JPH0248620A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To miniaturize the whole lens system, and also, to attain a satisfactory aberration correction extending over the whole variable power range by setting moving conditions of a second group and a fourth groove in a refractive index and variable power of four lens groups. CONSTITUTION:The zoom lens has four lens groups of a first group I of positive refractive power, a second group II of negative refracting power, a third group III of positive refracting power and a south group IV of positive refracting power in order from an object side. A third group III has a III-1 group of positive refracting power and an opening diaphragm SP and a III-2 group of positive or negative refracting power. In such a state, at the time of varying power from a wide angle end to a telephoto 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. Moreover, focusing is executed by moving a fourth group IV, and also, while miniaturizing the whole lens system by setting refracting power, etc. of each lens group to satisfying five conditional expressions, an aberration fluctuation at the time of focusing is corrected satisfactorily by using a fourth group.

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 in which the variable power section is composed of three or more lens groups, and focusing is performed 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.

In addition, when the variable power ratio of such a zoom lens 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 focusing method, and when achieving a large aperture ratio and high variable power, it prevents the overall size of the lens system from increasing, and allows 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.
It has four lens groups: a third group with positive refractive power, and a fourth group with positive refractive power, and the third group has a third lens group with positive refractive power.
It has a 1st group, an aperture stop, and a 3rd-2nd group with positive or negative refractive power.The 2nd group is moved to change the magnification, and the image plane fluctuation caused by the change in magnification is corrected by the 4th group. The focal length of the first group is fi, the focal length of the 3-1st group and the entire system at the wide-angle end is f3-1, respectively. fw, the zoom ratio is 2, and the shooting magnification at the telephoto end of the second group is β2T, then 0.9<|f2/fwl<1.3 (1)
0.9<lβ2 T/ fi I<1, 4 ・
...(2) 1.8< f4/fw <2.4 -
--(3) 2.8< f3/fw <3.7
...(4) 1.6<f3-1/f3<2.6 ・
...(5) 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 l11-1 with positive refractive power.
, an aperture stop SP, and a third-second group lll-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 the first group is extended to focus, the effective diameter of the first group can be effectively increased by adopting a rear focusing method similar to the one described in the above. It is prevented.

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 subsurface lens diameter. 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 refractive power of each lens group as shown in conditional expressions (1) to (5) in the above-mentioned lens configuration, 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 (1) relates to the refractive power of the second lens group, and is intended to effectively obtain a predetermined zoom ratio while reducing aberration fluctuations accompanying zooming. If the refractive power of the second group exceeds the lower limit and becomes too strong, it will be easier to downsize the entire lens system, but the Petzval sum will increase in the negative direction, the curvature of field will increase, and aberrations will fluctuate with zooming. is getting bigger. If the upper limit is exceeded and the refractive power of the second group becomes too weak, aberration fluctuations due to zooming will be reduced, but the amount of movement of the second group to obtain the specified zoom ratio will increase, and the overall length of the lens will become longer. It's not good because it's getting worse.

Conditional expression (2) relates to the imaging magnification of the second group at the telephoto end relative to the zoom ratio. If the lower limit value is exceeded and the photographing magnification becomes too small, the amount of movement of the second group to obtain a predetermined variable magnification ratio becomes large, resulting in an increase in the total length of the lens. If the maximum magnification is exceeded and the photographic magnification becomes too large, the overall length of the lens will be shortened, but the movement trajectory of the fourth group near the telephoto end for an object at infinity will change rapidly, causing a load on the driving means such as the motor. It's not good because it gets bigger.

Conditional expression (3) relates to the positive refractive power of the fourth group, and is mainly used to satisfactorily correct aberration fluctuations during zooming and focusing. If the positive refractive power of the fourth group exceeds the lower limit and becomes too strong, spherical aberration will be insufficiently corrected, and aberration fluctuations due to zooming, especially fluctuations in lateral chromatic aberration, will increase, and it is important to properly correct this. It's getting difficult. Furthermore, if the positive refractive power of the fourth group becomes too weak by exceeding the upper limit, the amount of movement of the fourth group during zooming and focusing becomes too large, which is not good, as the overall length of the lens increases.

Conditional expression (4) relates to the positive refractive power of the third group, and as the positive refractive power of the third group exceeds the lower limit and becomes stronger, the spherical aberration at the zoom position on the wide-angle side tends to be undercorrected. In addition, in order to obtain a predetermined back focus, the positive refractive power of the fourth group must be weakened while the positive refractive power of the third group is strengthened, and as a result, the amount of movement of the fourth group during focusing increases. It's not good because it comes. Furthermore, if the upper limit is exceeded and the positive refractive power of the third group becomes too weak, the spherical aberration on the wide-angle side tends to be overcorrected, which is not good.

Conditional expression (5) concerns the positive refractive power of the 3-1st group, which corresponds to the front group in front of the aperture stop in the 3rd group, and the positive refractive power of the 3rd-1st group is strong beyond the lower limit. If it becomes too large, fluctuations in coma aberration will increase from the wide-angle end to intermediate zoom positions, making it difficult to correct this well.

Furthermore, if the upper limit is exceeded and the positive refractive power of the 3-1st group becomes too weak, the aperture diaphragm and the effective diameters of the 3-2nd and subsequent lens groups become large, which is not good.

Next, numerical examples of the present invention will be shown. R on the numerical example
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 face plate.

Numerical Example 1 f-1, Q ~ 5.573 1t I-10, +29 R2-3,504 83--7,711 84-2,766 85-10,898 86--15,961 87-1, 124 R8--1,538 R9-1,538 810-836,382 Old 1 3.814 Ri2- 22.021 R13 ~ Aperture stop R14-4,298 Ri5- -2.17Z R16--1,437 Ri7 - -4.832 18- 1.443 Old 9- 1.724 R20-3,443 FNoJ: 1.45~1.95 2ω=49.13°~
9.318゜D I= 0.1506 N I
-1,805+8 v 1=25.402-0.6
131 N 2-1.51633 ν 2-64.
1D 3 palanquins 0.0161 0 4w 0.3980 N 3-1.65844
v 3-50.9D5・Variable D 611O,0860N 4-1.83400 v
4-37.2D 7 Snow 0.3046 D 8-0.086085-1.51633ν 5-6
4. ID 9 = 0.279686-1.84666
v 6-23.9010- variable old 1 = 0.2366 N 7 proverbs 1.70 + 54
v 7-41.2D12-0.1613 013-0.2151 014-0.4302 N 8=1.57099 v
8-50.815- 0.3372 DI8= 0.1076 N 9J, 80518
v 9-25.4 DI7-0.0161 DI8= 0.3227 Nl0-1.60311
v 1O-60,7DI9- variable 020-0.0860 N+1-1.84666shi11
-23.91.713 4.183 -2.391 1.923 -9.909 Numerical Example 2 f-1,0~5.563 RI-9,777 82-3,521 R3--6,819 R4 -2,718 R5-8,507 R6--18,388 2l- 022= 0.1233 0.3442 N12-1.48749 Shi12 proverb 70.20.011i1 0.3872 Ni3-1.60311 Shi13 proverb 60 .70.5378 0, fi453 N14-1.51633 Si14
-64.1FNO-1: 1.45-1.95 2ω-4
9,12' to 9.393'D I-0,1505
N l-1,80518ν l25.4D 2=
0. [1129N 2-1.51633 v
2-64. ID 3- 0.0161 D 4- 0.3763 N 3-1.6584
4 ν 3-50.9D5- Variable D 6- 0.0860 N 4 1.8340
0 ν 4-37.2R7-1,176 R8--1,5:16 8 9- 1.536 8IO--11,910 811-4,13:l R12-119,866 Old 3-Aperture stop Ri4- 8.43O Ri5 Dew -2,127 Ri6- -1.501 R17--6,31O Ri8- 1.521 Ri9- 1.896 R20-3,908 R21-1,717 R22-13,508 R23--2 ,828 R24■ 1.998 R25--5,565 R26-o.

R27-a.

D 7- 0.3011 D 8- 0.0860 N 5-1.6031
1 ν 5-60.70 9- 0.2796 8
6-1.84666 ν 6-23.9010 variable Dll grass 0.2151 8 7-1.72342
ν 7-38.0012- 0.1075 013- 0.2151 D14- 0.3871 N 8-1.72000
ν 8-50.2015- 0.2.492 Di8- 0.1075 N 9-1.84666
ν 9-23.9017- 〇, 0161 Di8- 0.2473 NIO Mati 1.65844
SI10-50.9DI9-Variable 020=0.0860 N11-1.84666
v II=23.9021- 0.1559 022- 0.3011 Nl2-1.51631
C12-64. ID23 embroidery 0.0161 024- 0.4409 N13-1.69680
1: ]-55,5025 0.5376 02B-0,6452N14-1.51633 ν1
4-64.1 Numerical Example 3 F-1,0~5.583 RI-9,404 R2-3,484 R3--8,:140 1(4.2.766 R5-10,898 n 6- -27.741 R7-1,096 R8--1,523 R9-1,523 RIO--28,748 811・3.494 RI2- 12.424 R13・Aperture stop FNo-1: 1.45~1. 952ω-49,13°~
9.360゜D I-0,1506N +-1,8
05+8 ν]-25,402-0,6131N
2-1.5+633 ν 2-64° ID 3
- 0.0161 D 4- 0.3980 N 31.65844
v 3-50.9D5・Variable D 6-0.0860 N 41.83400 v 4
-:+7.2D 7-0.3046 D B-0,0860N 5-1.5163:lν 5
-64. ID 9-0.279686-1.84666
Sea 6-23.9010- Variable Dll-0, 236687-1, 62004 Sear 36
．． 3012-0.1613 013-0.2151 R14 Rl6 RIO R2O- Ro21- 4.301 -2.172 -1.470 -5.630 1.451 1.709 3.293 +, 726 4. 183 -2.428 1.676 -63.708 014- 0.4302 N 8-1.61484
ν 8-51.2015- 0.3110 DI6- 0.1076 N 9-1.805+8
ν 9-25.4D17- 0.0161 018~0.3227 Nl0-1.603+1
C10-60.709/Variable 020-0.0860 N11-1.84666 C11
-23.9021-0.1300 D22-IO, 3442N12-1.49831 v
12-65.0D23- 0.0161 024-0.3872 N+3-1.60311 Si1
3-60.7D25-0.5378 D26-0.6453 N14-1.51633shi14
-64.1 Table-1 (Effects of the Invention) According to the present invention, as described above, the refractive powers of the four lens groups and the movement conditions of the second and fourth groups during zooming are set, and the By adopting a lens configuration in which the four groups are moved, the entire lens system is made smaller, achieving a zoom ratio of about 6 and good aberration correction over the entire zoom range, while minimizing aberration fluctuations during focusing. It is possible to achieve a rear focus type zoom lens with an F number of 1.4 and a large aperture ratio with high optical performance.

[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 FIG. 2, I, II, III, rV are numbered 1. Second. Third. In the fourth group, d is the d-line, g is the g-line, ΔM is the meridional image plane, ΔS is the sagittal image plane, and sp is the aperture stop.

Claims (1)

[Claims] (1) In order from the object side, the first group has a positive refractive power, the second group has a negative refractive power, the third group has a positive refractive power, and the fourth group has a positive refractive power. It has four lens groups, and the third group is a 3-1st group with positive refractive power, an aperture stop, and a 3rd lens group with positive or negative refractive power.
- It has two groups, and the second group is moved to change the magnification, and the fourth group is moved to correct image plane fluctuations caused by the change in magnification, and the fourth group is moved to focus. and the i-th
When the focal length of the group is fi, the focal lengths of the 3-1st group and the entire system at the wide-angle end are f3-1 and fw, respectively, the zoom ratio is z, and the imaging magnification of the 2nd group at the telephoto end is β2T. 0.9<|f2/fw|<1.3 0.9<|β2T/√(z)|<1.4 1.8<f4/fw<2.4 2.8<f3/fw<3. 7 A rear focus type zoom lens that satisfies the following condition: 1.6<f3-1/f3<2.6.