JPH02201409A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To reduce aberration variation in focusing and to obtain high optical performance while reducing the size of a lens system on the whole by specifying three lens groups of a zoom lens when putting the zoom lens consisting of the three lens groups with specific refracting power in focus by moving the 3rd group. CONSTITUTION:The zoom lens has the 1st group I with negative refracting power, the 2nd group II with positive refracting power, and the 3rd group III with negative refracting power in order from the object side; and the three lens groups are moved to the object side to vary the power from the wide-angle end to the telephoto end and when the 3rd group III is moved for the focusing, the 3rd group III consists of a 31st positive lens and a 32nd negative lens. Then 1 < f31/¦f3¦ < 2, where f3 and f31 the focal lengths of the 3rd group and 31st lens. Consequently, the aberration variation in the focusing is small, the whole lens system is easily reduced in size, and the rear focus type zoom lens which has high optical performance suitable to, for example, a camera which has an automatic focusing device is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rear focus type zoom lens, and particularly to a rear focus type zoom lens, which is suitable for focusing on cameras equipped with automatic focus detection devices, such as photographic cameras and video cameras. The present invention relates to a rear focus type zoom lens having good optical performance with a short overall lens length and little fluctuation in aberrations.

(Prior Art) Conventionally, zoom lenses for photographic cameras, video cameras, etc. have adopted 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. This method has been proposed in Japanese Patent Application Laid-open No. 58-91421, Japanese Patent Application Laid-open No. 60-39613, etc.

In general, the rear focus type uses a relatively small and lightweight lens group that moves, so the driving force for the lens group is small, and it has the advantage of being able to quickly focus, especially when performing automatic focusing.

Additionally, compared to the so-called front focus type, in which focusing is performed by moving the first lens group on the object side, the effective diameter of the lens group on the object side is smaller, making it easier to downsize the entire lens system. .

As a rear focus type zoom lens, in the aforementioned Japanese Patent Laid-Open No. 58-91421, focusing is performed using a part of the lens group of a variable power system.

However, since the entire zoom lens disclosed in this publication has a so-called retrofocus type lens configuration, the telephoto ratio tends to be relatively large and the entire lens system tends to be large.

Further, in Japanese Patent Application Laid-open No. 60-39613, focusing is performed using three lens groups in a variable power system, and the difference in the amount of extension of the focusing lens group when changing the power of the same object is reduced. It consists of

However, all rear focus zoom lenses that utilize some of the lens groups in these variable magnification systems cover a wide range of object distances, from objects at infinity to objects at close range.
Aberration fluctuations in focus tended to be relatively large. In particular, fluctuations in spherical aberration, astigmatism, and coma aberration are large, and it is very difficult to properly correct these various aberrations.

(Problems to be Solved by the Invention) The present invention comprises a zoom lens as a whole consisting of three lens groups having a predetermined refractive power, and all three lens groups are moved to change the magnification. A rear focus with high optical performance that has little aberration variation when focusing by moving some lens groups, and the entire lens system can be easily miniaturized, making it suitable for, for example, cameras with automatic focusing devices. The purpose is to provide a zoom lens of the following type.

(Means for Solving the Problems) The rear focus type zoom lens of the present invention includes, in order from the object side, a first group having a negative refractive power, a second group having a positive refractive power, and a second group having a negative refractive power. It has three lens groups of three groups, and the three
When moving two lens groups toward the object side to change the magnification from the wide-angle end to the telephoto end, and when focusing by moving the third group, the third group is combined with the positive 31st lens and the negative 31st lens. It is composed of 32 lenses, and the focal lengths of the third group and the 31st lens are f3. When f31, 1 < f31/ | f3 | < 2 (1
) is characterized by satisfying the following conditions.

(Example) FIG. 1 is a schematic diagram of an example showing the paraxial power arrangement of the present invention. Figures 2 to 5 are numerical examples 1 to 4, which will be described later.
FIG. 3 is a sectional view of the lens.

In the figure, (A) and (B) are right at the wide-angle end and telephoto end, respectively, and when focusing on an object at infinity, (C) and (D
) indicates the position of each lens group when focusing on a short-distance object (for example, a distance 111 that is approximately 20 times the focal length of the entire system) to the right of the wide-angle end and the telephoto end.

Arrows indicate the moving direction of each lens group when changing magnification or focusing. Symbol 1 is the first group with negative refractive power, ■ is the second group with positive refractive power, and ■ is the third group with negative refractive power.

In this embodiment, all three lens groups, the first group to the third group, are independently moved toward the object side when changing the magnification from the wide-angle end to the telephoto end. Furthermore, focusing from an object at infinity to a near object at any position within the variable magnification range is achieved by moving the third group having negative refractive power toward the image plane. At the telephoto end, as shown in the same figure (D), the third group ■ is further moved toward the image plane to the position indicated by the broken line, which allows for ultra-close-up photography (
I try to do so-called macro photography.

In addition, in this embodiment, the zoom lens is set in order from the object side to negative, positive,
It is composed of three lens groups with negative refractive power, and each lens group is moved toward the object side as shown by the arrow when changing the magnification from the wide-angle end to the telephoto end. At the wide-angle end, the lens has a retrofocus type lens configuration, and at the telephoto end, it has a telephoto type lens configuration, making it easier to widen the angle of view at the wide-angle end, and shortening the back focus at the telephoto end. At the same time, the overall length of the lens is effectively shortened.

The lens is set to focus from an object at infinity to an object at a short distance by moving the third group having negative refractive power toward the image plane, thereby making effective use of the back focus space.

For this reason, the present invention is extremely advantageous in reducing the size of the photographing system, particularly in lens shutter cameras with short back focal lengths.

In addition, as shown in Figure 1 (D), the third group is configured to be located closer to the object side at the telephoto end than at the wide-angle end, and can be moved more toward the image plane than at the wide-angle end. This makes it possible to perform so-called super close-up photography with a high magnification.

In this embodiment, the third group that performs focusing is composed of two lenses, a positive 31st lens and a negative 32nd lens, in order from the object side, and especially at close range at the wide-angle end, the light enters the negative 32nd lens. By lowering the incident height of off-axis rays, the lens diameter can be reduced and the weight of the third group can be reduced, thereby improving drive operability when focusing with the third group.

Next, the technical meaning of the above-mentioned conditional expression (1) will be explained. Conditional expression (1) relates to the refractive power ratio of the third group and the positive 31st lens in the third group.

In a rear focus type zoom lens, there are generally large aberration fluctuations during focusing, such as astigmatism at the wide-angle end and spherical aberration at the telephoto end, making it difficult to satisfactorily correct these various aberrations.

That is, as the object distance becomes short from infinity, astigmatism at the wide-angle end tends to become undervalued, and spherical aberration at the telephoto end tends to become undervalued. The positive lens in the third group has the effect of under-reducing astigmatism at the wide-angle end and over-setting spherical aberration at the telephoto end during focusing.

Conditional expression (1) is used to appropriately set the refractive power of the positive 31st lens in the third group and to correct the aberration fluctuations during focusing of these various aberrations in a well-balanced manner.

When the focal length of the positive 31st lens exceeds the upper limit of conditional expression (1), the effect of correcting spherical aberration at the telephoto end becomes weaker, and the spherical aberration becomes undervalued at close range, making it difficult to correct it. becomes. If the lower limit of conditional expression (1) is exceeded and the focal length of the positive 31st lens becomes small, the fluctuation of astigmatism at the wide-angle end becomes even smaller, making it difficult to correct it.

By satisfying the above-mentioned conditions, the present invention achieves a compact rear focus type zoom lens with high optical performance with little aberration fluctuation during focusing. In order to obtain compact and high optical performance over a wide range, the following conditions should be satisfied.

The refractive index of the material of the 31st lens and the 32nd lens is N31. N32, when the radius of curvature of the first lens surface counting from the object side of the third group is R111i, N31 < N32 ・
...... (2) 1 < Rm2/RI
II3 <2゜(RI[I2<O, RI[I
3<0)...The following condition (3) must be satisfied.

Conditional expression (2) is based on the positive 31st lens and the negative 32nd lens in the third group.
Regarding the refractive index of the material of the lens, since the third group has negative refractive power, if conditional expression (2) is violated and the refractive index of the material of the positive lens becomes larger than that of the material of the negative lens, the refractive index of the third group Since the Petzval sum becomes large in the negative direction, the image surface characteristics deteriorate.

Therefore, if conditional expression (2) is not satisfied, it becomes difficult to satisfactorily correct the image plane characteristics over the entire magnification range and focus range.

Conditional expression (3) relates to the ratio of the radius of curvature of the image plane side lens surface of the positive 31st lens in the third group to the object side lens surface of the negative 32nd lens.

The object-side lens surface of the negative 32nd lens in the third group is a convex surface (
In other words, if RIII3>O), the lens surface on the image side of the 32nd lens becomes a strongly concave surface and large distortion aberration occurs at the wide-angle end.In this example, the lens surface on the object side of the negative 32nd lens is The surface is convex (ie, Rm3<O). At this time, spherical aberration occurs at the telephoto end on the object side lens surface of the negative 32nd lens, so in order to properly correct this,
The lens surface on the image plane side of the positive 31st lens is a convex surface (that is, Rm
2<O).

In addition, the lens surface on the image side of the positive 31st lens has the effect of over-correcting spherical aberration at the telephoto end as the object distance becomes closer from infinity, and the lens surface on the object side of the negative 32nd lens The lens surface has the effect of over-correcting astigmatism at the wide-angle end.

Conditional expression (3) was specified taking such points into consideration. If the upper limit of conditional expression (3) is exceeded and the curvature of the image-side lens surface of the positive 31st lens becomes gentler than the curvature of the object-side lens surface of the negative 32nd lens, spherical aberration will occur at the telephoto end. The fluctuation of the image becomes large, resulting in a large undershoot at close range, which becomes difficult to correct. Conversely, if the lower limit of conditional expression (3) is exceeded and the curvature of the object-side lens surface of the negative 32nd lens becomes gentler than the curvature of the image-side lens surface of the positive 31st lens, the Astigmatism fluctuates greatly and becomes significantly under-represented at close distances, making it difficult to correct it.

In addition, in this embodiment, while trying to downsize the entire lens system,
In order to better correct aberration fluctuations caused by zooming, the first group is configured to have two lenses, a negative lens and a positive lens, in order from the object side, and the second group is configured to have two positive lenses, a negative lens, and a negative lens. It is preferable to configure it so that it has four positive lenses.

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 υi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side. The value outside () of the variable interval 013 is an object at an infinite distance, and the value inside () is a value for an object distance lll11m.

The aspherical shape has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis,
The traveling direction of light is positive, R is the paraxial radius of curvature, aI +
It is expressed by the formula + a411' + a5H10, where 2 + ''· and a6 are respectively aspherical coefficients.

Further, for example, the display "re-03" means NO-'J.

Numerical Example I F-28,8 to 78 RI--84,53 It 2- 18.21 113- 21.31 R4-150,23 R5-39,85 It 6--51.65 R71-th, 63 R11-8638,77 It 9--41.74 10- 15.67 Old 1- 29.07 812--21.54 Ri3- Aperture old 4--83.44 R15--19,54 Old 6--13 .04 R17-1897,07 FNo=1:3.5 2ω-73,8° ~:1
1.00~8.24 DI-1,50 02-2,17 03-4,58 D4/Variable D 5- 3.04 D 6-0.15 D7 3.25 08-0.57 09- 5.00 0 O-0,45 DI-3,32 02 Maro! , 00 013 Heat variable D 4-3.83 05-3.19 016- 1.5O N I-1,77250ν 1-49.6N 2-
1.68893 ν 2-31.1N 3-1.4
9831 ν 3-65. ON 4-1.57250
ν 4-57.8N 5-1.805+8 ν
5-25.4N 6-164769 ν 6-33.
8N 7”1.58500 v 7-29.3N
8=1.77250 v 8-49.6 Aspherical surface: R
15 a, = O a , = -1,060-05 a , = -1,680-07 R4 = 1.570-09 8 g = -1,330-11 f31 / If31 = 1.40Rn12
/ 8m3 = 1.5 Numerical Example 2 Fso 28.8 ~ 78 R1 Kai -71,65 R3 San R4 Kai R7 Kai 18.30 22.20 17+, 88 49.94 -43.75 To L25 FNo=1 :3.6 26) -73,8° ~3
1.0'~8.8 1-1.70 N 1m1.77250v
1-49.62- 2.65 3- 4.64 N 2-1.68893 ν
2-31.14・Variable 5-3.00 N 3-1.51633 v 3-
64.16 Maro 0.15 7-3.08 N 4-1.56384 ν 4-6
0.7R8 235.18 R9--46,18 1110-15,53 R11 So 26.56 RI2 -2:1.33 Old 3- Aperture RI4 -39,19 R15 vote -17,18 ft+6-- 12.98 RI7-146.58 D 8- 0.45 0 9- 5.03 010- 0.31 Dll-: 1.30 01201.00 D131-Variable D14-3.16 D15-3.71 016 Self 1 .5O N 5-1.80518 ν 5-25.4N
6-1.68680 ν B-33, ON 7=1
．． 58500 v 7s29.3N 8-1.7
7250 ν 8 Snow 49.6 Aspherical surface: 2.34D-07 -1,070-07 1,780-09 -1,580-11 +31 / 1f31 = 1.57Rm2
/ 8m3 = 1.32 Numerical Example 3 F-28,8 ~ 81.6 R1--83,96 R2-20,24 R3-25,12 R4-180,77 R5-34,03 R6--79, 83 87-16,19 R8-430,50 R9--49,84 Old 1■ 14.22 R11-17,59 R12--22,50 R13-Aperture R14■-45,28 815--18,91 R16 --12,77 817-540,53 73,8' ~ 29.7' FNoll: 3.4 2ω- ~ 8.24 D I-1,70N l-1,77250υD
2- 3.04 D 3- 4.75 D4/Variable D 5- 3.44 D 6-0.15 D 7- 4.63 D 8-0.49 D 9-5.00 01G-0,28 Dll -3,77 012-1,00 013-Variable D14- 3.23 015-4.06 D16■ 1.5O N 8 1.71299 ν N 6 1.58500 υ N 5 1.84666 1 N 7 -1.58500 υ N 4-1.66384 ν N 2 1.69895 ν N 3 Exposure 1.51633 ν 1-49.6 2 30.1 Aspheric: R12 Aspheric: 3 64.1 4- 60゜7 5-23.9 al = O R2 = 7.570-06 83 = 1.330-08 a 4 = -3,820-10 a m = -2,990-11 f31 / 1f31 = 1.75RIII2
78m3 = 1.48-9.62D-06 -1,460-07 4,570-10 -6,220-12 6-29,3 Numerical Example 4 7-29.3 8-53.8 ~102 Rl --55,52 12-20,98 13-23,04 84-778813,68 R5-33,41 R6-119,99 Jl 7- 17.10 FNo=l:3.5 2(1)-62, 0° ~
24.0'~8.24 D I-1,50N 1-1.71299 ν
t-R3, '82 meeting 1.35 3- 4.20 4.Variable 5-2.42 6 O,tS 7- 4.35 N 2-1.64769 ν 2-33.8N
3-1.51633 v 3-64.1N 4-
1.56384 ν 4-60.7R8-199,6
4 R9--65,04 RIO-16,03 R11-28,+8 RI2--34.44 813- Aperture R4--86,76 RI5--19.83 R16-13,97 R17-1037,24 Non Spherical surface: D 8- 0.25 0 9- 5.05 DO 0.55 Dll-3,77 012-1,00 D3・Variable 04-3.39 D5-3.06 D 6- 1.5O N 5-1.80518 N 6-1.84769 N 7-1.5850O N 8-1.71299 -1.190-05 -5.05D-09 -5,950-10 -1,310-12 +31 / 1f31 Fuji 1.118m2 /
Rm3 = 1.42 ν 5 25.4 v 6-33.8 ν 7-29.3 v 8-53.8 (Effect of the invention) According to the present invention, a zoom consisting of three lens groups with predetermined refractive powers When focusing by moving the third group of the lens, by setting each lens group as described above, there is less aberration variation during focusing, and the rear focus type has high optical performance with a miniaturized overall lens system. can be achieved with a zoom lens.

Furthermore, it is possible to achieve a rear focus type zoom lens having features such as the ability to easily perform extremely close-up photography at the telephoto end.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment showing the paraxial refractive power arrangement of the present invention, FIGS. 2 to 5 are cross-sectional views of lenses of numerical embodiments 1 to 4 of the present invention, and FIGS. 6 to 9 are numerical examples 1 to 4 of the present invention
It is a diagram of various aberrations. In the lens cross-sectional view and aberration diagram (A
), (B) are wide-angle end and telephoto end when focusing on an object at infinity, (C) and (D) are object ratio 11t 1
The wide-angle end and telephoto end at m are shown, respectively. In the figure, r, n, m are numbered in order. Second. In the third group, arrows indicate the moving direction of the lens group during zooming or focusing, d indicates the d-line, g indicates the g-line, ΔM indicates the meridional image plane, and ΔS indicates the sagittal image plane.

Claims (3)

[Claims] (1) It has three lens groups in order from the object side: a first group with negative refractive power, a second group with positive refractive power, and a third group with negative refractive power, and these three lens groups are connected to the object. When the lens is moved to the side to change the magnification from the wide-angle end to the telephoto end, and the third group is moved to focus, the third group is composed of a positive 31st lens and a negative 32nd lens. , where the focal lengths of the third group and the 31st lens are f3 and f31, respectively, and the following condition is satisfied: 1<f31/|f3|<2. (2) When the refractive index of the material of the 31st lens and the 32nd lens is N31 and N32, respectively, and the radius of curvature of the i-th lens surface counting from the object side of the third group is RIIIi, N31<N32 1 2. The rear focus type zoom lens according to claim 1, which satisfies the following conditions: <RIII2/RIII3<2, (RIII2<0, RIII3<0). (3) Claim 1 characterized in that the third group is moved at the telephoto end to enable extremely close-up photography.
The rear focus zoom lens described.