JPH0248621A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To attain a satisfactory aberration correction, while miniaturizing the whole lens system by setting refracting power of four lens groups and moving conditions of a second group and a fourth group at the time of variable power and lens shapes of each lens group, etc., and also, executing focusing by moving a fourth group. CONSTITUTION:The zoom lens has four lens groups of a first group I of positive refracting power, a second group II of negative refracting power, a third group III of positive refracting power and a fourth group IV of positive refracting power in order from an object side. A third group III consists of a lens III-1 of positive refracting power and an opening diaphragm SP and a lens III-2 of positive or negative refracting power, and at the time of varying power from a wide angle end to a telephone end, a second group II is moved to an image surface side, and also, an image surface fluctuation which follows up variable power is corrected by moving a fourth group IV. Also, focusing is executed by moving a fourth group IV, and also, a lens shape, etc. of each lens group are set to satisfy four conditional expressions, and while miniaturizing the whole lens system, variable power is executed and by using a fourth group IV, an aberration fluctuation at the time of focusing is corrected satisfactorily.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rear focus type zoom lens, in particular a lens with a variable power ratio of 6 and an F number of 1.4, which is used in photographic cameras, video cameras, broadcasting cameras, etc. The present invention relates to a rear focus type zoom lens suitable for a zoom lens having a relatively large aperture ratio and a high zoom ratio.

(Prior Art) Various types of zoom lenses for photographic cameras, video cameras, etc. have been proposed that adopt the so-called rear focus system, in which focusing is performed by moving lens groups other than the first lens group on the object side. ing.

In general, the rear focus type uses a relatively small and lightweight lens group that moves, so it has the advantage of requiring less driving force for the lens group and allowing quick focusing.

For example, in order from the object side, there is a first group with positive refractive power, a second group with negative refractive power for zooming, a third group with negative refractive power to correct image plane fluctuations due to zooming, and then In a so-called four-group zoom lens consisting of four lens groups with a fourth group having a refractive power of , a rear focus type zoom lens has been proposed in which focusing is performed by moving the third group.

Furthermore, Japanese Patent Laid-Open No. 58-136012 proposes a rear focus type zoom lens that is composed of three or more lens groups including a variable magnification section, and focuses by moving some of the lens groups. There is.

However, with these zoom lenses, for example, even for the same object distance, the amount of extension of the focus lens group differs depending on the zoom position, that is, the difference in focal length, and the amount of extension is quadratic or discontinuous. may change.

Furthermore, in such a zoom lens, if the variable power ratio is increased, a large amount of space must be reserved for movement of the focus lens group on the wide-angle side, resulting in an increase in the size of the lens system. In addition, if a rear focus system similar to that described above is adopted, the amount of extension of the focus lens group for the same object distance may be significantly larger at the telephoto end than at the wide-angle end.

In addition, in such a zoom lens, the amount of movement of the image plane relative to the amount of movement of the focus lens group, that is, the sensitivity increases at the telephoto end, and when this value increases to a certain extent, it becomes mechanically difficult to control the movement of the focus lens group. It's coming.

On the other hand, if the sensitivity at the telephoto end is set to a controllable value, the sensitivity at the wide-angle end becomes too small, requiring a lot of space for moving the focus lens group, and the lens system increases. There were some problems, such as the fact that the

(Problems to be Solved by the Invention) The present invention adopts a rear focus method, and when achieving a large aperture ratio and high variable power, it prevents the overall size of the lens system from increasing, while allowing the lens to move from the wide-angle end to the telephoto end. It is an object of the present invention to provide a rear focus type zoom lens having a simple structure, which has good optical performance over the entire zooming range up to , and which facilitates mechanical control of a focusing lens group.

(Means for solving the problem) From the object side, the first group has positive refractive power, and the second group has negative refractive power.
group, a third group with positive refractive power, and a fourth group with positive refractive power, the third group having a positive 31st lens,
It has an aperture stop, a 32nd lens with both lens surfaces convex, a 33rd lens with a negative meniscus shape with its convex surface facing the image side, and a positive 34th lens, with the fourth group facing the object side. A 42nd lens whose 41st lens surface is a convex negative meniscus, and a 43rd lens whose both lens surfaces are convex are moved. The image plane fluctuation caused by the magnification is corrected by moving the fourth group, and the fourth group is moved to perform focusing, and the focal length of the i-th group is set to fi, and the j-th lens surface of the i-th group is 1.2 <|R2, 1/f2|<27 - (1) 0 where the radius of curvature Ri, j and the Abbe numbers of the materials of the positive and negative lenses in the fourth group are νP and νN, respectively.
．． 4 <|R3, 5/f3|<0.52...(2)
0.7|<l R4,27f4|<0.93-(3)3
2 Ku νP −νN ・
......-(4) is to be satisfied.

(Example) FIG. 1 is a schematic diagram of an example showing the paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention.

In the figure, ■ is the first group with positive refractive power, and ■ is the second group with negative refractive power.
The group ■ is the third group with positive refractive power, and ■ is the fourth group with positive refractive power. The third group m is the 3-1st group 111-1 with positive refractive power.
, an aperture stop SP, and a 3-2nd group 111-2 having positive or negative refractive power.

When changing the magnification from the wide-angle end to the telephoto end, the second group is moved toward the image plane side, and the fourth group is moved to correct image plane fluctuations caused by the change in magnification.

Additionally, a rear focusing system is adopted in which focusing is performed by moving the fourth group on the optical axis. The solid line curve 4a and the dotted line curve 4b of the fourth group shown in the figure represent image plane fluctuations when changing the magnification from the wide-angle end to the telephoto end when focusing on an object at infinity and a close object, respectively. It shows the movement trajectory for correction.

Note that the first group and the third group are fixed during zooming and focusing.

In this embodiment, the fourth group is moved to correct image plane fluctuations due to zooming, and the fourth group is also moved to perform focusing. In particular, as shown by curves 4a and 4b in the figure, when changing the magnification from the wide-angle end to the telephoto end, the lens is moved so as to have a convex locus toward the object side. This makes effective use of the space between the third and fourth groups and effectively shortens the overall length of the lens.

In this embodiment, for example, when focusing from an object at infinity to a close object at the telephoto end, straight line 4C in the figure is used.
This is done by rolling the fourth group forward as shown in the figure.

In this example, compared to a conventional 4-group zoom lens in which focusing is performed by extending the first group, the rear focusing method described above is used to effectively prevent an increase in the effective diameter of the first group. are doing.

Placing the aperture diaphragm between the front and rear groups of the third group reduces aberration fluctuations due to the movable lens group, and shortens the distance between the lens groups in front of the aperture diaphragm to reduce the diameter of the front lens. This can be easily achieved.

Further, by arranging the 3-1st lens group with positive refractive power in front of the aperture stop, the diameter of the aperture stop is reduced. By setting the lens shape etc. of each lens group in the above-mentioned lens configuration as shown in conditional expressions (1) to (4), the entire lens system can be miniaturized while changing magnification and focusing using the fourth group. The aberration fluctuations are well corrected.

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

Conditional expression (+) relates to the radius of curvature of the lens surface closest to the object side of the second group, and is expressed by conditional expression 2b, which satisfactorily corrects fluctuations in distortion mainly due to zooming. If the lower limit of the conditional expression (2) is exceeded, fluctuations in distortion due to zooming will increase; on the other hand, if the lower limit is exceeded, fluctuations in coma aberration from the center to the periphery of the screen will increase, which is not good.

Conditional expression (2) relates to the radius of curvature of the object-side negative refractive power lens surface of the negative 33rd lens of the third group. If the negative refractive power exceeds the lower limit and becomes too strong, spherical aberration will be over-corrected over the entire magnification range, and if the upper limit is exceeded and the negative refractive power becomes too weak, the spherical aberration will be over-corrected over the entire magnification range. As the aberration becomes insufficient, coma aberration increases from the wide-angle end to the intermediate zoom range, and it becomes difficult to properly correct this aberration.

Conditional expression (3) concerns the radius of curvature of the lens surface with negative refractive power on the image plane side of the negative meniscus-like fourth lens of the fourth group, and if the negative refractive power becomes too strong beyond the lower limit, it becomes positive. If the negative refractive power exceeds the upper limit and the negative refractive power becomes too weak, the negative astigmatism will increase and it will become difficult to properly correct it.

Conditional expression (4) is used to appropriately set the Atsube number of the materials of the positive and negative lenses in the fourth group, and to properly correct lateral chromatic aberration over the entire zoom range. This is not good because the fluctuations in lateral chromatic aberration become large.

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.

Further, Table 1 shows the relationship with each conditional expression in each numerical example. Note that R26 and R27 are glass materials such as a face plate.

Numerical Example 1 f-1,0~5.573 RI-10,129 R2-3,504 83--7,711 04-2,766 R5-10,898 R6--15,961 R7 Liao 1. +24 1+ 8- -1.538 8 9- 1.538 R10 -836,362 811-3,614 1112-22,021 R13-Aperture stop R14-4,298 R15--2,172 R16--1, 437 R]7- -4.832 RI8- 1.443 1119- 1.724 R20-3,443 FNo-1: 1.45-1.95 2ω=49.13'
~9.318'D I-0,1506N I-
1,805+8 ν l-25,402-0,613
182-1,5+633 ν 2 items 64. ID 3-
0.0161 D 4- 0.3980 N 3-1.6584
4 ν 3-50.9D5・Variable DB-0,086084-1,83400ν4-37
．． 2D 7-0.3046 D 8-0.0860 N 5-1.51633 5-
64. ID 9-0.2796 N 6-1.8466
6shi6-23.9010- Variable DI! -0,2366N 7-1.70+54 ν
7-41.2012- 0.16+3 DI3-0.2151 DI4= 0.4302 N 8-1.57099 v
8-50.8015-0. :]372 016-0.1076 N 9-1.80518ν9-
25.4017- 0.0151 018-0.3227 N10-1.60311shi10
-60.7019- Variable D20-0.0860 N11-1.84866 C11
-23.9R21 meeting 2B- 1,713 4,183 -2,391 1,923 -9,909 Numerical example 2 f-1,0~5.563 8 I-9,777 82-3,521 R3 Maro -6,819 R4-2,718 R5-8,507 86--18,368 〇24- D25~ 0.1233 0.3442 N+2-1.48749 C12-
70.20.0161 0.3872 N13-1.60311 C13-
60.70.5378 0.6453 N14-1.51633 C14-
64.1FNo=I:1.45~1.95 2ω-49
,12° ~9.393°D I-0,1505N
I-1,80518ν l-25,402-0,81
29N 2-1.51633 ν 2-64. ID
3- 0.0161 D 4- 0.3763 N 3-1.6584
4 ν 3C 50.9D5- Variable D 6= 0.0860 N 4-1.834
00 v 4-37.2R7-1,176 88--1゜536 R9-Summer, 536 fllO--11,910 R11-4,+33 R12-119,866 RI3-Aperture stop RI4- 8.43O R+5- - 2. +27 RI6～～1.501 R17・-6,:]l0 R18-1,521 RI9- 1.896 R20-3,908 021-1,717 R22-13,508 1123--2,828 R24-1, 998 R25@-5,565 026I-■ R27@ ■ D 7- 0.3011 D 8= 0.0860 N 5m1.603
11 v 5-60.70 9- 0.2796
N 6-1.84666 υ 6-23.90I
G-Variable DI! = 0.2151 N 7-1.72342 v
7-38.0012-0.1075 DI3-0.2151 014-0.3871 N 8-1.72000shi8-
50.2015-0.2492 DI6- 0.1075 N 9-1.84666
ν 9 Proverbs 23.90+7-0.0161 018-0.247:f N10-1.65844shi1
0-50.9019-variable 020-0.0860 N+1-1.84666 11
-23.9021- 0.1559 022-0.3011 N+2-1.51633shi12
-64.1023- 0.0161 D24-0.4409 N+3-1.69680ν13
-55.5025-0.5376 026-0.6452814-1.51633υ14-
64.1 Numerical Example 3 f-1,0~5.583 RI-9,404 R2-3,484 R3--8,340 R4-2,766 R5-10,898 86--27,741 R7- 1,096 88 Maro -1,523 R901,523 8IO--28,748 811-3,494 R12-12,424 813-Aperture stop FNo.I: 1.45-1.95 2ω-49,13°
~9.360°D I-0,15068+-1,8
05+8 ν 1-25.40 2- 0.6131
8 2-1.5+633 ν 2-64.103 building 0.0161 D 4- 0.3980 N 3-1.6584
4 ν 3-50.9D5- Variable D 6-0.0860 N 4-1.83400ν4-
37.20 7- 0.3046 D 8- 0.0860 N 5 irises 1.5+63
3 υ 5-64. Io 9-0.2791i N
6-1.84666 Sea 6-23.9010- Variable DI+-0,2366N 7-1.62004 Sea 3
6.3012-0.1813 013-0.2151 II4- R15- l6- RI7~ II8 Trout R2O/R21/R26 Midori 4.301 -2.172 1.470 -5.6:10 1.451 +, 709 3.293 +, 726 4.183 -2.428 1.676 -63.708 014 016 017 D2〇- D23 0.4302 N 8-1.81484 v
8-51.20.3110 0.1076 N 9-1.805+8 ν 9
-25.40.0161 0.3227 Nl0-1.60311 C10-
60.7 variable 0.0860 N11-1.84866 sill-23,
90,1:100 0.3442 Nl2=1.498:II C12-6
5.00.0+61 0.3872 N+3-1.60111 C13-60
．． 70.5378 0.8453 N+4-1.51833ν14-64.
Table 1-1 (Effects of the Invention) According to the present invention, as described above, the refractive powers of the four lens groups, the movement conditions of the second and fourth groups during zooming, and the lens shape of each lens group are set. At the same time, by adopting a lens configuration in which the fourth group is moved during focusing, the entire lens system can be miniaturized while achieving a zoom ratio of about 6 and good aberration correction over the entire zoom range. It is possible to achieve a rear focus type zoom lens with an F number of 1.4 and a large aperture ratio, which has high optical performance with little variation in aberrations.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment showing the paraxial refractive power arrangement of the present invention, FIG. 2 is a cross-sectional view of a lens of Numerical Example 1 of the present invention, and FIGS. 3 to 5 are numerical implementations of the present invention. FIG. 3 is a diagram of various aberrations of Examples 1 to 3. In the aberration diagram, (A) is at the wide-angle end, (B) is at the middle,
(C) is an aberration diagram at the telephoto end zoom position. 1st. In Figure 2, I, II, III, IV
are first in order. Second. Third. 4th group, d is d-line, g is g-line, ΔM is meridional image plane, ΔS is sagittal image plane, S
P is an aperture stop.

Claims (1)

[Claims] (1) It has four lens groups, in order from the object side: the first group with positive refractive power, the second group with negative refractive power, the third group with positive refractive power, and the fourth group with positive refractive power. However, the third group includes a positive 31st lens, an aperture diaphragm, a 32nd lens whose both lens surfaces are convex, a negative meniscus-shaped 33rd lens with a convex surface facing the image plane side,
The fourth group includes a negative meniscus-shaped 41st lens with a convex surface facing the object side, a 42nd lens with both lens surfaces convex, and a 42nd lens with both lens surfaces convex. It has a 43rd lens, moves the second group to change the magnification, moves the fourth group to correct image plane fluctuations caused by changing the magnification, and moves the fourth group to focus. , the focal length of the i-th group is fi, the radius of curvature of the j-th lens surface of the i-th group is Ri,j, and the Abbe numbers of the materials of the positive and negative lenses in the fourth group are νP, respectively. , νN 1.2<|R2, 1/f2|<27 0.4<|R3, 5/f3|<0.52 0.71<|R4, 2/f4|<0.93 32< A rear focus type zoom lens that satisfies the following condition: νP−νN.