JPS63265211A - Variable power photographing lens of long back focus - Google Patents

Abstract

PURPOSE:To obtain a variable power photographing lens of a long back focus with which aberrations are well corrected by specifying the number of lens elements to about 7 pieces and specifying the respective lenses. CONSTITUTION:A fixed stop is disposed between both lens groups. The 1st group, successively from an object side, has a negative meniscus lens-like 1st lens having the face of a strong curvature on an image plane side, a negative 2nd lens and a positive 3rd lens. The 2nd group has a positive 4th lens, a positive 5th lens having the face of a strong curvature on the object side, a negative 6th lens having the face of a strong curvature on the image side and a positive 7th lens. The conditions expressed by equation I are satisfied when the focal lengths of the 1st and 2nd groups are respectively designated as fI, fII the focal length of the 6th length is designated as f6, the focal length at the wide angle end of the entire system as fw, the radius of curvature of the i-th lens fore counted from the object side as Ri, and the refractive index of the medium of the i-th lens as Ni. The variable power photographing lens of the long back focus with which the aberrations are well corrected and which has the high optical performances is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a variable magnification photographic lens with a long back focus, and in particular a lens with an F number of 2.0 to 2 suitable for video cameras, electronic still cameras, photographic cameras, etc. The present invention relates to a variable magnification photographing lens with a long back focus, which has a large aperture ratio of .5, has a simple structure with about seven lenses, and has good optical performance.

(Prior Art) As a standard zoom lens having a standard angle of view, for example, Japanese Patent Publication No. 55-13334 and Japanese Patent Publication No. 60-46
As proposed in Publication No. 688, etc., it has two lens groups, a first group with negative refractive power and a second group with positive refractive power, in order from the object side, and magnification can be changed by moving both lens groups. The so-called 2
There is a group zoom lens.

This two-group zoom lens has a relatively short overall lens length, which makes it easy to downsize the entire lens system, and also has the advantage of easily achieving a variable power ratio of about 2 degrees. For this reason, two-group zoom lenses have traditionally been used in still photography cameras that use silver halide film.

However, a two-group zoom lens generally has a short back focus at the wide-angle end zoom position, where the second group is located closest to the image plane, and is approximately 0 if the diagonal length of the effective screen is L.
．． It was about 8L. Also, the F number is 2.8 to 3.5
There are some disadvantages such as it is relatively dark and difficult to brighten.

On the other hand, recent video cameras, electronic still cameras, and the like (hereinafter referred to as "electronic still cameras") require smaller overall devices and variable-magnification photographing lenses that are simple in construction and have good optical performance. The effective screen of a general electronic still camera is smaller than that of a single-lens reflex camera using 35mm film, and its diagonal length is approximately 8 m.
m”-11 mm.

Furthermore, electronic still cameras have various glass materials such as low-pass filters and color filters placed in front of the photosensitive surface to adjust various optical characteristics. For this reason, electronic still cameras are required to have a photographic lens with a considerably long back focus compared to the effective screen size.

Generally, in order to ensure a long back focus, it is necessary to construct a lens system so that it is an inverted telephoto type, with the front lens group having strong negative refractive power and the rear lens group having positive refractive power.

However, as the refractive power is strengthened from the front lens group with negative refractive power, more negative distortion, spherical aberration, and coma aberration occur, and if you try to correct these various aberrations well, the lens structure becomes complicated. come. Furthermore, if an attempt is made to correct various aberrations that are insufficiently corrected in the front lens group with the rear lens group having a positive refractive power, the lens configuration of the rear lens group will become complicated, as in the case of the front lens group. In particular, when attempting to increase the aperture ratio of a lens system, this tendency becomes more pronounced, and the entire lens system tends to become larger.

(Problems to be Solved by the Invention) The present invention has a total number of lenses of about 7, a standard shooting angle of view, a relatively large aperture ratio, and a back focus of about 1.3 degrees of the diagonal length of the effective screen. Moreover, it is an object of the present invention to provide a variable magnification photographing lens with a long back focus, which is particularly suitable for electrophotographic cameras, and which performs aberration correction well over the entire screen.

(Means for solving the problem) From the object side, the first group has a negative refractive power and the second group has a positive refractive power.
When changing the distance between the two lens groups to change the magnification, a fixed diaphragm is placed between the two lens groups, and the first group is arranged in order from the object side to the image plane. It has three lenses: a negative meniscus first lens with a strong curvature surface on the side, a negative second lens, and a positive third lens, and the second group has a positive fourth lens and an object side. It has four lenses: a positive fifth lens having a surface with a strong curvature on the side, a negative sixth lens having a surface with a strong curvature on the image plane side, and a positive seventh lens. Let the focal lengths of fI and fH be respectively
1 Set the focal length of the sixth lens to f6. When fw is the focal length of the entire system at the wide-angle end, Ri is the radius of curvature of the i-th lens surface counting from the object side, and Ni is the refractive index of the medium of the i-th lens, 2, 1 < | fI /fw l <2.6 - (1)
0.57<|f6/fllll<0.65...(2
)2,, 5 < R8/R10 <10...
・(3)0.95<R10/fw<1.4
-(4) 1.7 < n4. n7
・-(6) is to be satisfied.

Note that a surface with a strong curvature on the image plane side means that the surface has a strong curvature compared to the curvature on the object side. The same holds true for surfaces with strong curvature on the object side.

(Example) Figures 1 to 3 are cross-sectional views of lenses of numerical examples 1 to 3 of the present invention. In the figure, numeral 1 indicates the first group with negative refractive power, and ▪ indicates the second group with positive refractive power. In this embodiment, by specifying each lens as described above in a lens configuration of 7 elements in 7 groups, a variable magnification photographing lens with a long back focus and excellent aberration correction is achieved.

In particular, this embodiment is composed of two lens groups, a first group with a negative refractive power and a second group with a positive refractive power, with an aperture in between, and the distance between both lens groups is By changing the magnification by changing the magnification, the entire lens system is configured to be of a retrofocus type at all times during the magnification change, thereby increasing the back focus.

In the two-group zoom lens according to this embodiment, since the height of incidence of the axial ray into the first group is low, the amount of aberrations generated is relatively small. For this reason, as mentioned above, the first group is made of three lenses with a predetermined shape.
It consists of a group of three lenses. On the other hand, since the incident height of the axial ray is high in the second group, it is composed of four lenses in four groups having a predetermined shape, and aberrations are corrected well.

Further, by configuring the first group with three elements in three groups as described above, the amount of axial and off-axis aberrations generated is reduced while increasing the back focus.

Furthermore, by configuring the second lens group with four lenses in four groups as described above, spherical aberration, coma aberration, etc. are favorably corrected.

A fixed diaphragm is placed between the first and second groups to maintain an appropriate balance between the lens outer diameters of the first and second groups before and after the diaphragm, thereby reducing the overall size of the variable magnification photographic lens. ing.

Next, the technical meaning of each of the above conditional expressions will be explained.

Conditional expression (1) regulates the refractive power of the first group,
In particular, this is intended to shorten the overall lens length on the telephoto side, reduce the amount of movement of the first group during zooming, and further lengthen the back focus as much as possible at the wide-angle end.

If the negative refractive power of the first lens group becomes strong beyond the lower limit of conditional expression (1), the divergence of the first lens group becomes too strong, which is not good, especially since the distortion aberration increases. Furthermore, if the negative refractive power of the first group exceeds the upper limit and becomes weaker, it becomes difficult to secure a predetermined amount of back focus, and the amount of movement of the first group increases during zooming. In this case, the diameter of the front lens determined by the off-axis oblique light beam on the wide-angle side increases, which is not good because the entire lens system becomes larger. In particular, in the present invention, the conditional expression (1
) by fixing the aperture between the first and second groups, the diameters of the front and rear lenses can be maintained in a well-balanced manner, making it possible to efficiently downsize the entire lens system. Is going.

Conditional expression (2) relates to the refractive power of the sixth lens, which is the only negative lens in the second group. If the lower limit is exceeded and the negative refractive power becomes too strong, it will be advantageous for Petzval sum correction, but the curvature of the lens surface on the image side will become stronger, resulting in higher-order field curvature and astigmatism. These aberrations occur frequently, and it becomes difficult to satisfactorily correct these various aberrations. Furthermore, when the upper limit is exceeded and the negative refractive power becomes weaker, it becomes difficult to secure a predetermined amount of back focus.

Conditional expression (3) concerns the ratio of the radius of curvature of the lens surface on the object side of the fourth lens to the radius of curvature of the lens surface on the object side of the fifth lens. If the fluctuation of aberration increases and exceeds the upper limit, the second
Since the front principal point of the lens group moves too far toward the image plane, the first and second lens groups may come too close to each other on the telephoto side during zooming, which is not good.

Conditional expression (4) limits the refractive power of the object-side lens surface of the fifth lens based on conditional expression (3). Fluctuations in point aberration become large. Furthermore, if the upper limit is exceeded and the refractive power becomes too weak, spherical aberration becomes overcorrected over the entire zoom range.

Conditional expression (5) concerns the shape of the negative air lens formed by the fifth and sixth lenses, and if the lower limit is exceeded, higher-order spherical aberrations will be overcorrected and the back focus will become shorter. not good. On the other hand, if the upper limit is exceeded, higher-order spherical aberrations will be insufficiently corrected, and it will become difficult to correct them satisfactorily.

Conditional expression (6) relates to the refractive index of the medium of the positive fourth lens and the positive seventh lens, and by using a medium with a high refractive index of 1.7 or more for both, the positive Petzval sum of the second group can be obtained. At the same time, the curvature of the lens surface is made gentler, reducing the amount of various aberrations that occur when increasing the aperture ratio, and achieving high optical performance.

In this example, in order to further reduce aberration fluctuations during zooming and obtain good optical performance over the entire zooming range, the first lens is a meniscus lens with a convex surface facing the object side, and the second lens and sixth lens are The lens is a biconcave lens with both concave lens surfaces, the third lens is a meniscus lens with a convex surface facing the object side, the fourth and seventh lenses are biconvex lenses with both lens surfaces convex, and the fifth lens is a convex surface facing the object side. It is preferable to use a meniscus lens or a biconvex lens in which both lens surfaces are convex.

Focusing in this embodiment is achieved by moving the first group because it reduces aberration fluctuations, but it may also be achieved by moving only the second group or the entire lens system.

Furthermore, in this embodiment, when photographing, instead of using the entire zoom range, only the zoom positions at both the wide-angle end and the telephoto end may be used. This case is preferable because the movement state of each lens group becomes simple and the lens barrel structure can be simplified.

Next, numerical examples of the present invention will be shown. In numerical examples R
i is the radius of curvature of the i-th lens surface in order from the object side, D
i is the i-th lens thickness and air distance from the object side, Ni
and νi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side.

j:,,+ Numerical Example I F-9,18FNo-2,02ω-47° ~ 28.6° ~ 15.138 ~ 2.5 R1= 14.97 D I-1,40N I=1
．． 7] 300v l-53,8R2-9,48D
2-2.19 R3=-125,39D 3-1.20 N 2-1
．． 6393Q v 2-44.9R4-10,08D
4-1.58 R5-11,8605-2,00N 3-1.805+
8 ν 3-25.4R6-28,17D 6-Variable R7-Aperture Di-Variable R8-30,1308-2,10N 4-1.8040
0 ν 4-46.6R9--42,05D 9-0
．． 15 R10-11, 80D10-2, 90N 5-1.7]
300 ν 5=53.8R11-173,25DI
+-0,37RI2--23.90 Di2-4.0
7 86-1.84666 ν 6=23.9R13
-10,411113-0,74RI4- 44.95
Di4-2.00 N 7-1.83481
ν 7=42.7R]5--13.12 Di5-variable R16-co 016-7.38 N 8=
1.51633 v 8=64. lR17= (
1) l f I /fw l = 2.21 f6 /
f II l = 0.61 Numerical Example 2 F = 8.71 FNo-1,82ω-49,4° ~
30.2° ~ 14.79 ~ 2.2 RI-21,86D I-1,40N l-1,713
00ν 1-53.8R2-7,65D 2-2.61 R3 proverb -45,25D 3- 1.20 N
2-1.64769 ν 2-33.884-
19.33 D 4-0.29R5= 13.2
0 05-2.60 83=1.80518
v 3-25.4R6-88, 43D 6-Variable R7/Aperture D7-Variable R8= 100.25 D 8- 2.50
N 4=1.88300 v 4-40
．． 8R9 Yoi-42,7009-0,15 R10= 10.16 D10-2,9085=1
．． 77250 Z/ 51149.6R11-869
,35Dll-0,34RI2=-42,17012
-4,35N 6-1.84666 ν 6-23.
9RI3- 8.32 D13-0.76R14 Proverbs
17.78 014 Tomi 2.0ON7~1.83
481 ν 7-42.7815--19.69
D15-variable R16-ω DI6-7.38
N 8-1.51633 C8-64. lR17-■ (f-engine/f-arm l=2.24 1f6/fllll= 0.60 Numerical Example 3 F-9,18FNo-22ω-47'~28.6'~
15.67 ~2.5 Rl-27,79D I-1,40N l-1,713
00ν 1-53.8R2-11,00D 2-2.0
2 R3-545, 29D 3-1.20 N 2-1.
65018 ν 2-39.4R4-10,47D
4 Tortoise 1.24 R5-12, 2905 Dew 2.00 N 3-1.80
518 Shi3-25.4R6-44,20D 6=Variable R7-Aperture D7・Variable R8 Maro 39.97 D 8 Bird 2.10
N 4-1.80400 ν 4-46.6
89--64.11 09-0.15 R10 8.95 D10 2.9ON5■1.7
1300 Shi5 Snow 53.8R11-103.61
Dll■0.34R12--51,80012-3,4
4N 6-1.84666 Jiro 23.9R13-
7,60013-0,93 RI4- 17.40 D14■ 2.0ON
7 Proverbs 1.83481 ν 7-42.7R15--
19,75D15-variable R16=oo 016-7.38 N 8
m11.51633 v 8-64. lR17-■ 1fI/f stomach 1=2.2 1 f6 /f■l=0.57 (Effects of the invention) As described above, according to the present invention, a back that has high optical performance with good aberration correction is obtained. A variable magnification photographic lens with a long focus can be achieved.

In addition, when applied to an inexpensive electronic still camera, if the lens is constructed as a so-called bifocal switching lens that uses both the wide-angle and telephoto ends of its variable magnification range, the movement of the first and second groups can be done in a straight line. This can be done by using a guide groove, and the structure can be greatly simplified.

In addition, since the aperture is fixed when changing magnification, it is also possible to mechanically replace the aperture, which provides sufficient exposure accuracy especially for electronic still cameras, where the small aperture diameter is extremely small in proportion to the effective screen. It has the following features:

[Brief explanation of the drawing]

FIGS. 1 to 3 are cross-sectional views of lenses of numerical examples 1 to 3 of the present invention, and FIGS. 4 to 6 are diagrams of various aberrations for an object at infinity in numerical examples 1 to 3 of the present invention. In the aberration diagrams, (A) is an aberration diagram at the wide-angle end, and (B) is an aberration diagram at the telephoto end. In the figure, SEM is the first group, ■ is the second group, ΔM is the meridional image plane, ΔS is the sagittal image plane, d is the d-line, and g is the g-line. Patent Applicant: Canon Co., Ltd. 1 Figure No. 21\] Figure 3 Chrysanthemum 4 Figure (A) 1°゛'-i, 7 0-0157. ．． 4□Ko 20-゛Heikyoku, 7 Kansha Misaki No. 5 [21 (A') -Q% rYa 9 L -o7. ,f',,HRt
”-”'4- [! I 9%C%) Figure 6 (A) (B)

Claims (1)

[Claims] (1) It has two lens groups, a first group with negative refractive power and a second group with positive refractive power, in order from the object side, and magnification can be changed by changing the distance between both lens groups. When performing this, a fixed diaphragm is placed between both lens groups, and in order from the object side, the first group includes a negative meniscus lens-shaped first lens having a surface with a strong curvature on the image plane side, and a negative lens group. The second group includes a positive fourth lens, a positive fifth lens having a surface with a strong curvature on the object side, and a surface with a strong curvature on the image plane side. It has four lenses, a negative sixth lens and a positive seventh lens, and the focal length of the first group and the second group is f.
I, fII, the focal length of the sixth lens is f6, the focal length of the entire system at the wide-angle end is fw, the radius of curvature of the i-th lens surface counting from the object side is Ri, the medium of the i-th lens When the refractive index of Ni is 2.1<|f I /fw|<2.6 0.57<|f6/fII|<0.65 2.5<R8/R10<10 0.95<R10/ fw<1.4 -2<(R12+R11/R12-R11)<01.7
<n4, n7 A variable magnification photographing lens with a long back focus, which satisfies the following conditions. (2) The moving state of the first group and the second group during zooming is set to be used only at zoom positions at both the wide-angle end and the telephoto end. A variable magnification lens with a long back focus.