are cemented in order from the object side, a negative lens having a strong concave surface facing the object side, and at least two positive lenses. At least one lens is included in the rear group. It is a large aperture medium telephoto lens that uses a gradient index lens.

In order to improve the performance of a Gauss type large aperture medium telephoto lens as in the present invention, correction of spherical aberration and sagittal coma becomes a problem.

However, the spherical aberration and the sagittal coma aberration, which are characteristic of the Gauss-type lens and are generated in the concave surfaces facing each other with the diaphragm interposed therebetween, are generated in the directions opposite to the case where they are simultaneously corrected. It is not possible to sufficiently correct sagittal coma aberration while satisfactorily correcting.

That is, in order to correct the spherical aberration that largely occurs in the negative direction in the entire system, it is necessary to make the curvature of the concave surfaces facing each other across the stop tight and generate positive spherical aberration in these concave surfaces. .. However, the flare component of the sagittal coma aberration generated on the concave surface at that time becomes too large to correct the entire system. On the contrary, when the curvature of these concave surfaces becomes too loose, not only the spherical aberration is corrected but also the Petzval sum becomes a large positive value, and the image surface falls to the object side. Therefore, it is difficult to correct spherical aberration and sagittal coma aberration at the same time while maintaining a good image surface.

The third-order spherical aberration is 4 which is the height of the axial ray.
Since it is proportional to the power, in the present invention, by arranging at least three positive lenses in the front group on the object side of the diaphragm having a high ray height of the marginal ray that contributes to spherical aberration in order from the object side, the negative The spherical aberration of is suppressed to a smaller value.

Further, at an angle of view equivalent to that of the lens system of the present invention, not only spherical aberration but also axial chromatic aberration has a large effect on the performance at the center of the screen. Therefore, the chromatic aberration generated by the three positive lenses of the front group is preferably as small as possible, and it is desirable to use a low dispersion glass having a large Abbe number.

However, when trying to use a low dispersion glass having a larger Abbe number, it is unavoidable to use a glass having a low refractive index. Use of three positive lenses made of glass having a low refractive index in the front group is effective for suppressing negative spherical aberration, but the Petzval sum becomes a large positive value and the image plane falls toward the object side, which is not preferable. ..

In the present invention, by disposing at least one negative lens on the image side of the cemented lens in which the biconcave lens and the biconvex lens are cemented, in the rear group on the image side of the diaphragm, the front lens group is formed by this negative lens. The spherical aberration and axial chromatic aberration generated by the three positive lenses used in 1. are corrected for the whole system by generating spherical aberration and axial chromatic aberration of opposite signs to the axial aberration, and at the same time the Abbe number is large in the front group for comparison. The Petzval sum, which is deteriorated by using many positive lenses made of glass having a low refractive index, is properly corrected. Further, with the above-mentioned configuration, the spherical aberration of the entire system can be favorably corrected without tightly curving the concave surfaces facing each other with the diaphragm interposed therebetween, and at the same time, the occurrence of sagittal coma aberration can be suppressed to a small level. ..

As described above, a lens system in which various aberrations are satisfactorily corrected can be obtained, but in order to obtain a lens system of higher performance, a gradient index lens is introduced in the present invention.

The gradient index lens can be roughly classified into a radial type having a refractive index distribution in the radial direction and an axial type having a refractive index distribution in the optical axis direction.

The radial type gradient index lens is capable of correcting Petzval sum and chromatic aberration with the medium, and is extremely advantageous for aberration correction. However, it has a drawback that it is difficult to increase the diameter.

Therefore, for a lens system having a large aperture such as the lens system of the present invention, it is preferable to use an axial type gradient index lens in view of lens production and cost.

Further, in the lens system according to the present invention, in order to secure a necessary back focus, the refractive power of the front group on the object side of the diaphragm is made relatively weak so that the refractive power of the rear group on the image side of the diaphragm is reduced. Since the configuration is made stronger, the amount of aberration that occurs in the rear group is larger. Therefore, in order to maximize the effect of the axial gradient index lens, it is effective to use at least one axial gradient index lens in the rear group.

By configuring the lens system of the present invention as described above, the spherical aberration of the entire system can be corrected more satisfactorily, and as a result, the position where the spherical aberration correction burden is lightened is placed across the diaphragm. The concave surface is shaped to be advantageous for correcting sagittal coma, and a high-performance lens system in which both are simultaneously corrected can be obtained.

Here, the axial type gradient index lens having a refractive index distribution in the optical axis direction used in the present invention has a refractive index distribution represented by the following formula. n (x) = N ₀ + N ₁ x + N ₂ x ² + N ₃ x ³ + ... where x is the distance from the object side surface of the axial gradient index lens in the optical axis direction, and n (x) is Refractive index on the optical axis at a distance of x, N ₀ is the refractive index on the optical axis of the object side surface of the gradient index lens, N ₁ , N ₂ , N ₃ ...
Are first-order, second-order, third-order, ... Coefficients. Although all the references are on the object side surface of the gradient index lens, it is also possible to use the reference inside the gradient index lens.

In the lens system of the present invention having the above-described structure, at least one negative lens used on the image side of the cemented lens of the biconcave lens and the biconvex lens in the rear group is excellent in off-axis aberration, especially astigmatism. In order to keep the aperture, it is desirable that the aperture has a concentric shape, and it is preferable that the meniscus lens has a concave surface facing the object side. Further, if the negative lens is arranged closer to the image side, it becomes difficult to correct spherical aberration, so it is effective to dispose the negative lens close to the cemented lens of the biconcave lens and the biconvex lens.

From the above, it is preferable for the negative lens to satisfy the following condition (1). (1) 0.2 <R ₀ / R ₁ <1.0 where R ₀ is the curvature of the object-side surface of the negative lens used at least one on the image side of the cemented lens of the biconcave lens and the biconvex lens in the rear group. The radius R ₁ is the radius of curvature of the image-side surface of the negative lens.

The above condition (1) is a condition for correcting astigmatism in particular while maintaining good spherical aberration, Petzval sum, and axial chromatic aberration. When the lower limit is exceeded, the concentricity of the negative lens with respect to the diaphragm. Cannot be corrected well, and if the upper limit is exceeded, the refractive power of the negative lens becomes small, and spherical aberration, Petzval sum,
The axial chromatic aberration cannot be corrected.

Further, in the lens system of the present invention, it is preferable to satisfy the following conditions in order to satisfactorily correct aberrations. (2) f · φ · R · N ₁ <0 (3) 1.55 <(n ₁ + n ₂ + n ₃ ) / 3 (4) 0.5 <f _1-3 /f<0.9 (5) 0.5 <f _R /f<0.9 where f is the combined focal length of the entire system, φ is the power of the surface of the axial type gradient index lens on the side having the refractive index distribution, and R is the above The radius of curvature of the surface, N ₁ is the first-order coefficient of the refractive index distribution formula, and n ₁ , n ₂ , and n ₃ are the refraction of the positive lens of the first lens, the second lens, and the third lens in order from the object side of the front group. The ratio, f _1-3, is the combined focal length of the first lens, the second lens, and the third lens, and f _R is the combined focal length of the rear group.

The condition (2) is a condition provided to effectively generate a positive spherical aberration in the axial type gradient index lens. If the upper limit is exceeded, the surface having a refractive index distribution has a positive spherical surface. Aberration cannot be generated and spherical aberration in the entire system cannot be corrected.

The condition (3) is a condition provided for keeping the Petzval sum of the entire system in a good condition. When the lower limit is exceeded, the Petzval sum of the entire system takes a large positive value, and the image surface is on the object side. It collapses and cannot be corrected.

Condition (4) is a condition provided to reduce the amount of negative spherical aberration generated. If the lower limit is exceeded, the refractive power of the three positive lenses arranged in order from the object side of the front group will become strong. As a result, the amount of negative spherical aberration generated becomes too large to correct. If the upper limit is exceeded, the amount of negative spherical aberration generated will be small, but the lens system will become large, which is not preferable.

The condition (5) is a condition for ensuring the required back focus while keeping the lens system compact, and if the lower limit is exceeded, the refractive power of the rear group becomes too large and the necessary back focus is secured. Can not do. On the other hand, if it exceeds the upper limit, the refractive power of the rear group becomes too small, and the lens system becomes large, which is not preferable.

Further, in the lens system of the present invention, the negative lens used in the rear group, at least one lens on the image side of the cemented lens of the biconcave lens and the biconvex lens, satisfies the following condition (6) so that spherical aberration, It is preferable because Petzval sum and axial chromatic aberration can be corrected more satisfactorily. (6) −2.5 <f _n /f<−0.5 where f _n is the focal length of at least one negative lens used on the image side of the cemented lens of the biconcave lens and the biconvex lens in the rear group.

When the lower limit of the condition (6) is exceeded, the refracting power of the negative lens becomes too weak and the spherical aberration, Petzval sum, and axial chromatic aberration are insufficiently corrected. Girl sum and axial chromatic aberration are overcorrected, which is not preferable.

Further, in the lens system of the present invention, by providing a so-called floating mechanism for changing at least one of the air gaps in the rear group on the image side of the diaphragm during focusing, the photographing magnification is about -1/7. It is possible to satisfactorily correct various aberrations in the entire focusing area up to the double object point.

[0036]

EXAMPLES Next, examples of the lens system of the present invention will be shown. Example 1 f = 1 mm, F / 1.44, 2ω = 28.8 ° r ₁ = 2.1065 d ₁ = 0.0514 n ₁ = 1.69680 ν ₁ = 56.49 r ₂ = 2.5980 d ₂ = 0.0015 r ₃ = 0.7291 d ₃ = 0.0977 n ₂ = 1.49700 ν ₂ = 81.61 r ₄ = 7.9280 d ₄ = 0.0015 r ₅ = 0.5414 d ₅ = 0.0749 n ₃ = 1.69680 ν ₃ = 56.49 r ₆ = 1.0711 d ₆ = 0.0015 r ₇ = 0.4014 d ₇ = 0.0824 n ₄ = 1.69680 ν ₄ = 56.49 r ₈ = 0.3770 d ₈ = 0.0617 r ₉ = 0.9742 d ₉ = 0.0266 n ₅ = 1.75520 ν ₅ = 27.51 r ₁₀ = 0.2913 d ₁₀ = 0.1029 r ₁₁ = diaphragm d ₁₁ = 0.0566 r ₁₂ = -0.5037 d ₁₂ = 0.0206 n ₆ = 1.60342 ν ₆ = 38.01 r ₁₃ ＝ 0.7910 d ₁₃ ＝ 0.1330 n ₇ = 1.88300 ν ₇ = 40.78 r ₁₄ ＝ -0.7330 d ₁₄ ＝ 0.0226 r ₁₅ ＝ -0.5124 d ₁₅ ＝ 0.0432 n ₈ (Refractive index distribution Type lens) r ₁₆ = -1.6573 d ₁₆ = D ₁ r ₁₇ = 3.9255 d ₁₇ = 0.0601 n ₉ = 1.74100 ν ₉ = 52.68 r ₁₈ = -0.7865 d ₁₈ = D ₂ r ₁₉ = 2.4150 d ₁₉ = 0.0662 n ₁₀ = 1.69680 ν ₁₀ = 56.49 r ₂₀ = -3.4322 ( floating) (Refractive index distribution type lens) 0.0154 mm from the object side surface to the image side
A homogeneous glass with a constant refractive index of N _{0 up} to. Follow the refractive index distribution formula with 0.0154 mm as a reference (x = 0).

[0037] R ₀ / R ₁ = 0.309, f ・ φ ・ R ・ N ₁ = -0.602, (n ₁ +
n ₂ + n ₃ ) / 3 = 1.63 f _1-3 /f=0.750, f _R /f=0.753, f _n /f=-1.192 Example 2 f = 1 mm, F / 1.44, 2ω = 28.8 ° r ₁ = 1.6296 d ₁ = 0.0494 n ₁ = 1.9680 v ₁ = 55.52 r ₂ = 3.0124 d ₂ = 0.0015 r ₃ = 0.7186 d ₃ = 0.0905 n ₂ = 1.43875 v ₂ = 94.97 r ₄ = 5.2012 d ₄ = 0.0015 r ₅ = 0.5313 d ₅ = 0.0756 n ₃ = 1.9680 ν ₃ ＝ 55.52 r ₆ ＝ 0.9715 d ₆ ＝ 0.0015 r ₇ ＝ 0.4037 d ₇ ＝ 0.0826 n ₄ ＝ 1.69680 ν ₄ ＝ 55.52 r ₈ ＝ 0.3605 d ₈ ＝ 0.0617 r ₉ ＝ 0.9306 d ₉ = 0.0271 n ₅ = 1.75520 ν ₅ = 27.51 r ₁₀ = 0.2993 d ₁₀ = 0.1029 r ₁₁ = Aperture d ₁₁ = 0.0566 r ₁₂ = -0.5007 d ₁₂ = 0.0190 n ₆ = 1.62588 ν ₆ = 35.70 r ₁₃ = 0.6408 d ₁₃ = 0.1329 n ₇ = 1.88300 ν ₇ ＝ 40.78 r ₁₄ ＝ -0.7075 d ₁₄ ＝ 0.0206 r ₁₅ ＝ -0.5129 d ₁₅ ＝ 0.0461 n ₈ (Gradation index lens) r ₁₆ ＝ -1.1129 d ₁₆ ＝ 0.0015 r ₁₇ ＝ 3.5472 d _{17 = 0.0586 n 9 = 1.74100 ν} 9 = 52.68 r 18 = - 0.8699 d ₁₈ = D ₁ r ₁₉ = 6.9188 d ₁₉ = 0.0559 n ₁₀ = 1.69680 ν ₁₀ = 55.52 r ₂₀ = -3.0132 (floating) (Refractive index distribution type lens) 0.0151 mm toward the image side from the object side surface
A homogeneous glass with a constant refractive index of N _{0 up} to. Follow the refractive index distribution formula with 0.0151 mm as a reference (x = 0).

N ₀ N ₁ N ₂ N ₃ d line 1.66680 0.70761 0.17083 × 10 ² 0.80550 × 10 ² C line 1.66091 0.79783 0.12060 × 10 ² 0.18740 × 10 ³ F line 1.68109 0.74520 0.19728 × 10 ² -0.10984 × 10 ² g line 1.69298 0.95417 0.15731 × 10 ² 0.51488 × 10 ² R ₀ / R ₁ = 0.461, f ・ φ ・ R ・ N ₁ = -0.472, (n ₁ + n ₂ + n ₃ ) / 3 = 1.61 f _1-3 / f = 0.746, f _R / f = 0.763, f _n / f = -1.559 Example 3 f = 1 mm, F / 1.44, 2ω = 28.8 ° r ₁ = 1.6507 d ₁ = 0.0494 n ₁ = 1.69680 ν ₁ = 56.49 r ₂ = 2.8068 d ₂ = 0.0015 r ₃ = 0.7601 d ₃ = 0.0844 n ₂ = 1.43875 ν ₂ = 94.97 r ₄ = 4.5581 d ₄ = 0.0015 r ₅ = 0.5225 d ₅ = 0.0758 n ₃ = 1.69680 ν ₃ = 56.49 r ₆ = 0.9767 _{d 6 = 0.0015 r 7 = 0.4018} d 7 = 0.0826 n 4 = 1.69680 ν 4 = 56.49 r 8 = 0.3571 d 8 = 0.0617 r 9 = 0.8653 d 9 = 0.0273 n 5 = 1.76180 ν 5 = 27.11 r 10 = 0.3004 d 10 = 0.1029 r ₁₁ = Aperture d ₁₁ = 0.0566 r ₁₂ = -0.4914 d ₁₂ = 0.0190 n ₆ = 1.60342 ν ₆ ＝ 38.01 r ₁₃ ＝ 0.6508 d ₁₃ ＝ 0.1335 n ₇ ＝ 1.88300 ν ₇ ＝ 40.78 r ₁₄ ＝ -0.6863 d ₁₄ ＝ 0.0206 r ₁₅ ＝ -0.5027 d ₁₅ ＝ 0.0441 n ₈ (Refractive index distribution type) Lens) r ₁₆ = -1.0356 d ₁₆ = D ₁ r ₁₇ = 3.0200 d ₁₇ = 0.0589 n ₉ = 1.74100 ν ₉ = 52.68 r ₁₈ = -0.8497 d ₁₈ = D ₂ r ₁₉ = 11.7531 d ₁₉ = 0.0551 n ₁₀ = 1.69680 ν ₁₀ = 56.49 r ₂₀ = -4.5542 (floating) (Refractive index distribution type lens) 0.0158mm from the object side to the image side
A homogeneous glass with a constant refractive index of N _{0 up} to. Follow the refractive index distribution formula with 0.0158 mm as a reference (x = 0).

N ₀ N ₁ N ₂ N ₃ d line 1.69895 0.68286 0.19 140 × 10 ² 0.80550 × 10 ² C line 1.69223 0.76992 0.135 12 × 10 ² 0.18740 × 10 ³ F line 1.71543 0.71913 0.22 105 × 10 ² -0.10984 × 10 ² g line 1.72933 0.95417 0.15731 × 10 ² 0.51488 × 10 ² R ₀ / R ₁ = 0.485, f ・ φ ・ R ・ N ₁ = -0.477, (n ₁ +
n ₂ + n ₃ ) /3=1.61 f _1-3 /f=0.768, f _R /f=0.768, f _n /f=−1.527 Example 4 f = 1 mm, F / 1.44, 2ω = 28.8 ° r ₁ = 1.6795 d ₁ = 0.0494 n ₁ = 1.9680 v ₁ = 56.49 r ₂ = 2.8017 d ₂ = 0.0015 r ₃ = 0.7497 d ₃ = 0.0844 n ₂ = 1.43875 v ₂ = 94.97 r ₄ = 5.4048 d ₄ = 0.0015 r ₅ = 0.5224 d ₅ = 0.0758 n ₃ = 1.9680 ν ₃ = 56.49 r ₆ = 0.9889 d ₆ = 0.0015 r ₇ = 0.4018 d ₇ = 0.0826 n ₄ = 1.69680 ν ₄ = 56.49 r ₈ = 0.3575 d ₈ = 0.0617 r ₉ = 0.8793 d _{9 = 0.0273 n 5 = 1.76180 ν 5} = 27.11 r 10 = 0.3005 d 10 = 0.1029 r 11 = stop _{d 11 = 0.0566 r 12 = -0.4839} d 12 = 0.0190 n 6 = 1.60342 ν 6 = 38.01 r 13 = 0.6563 d 13 = 0.1335 n ₇ = 1.88300 ν ₇ = 40.78 r ₁₄ = -0.6846 d ₁₄ = 0.0206 r ₁₅ = -0.5065 d ₁₅ = 0.0441 n ₈ (gradient distribution type lens) r ₁₆ = -1.0572 d ₁₆ = 0.0021 r ₁₇ = 4.3349 d _{17 = 0.0590 n 9 = 1.74100 ν} 9 = 52.68 _{18 = -0.8443 d 18 = D 1} r 19 = 5.0019 d 19 = 0.0551 n 10 = 1.69680 ν 10 = 56.49 r 20 = -4.9245 ( floating) (Refractive index distribution type lens) 0.0158mm from the object side to the image side
A homogeneous glass with a constant refractive index of N _{0 up} to. Follow the refractive index distribution formula with 0.0158 mm as a reference (x = 0).

N ₀ N ₁ N ₂ N ₃ d line 1.69895 0.69245 0.180 59 × 10 ² 0.605 27 × 10 ² C line 1.69223 0.78074 0.127 49 × 10 ² 0.1408 1 × 10 ³ F line 1.71543 0.72924 0.20856 × 10 ² -0.82535 × 10 g line 1.72933 0.95417 0.15731 × 10 ² 0.51488 × 10 ² R ₀ / R ₁ = 0.479, f ・ φ ・ R ・ N ₁ = -0.484, (n ₁ +
n ₂ + n ₃ ) / 3 = 1.61 f _1-3 /f=0.755, f _R /f=0.755, f _n /f=-1.513 where r ₁ , r ₂ , ... are the radius of curvature of each lens surface , D
₁ , d ₂ , ... Is the thickness of each lens and the lens interval, n
₁ , n ₂ , ... Are the refractive indices of the respective lenses, ν ₁ , ν ₂ ,.
Is the Abbe number of each lens.

Example 1 has the configuration shown in FIG. 1, and three positive lenses having convex surfaces facing the object side in order from the object side, a meniscus lens having a convex surface facing the object side, and a concave surface facing the image side. A concave meniscus lens, a diaphragm, a cemented lens in which a biconcave lens and a biconvex lens are cemented, a concave meniscus lens with a concave surface facing the object side, and a positive lens with a convex surface facing the image side 2
A concave meniscus lens having a concave surface facing the object side arranged on the image side of the cemented lens is an axial type gradient index lens. The refractive index distribution of this gradient index lens is attached only to the image side surface.

In this embodiment, a so-called double floating mechanism for independently moving the two positive lenses on the image side during focusing is provided, and the entire area from the object point at infinity to the object point at about -1/7 times. This is an example that makes it possible to obtain good performance over the entire range.

In FIG. 1, (A) is an object point at infinity, and (B) is
Is for an object point of about -1/7.

The aberrations of this embodiment at infinity and at an object point of about -1/7 are shown in FIGS. 5 and 6, respectively.

The second embodiment has the configuration shown in FIG. 2, and includes three positive lenses having convex surfaces facing the object side in order from the object side, a meniscus lens having a convex surface facing the object side, and a concave lens having a concave surface facing the image side. It consists of a meniscus lens, a diaphragm, a cemented lens cemented with a biconcave lens and a biconvex lens, a concave meniscus lens with a concave surface facing the object side, and two positive lenses with a convex surface facing the image side. A concave meniscus lens having a concave surface facing the object side disposed on the image side is an axial gradient index lens. The refractive index distribution of the axial type gradient index lens of this embodiment is attached only to the image side surface.

In this embodiment, a floating mechanism for independently moving one of the positive lenses closest to the image side during focusing is provided to obtain good performance over the entire range of the object point from infinity to about -1/7. This is an example that makes it possible.

In FIG. 2, (A) is an object point at infinity, and (B) is
Is for an object point of about -1/7.

The aberrations of this embodiment at infinity and about -1/7 are as shown in FIGS. 7 and 8, respectively.

In Example 3, as shown in FIG. 3, three positive lenses having convex surfaces facing the object side, a meniscus lens having convex surfaces facing the object side, and a concave surface facing the image side are arranged in order from the object side. It consists of a concave meniscus lens, a diaphragm, a cemented lens cemented with a biconcave lens and a biconvex lens, a concave meniscus lens with the concave surface facing the object side, and two positive lenses with the convex surface facing the image side. The concave meniscus lens having a concave surface facing the object side disposed on the image side of the cemented lens is an axial gradient index lens. The refractive index distribution of the axial type gradient index lens of this embodiment is attached only to the image side surface. In the third embodiment, a so-called double floating mechanism for independently moving two positive lenses on the image side during focusing is provided, and the lens on the most image side is always fixed in order to simplify the configuration of the lens frame. It was possible to obtain good performance over the entire range from the infinity point to about -1/7.

In FIG. 3, (A) is an object point at infinity, and (B) is
Is for an object point of about -1/7.

The aberrations of this embodiment at infinity and about -1/7 are as shown in FIGS. 9 and 10, respectively.

In Example 4, as shown in FIG. 4, three positive lenses having convex surfaces facing the object side, a meniscus lens having convex surfaces facing the object side, and a concave surface facing the image side are arranged in order from the object side. A concave meniscus lens, a diaphragm, a cemented lens in which a biconcave lens and a biconvex lens are cemented together, a concave meniscus lens with a concave surface facing the object side, and two positive lenses with a convex surface facing the image side. The concave meniscus lens having a concave surface facing the object side disposed on the image side of the cemented lens is an axial gradient index lens. The refractive index distribution of this axial type gradient index lens is attached only to the image side surface. In the fourth embodiment, a floating mechanism for independently moving one positive lens closest to the image side during focusing is provided, and the lens closest to the image side is fixed at all times in order to simplify the configuration of the lens barrel. Good performance can be obtained over the entire range of points from infinity to about -1/7.

In FIG. 4, (A) is an object point at infinity, and (B) is
Is for an object point of about -1/7.

The aberrations of the object point at infinity and about -1/7 in Example 4 are as shown in FIGS. 11 and 12, respectively.

[0055]

According to the present invention, the photographing field angle is about 29 °.
With an F number of about 1.4, in addition to correcting sagittal coma and spherical aberration, the curvature of the image surface is suppressed, astigmatism is corrected well, and the defocus area before and after the in-focus position is improved. It is possible to obtain a high-performance large-medium telephoto lens with good blurring performance.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is a sectional view of a fourth embodiment of the present invention.

FIG. 5 is an aberration curve diagram for an object point at infinity according to the first embodiment of the present invention.

FIG. 6 is an aberration curve diagram for an object point of about −1/7 times that of Example 1 of the present invention.

FIG. 7 is an aberration curve diagram for an object point at infinity according to Example 2 of the present invention.

FIG. 8 is an aberration curve diagram for an object point of about −1/7 times that of Example 2 of the present invention.

FIG. 9 is an aberration curve diagram for an object point at infinity according to Example 3 of the present invention.

FIG. 10 is an aberration curve diagram for an object point of about −1/7 times that of Example 3 of the present invention.

FIG. 11 is an aberration curve diagram for an object point at infinity according to Example 4 of the present invention.

FIG. 12 is an aberration curve diagram for an object point of about −1/7 times that of Example 4 of the present invention.

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[Procedure amendment]

[Submission date] January 20, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction content]

[0002]

2. Description of the Related Art In recent years, it has become mainstream to mount variable magnification lenses that can easily change the angle of view in both single-lens reflex cameras and lens shutter cameras. However, it is difficult for a variable power lens to be a bright lens system with a small F number, and aberration correction cannot be said to be sufficient as compared with a single focus lens. Therefore, the single-focus lens has a large aperture and high image quality in accordance with the voice of the user who demands brightness and image quality.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction content]

The shooting angle of view is about 29 ° and the F number is 1.4.
As a conventional example of a bright medium telephoto lens of the order of magnitude, Japanese Patent Laid-Open No.
There are lens systems described in JP-A No. 9-48723, JP-A No. 62-244010, and JP-A No. 63-205625 .

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

In the lens system described in Japanese Patent Laid-Open No. 1-302311, there is an example in which an aspherical surface is used and the photographing field angle is 29 ° and the F-number is about 1.2.
It is shown.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

However, in such a lens system, even the smallest lens has an effective diameter of about 40 mm, and the cost of the aspherical surface is high. The applicant has proposed the lens system disclosed in Japanese Patent Laid-Open No. 2-50116 . This lens system is a system in which spherical aberration and sagittal coma aberration are simultaneously corrected by using a gradient index lens having a gradient index distribution in the optical axis direction, which is easier to increase the aperture than an aspherical surface. However, in this conventional example, although spherical aberration and sagittal coma aberration are well corrected, the curvature of the image plane is relatively large.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

However, when trying to use a low dispersion glass having a larger Abbe number, it is unavoidable to use a glass having a low refractive index. If three positive lenses are used in the front group, it will be negative
Is effective in suppressing the spherical aberration of the
Abbe number to minimize chromatic aberration caused by
By using glass with a large
The bar sum becomes a large positive value, and the image plane falls to the object side and is preferable.
Not good.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction content]

Further, in the lens system of the present invention, it is preferable to satisfy the following conditions in order to satisfactorily correct aberrations. (2) f · φ · R · N ₁ <0 (3) 1.55 <(n ₁ + n ₂ + n ₃ ) / 3 (4) 0.5 <f _1-3 /f<0.9 (5) 0.5 <f _R /f<0.9 where f is the combined focal length of the entire system, φ is the power of the surface of the axial type gradient index lens on the side having the refractive index distribution, and R is the above The radius of curvature of the surface, N ₁ is the first-order coefficient of the refractive index distribution formula, and n ₁ , n ₂ , and n ₃ are the refraction of the positive lens of the first lens, the second lens, and the third lens in order from the object side of the front group. , F _1-3 is the combined focal length of the first lens, the second lens, and the third lens, and f _R is the combination of the rear lens group at the object point at infinity.
The focal length.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0034

[Correction method] Change

[Correction content]

[0034] the lower limit of the condition (6), a negative lens spherical aberration power is too weak for the Petzval sum, the axial chromatic aberration is corrected insufficiently, spherical aberration conversely exceeds the upper limit, Petzval In addition , the axial and chromatic aberrations are overcorrected, which is not preferable.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction content]

[0038] R ₀ / R ₁ = 0.461, f · φ · R · N ₁ = -0.4
72, (n ₁ + n ₂ + n ₃ ) /3=1.61 f
_{1-3 /f=0.746,f R /f=0.763,f n}
/F=-1.559 Example 3 f = 1 mm, F / 1.44, 2ω = 28.8 ° (floating) (Refractive index distribution type lens) 0.01 on the image side from the object side surface
A homogeneous glass with a constant refractive index of N ₀ up to 58 mm. 0.
The refractive index distribution formula is followed with 0158 mm as a reference (x = 0).

[Procedure Amendment 9]

[Document name to be amended] Statement

[Item name to be corrected] 0040

[Correction method] Change

[Correction content]

[0040] _{R 0 / R 1 = 0.479,} f · φ · R · N 1 = -0.4
84, (n ₁ + n ₂ + n ₃ ) /3=1.61 f
_{1-3 /f=0.755, f R /f=0.775, f} n
/F=−1.513 where r ₁ , r ₂ , ... Are the radii of curvature of the lens surfaces, and d ₁ ,
d ₂ , ... Is the thickness of each lens and the lens interval, n ₁ , n
₂ , ... Is the refractive index of each lens, and ν ₁ , ν ₂ , ... Is the Abbe number of each lens.

Claims (1)

[Claims] 1. A front lens group on the object side and a rear lens group on the image side with a diaphragm interposed therebetween. The front lens group is composed of at least three positive lenses each having a strong convex surface directed toward the object side from the object side, and a diaphragm. At least one negative lens having a strong concave surface facing the image side disposed in close proximity is included, and the front group as a whole is composed of five or more lenses. The rear group has a biconcave lens and a biconvex lens in order from the object side. A large-aperture medium-distance telephoto lens that includes a cemented lens, a negative lens with a strong concave surface facing the object side, and at least two positive lenses, and uses at least one gradient index lens in the rear group.