JPH02207210A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To reduce aberration variation at the time of focusing by setting the refracting power of four lens groups, the movement conditions of the respective lens groups in power variation, a zoom ratio, the refracting power of the whole system at the wide-angle end and telephoto end, etc., and moving the 3rd group to the object side for the focusing. CONSTITUTION:The zoom lens consists of the 1st group I with positive refracting power, 2nd group II with negative refracting power, 3rd group III with positive refracting power, and high group IV with negative refracting power. Figures (A) and (C) show the positions of the respective lens groups when the zoom lens is put in focus on an infinite-distance body at the wide-angle end and telephoto end and figures (D) and (F) show the positions of the respective lens groups when the zoom lens is put in focus on a short-distance body at the wide-angle end and telephoto end. When the power is varied from the wide-angle end to the telephoto end, the four lens groups are all moved as shown by arrows in the figures (A) and (C). Then when focusing from the infinite-distance body to the short-distance body is performed, the 3rd group is moved to the body side as shown in the figures (D) and (F). Consequently, the aberration variation at the time of focusing is made small and the lens system is reduced in the size on the whole.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a rear focus type zoom lens, and in particular, to a rear focus type zoom lens, which includes four lens groups preceded by a lens group with positive refractive power used in photographic cameras, video cameras, etc. This invention relates to a rear focus type zoom lens having good optical performance with a variable power ratio of about 3.5.

(Prior Art) Conventionally, zoom lenses for photo cameras, video cameras, etc. have adopted the so-called rear focus method, in which focusing is performed by moving lens groups other than the first lens group on the object side. Various proposals have been made in Japanese Patent Publication No. 58-136012, Japanese Patent Publication No. 56-53730, 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, for example, Japanese Patent Laid-Open No. 58-91421 discloses focusing using some lens groups of a variable power system.

Further, in Japanese Patent Application Laid-Open No. 60-39613, focusing is performed using three lens groups in a variable power system, and the lens group is configured to reduce the difference in the amount of extension of the focusing lens group when changing the power of the same object. ing.

However, rear focus zoom lenses that utilize some of the lens groups in these variable power systems cover a wide range of object distances, from infinity to close objects.
There was a tendency for aberration fluctuations in focus to become relatively large. In particular, fluctuations in spherical aberration, astigmatism, and coma aberration are large, and it is extremely difficult to properly correct these various aberrations.

(Problems to be Solved by the Invention) The present invention has a positive refractive power lens group that has little aberration fluctuation during focusing, allows for easy miniaturization of the entire lens system, and is particularly suitable for cameras with automatic focusing devices. It is an object of the present invention to provide a rear focus type zoom lens which has four leading lens groups and performs focusing by moving some of the lens groups of a variable power system.

(Means for Solving the Problems) A rear focus type zoom lens according to the present invention includes, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power,
It has four lens groups: a third group with positive refractive power and a fourth group with negative refractive power.
All four lens groups from the group to the fourth group are moved toward the object side, the distance between the first and second groups increases, the distance between the second and third groups decreases, and the distance between the third and third groups increases. The focusing is performed by moving the four groups so that the distance between them decreases, and the focusing is performed by moving the third group.

In particular, in the present invention, the focal length of the first group is f1, the focal lengths of the entire system at the wide-angle end and the telephoto end are fW and fT, respectively, and the total lens length and back focus at the wide-angle end are LW, respectively.
0.17 < LW/(Z-fTl <0.23
...Self... (1) 0.25 < S
KW/fl# <0.45 −・・・−・・・・
・(2) 1.9fW<fl<1
．． 5fT・・・・・・・・・(3) 0.06 <IM
II/fZ-fT)<0.13 ・Auto-・・・・・(4
)0.01 <1M21/(Z-fT)<0゜08
−−−−−−−−−−(5) 0.06 <|M4|/(
It is characterized by satisfying the following condition: Z-fT)<0.13-...+61.

(Example) FIG. 1 is a schematic diagram showing the paraxial refractive power arrangement of a zoom lens according to the present invention, and FIGS. 2 to 4 are lens cross-sectional views of numerical examples 1 to 3 of the present invention, which will be described later. .

In the figure, I is the first group with positive refractive power, and II is the second group with negative refractive power.
The group ■ is the third group with positive refractive power, and ■ is the fourth group with negative refractive power.

In Figure 1, (A) and (C) are when focusing on an object at infinity at the wide-angle end and the telephoto end, respectively, (D) and (F
) indicates the position of each lens group when focusing on a short-distance object (distance 50 times the focal length of the entire system) at the wide-angle end and the telephoto end, respectively.

Arrows indicate the moving direction of each lens group when changing magnification or focusing.

In this embodiment, when changing the magnification from the wide-angle end to the telephoto end, the first
As shown in Figures (A) and (C,), all four lens groups are moved as indicated by the arrows.

That is, the four groups from the first group to the fourth group are arranged so that the distance between the first group and the second group increases, and the distance between the second group and the third group and the distance between the third group and the fourth group decrease. The lens group is moved toward the object side to change the magnification from the wide-angle end to the telephoto end.

When focusing from an object at infinity to an object at a short distance, the third group is moved toward the object as shown in FIGS. 1(D) and (F).

At this time, the movement conditions of each lens group and the conditions regarding the lens configuration are set as shown in the above-mentioned conditional expressions (1) to (6), and the aberration during focusing is improved by satisfying the conditional expression (7) described later. Fluctuation ratio 3 while properly correcting fluctuations
．． A compact zoom lens with a high zoom ratio of about 5 and high optical performance over the entire zoom range is obtained.

Furthermore, as mentioned above, the entire lens system is composed of four lens groups having positive, negative, positive, and negative refractive powers in order from the object side, so that fluctuations in various aberrations due to zooming cancel each other out. I have to.

By moving the four lens groups toward the object side under the above-mentioned conditions, the magnification ratio is distributed to each lens group in a well-balanced manner, thereby easily achieving a high zoom ratio. .

Also, when changing the magnification from the wide-angle end to the telephoto end, by moving the first group toward the object side, the overall length of the lens at the wide-angle end can be shortened, and the diameter of the front lens and the final lens can be reduced to ensure off-axis light flux. This prevents an increase in the effective diameter of the lens, thereby reducing the size of the entire lens system. At the telephoto end, the overall length of the lens is longer than at the wide-angle end to provide a sufficiently large telephoto ratio and correct various aberrations in a well-balanced manner.

In the embodiments shown in FIGS. 2 to 4, the first group and the fourth group are moved integrally to simplify the mechanism, but it goes without saying that they may be moved independently.

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

Conditional expression (1) appropriately sets the ratio of the product of the zoom ratio to the total lens length at the wide-angle end and the focal length of the entire system at the telephoto end,
This is mainly intended to reduce the size of the entire lens system while increasing the zoom ratio. In order to shorten the total lens length beyond the lower limit, the refractive power of each lens group must be strengthened accordingly, which causes the sensitivity of each lens group to become too large.
This is not good because the movement precision of each lens group and the movement and holding mechanism become complicated. Furthermore, if the total length of the lens is increased beyond the upper limit, the diameter of the front lens will increase and the entire lens system will become larger, which is not good.

Conditional expression (2) is for appropriately setting the back focus and ensuring a predetermined zoom ratio while reducing the size of the entire lens system.

If the lower limit value is exceeded and the back focus becomes too short, the diameter of the rear lens will increase in order to secure the luminous flux around the screen, and if the upper limit value is exceeded and the back focus becomes too long, the diameter of the rear lens will increase to ensure the luminous flux around the screen. This is not good because the amount of movement of each lens group increases, the total length of the lens increases, and the entire lens system becomes larger.

Conditional expression (3) relates to the refractive power of the first group, and is mainly used to satisfactorily correct distortion. If the lower limit is exceeded and the refractive power of the first group becomes too strong, pincushion distortion will increase on the telephoto side, and if the upper limit is exceeded and the refractive power of the first group becomes too weak, the distortion will decrease. However, this is not good because the amount of movement of the first lens group to obtain a predetermined variable power ratio increases.

Conditional expressions (4), (5), and (6) represent the first group, the second group, and
By appropriately setting the ratio of the product of the focal length of the entire system at the telephoto end and the zoom ratio to the total amount of movement associated with zooming of the fourth group, it is possible to achieve a predetermined zoom ratio while reducing the size of the entire lens system. This is to effectively obtain the results.

If the lower limit values of conditional expressions (4) and (6) are exceeded and the amount of movement of the first and fourth groups is set to decrease, the amount of movement of the first and fourth groups
The refractive power of the group must be increased accordingly, and the sensitivity of both will increase. This is not a good idea because the movement mechanism for moving these lens groups with high precision becomes complicated.

In addition, when the upper limits of conditional expressions (4) and (6) are exceeded, the first and fourth groups
If you set the groups to move more, the overall length of the lens will become longer, and each lens group will become too close together at the telephoto end.
This is not a good idea because the mechanical mechanism becomes more complicated.

If the lower limit of conditional expression (5) is exceeded and the amount of movement of the second lens group is set to be small, the distance between each lens group must be widened at the wide-angle end, and the overall length of the lens increases at the wide-angle end. come. Furthermore, if the amount of movement of the second lens group is set to increase beyond the upper limit, the amount of change in the air distance between the second lens group and the first lens group due to zooming will become too small, resulting in less effect of zooming, and it will not be possible to achieve a predetermined zoom ratio. It's not good because it becomes difficult.

In this embodiment, when the focal length of the third group for focusing is f3, 0.6<f3/fW<0.85 (
7) to reduce aberration fluctuations during focusing.

If the lower limit of conditional expression (7) is exceeded and the refractive power of the third group becomes too strong, the amount of movement during focusing will decrease, but aberration fluctuations will increase, which is not good. Also, if the upper limit is exceeded and the refractive power of the third group becomes too weak, the amount of movement of the third group during focusing will increase, and the distance between each lens group must be widened in advance, which increases the overall length of the lens. It's not good because it will come.

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

Furthermore, Table 1 shows the relationship between each of the above-mentioned conditional expressions and the word values in the numerical examples.

Numerical Example 1 F-39 to 131 Rl = 41.33 R2 = 29.80 R3 = 30.83 R4 = -229,89 R5 = -52,26 R6 = 23.95 R7-24,30 R8 = 120. 85 R9= 34.51 RIO= (Aperture) R11= 312.37 R12= -26,9O R+3= 13.10 R14= -68,03 R15= -24,93 RI6= 17.15 R17= 31゜14 R18 = -16,16 R19= -23,30 R20= -14,94 R21= -13,61 R22= -33,69 R23= -16,79 R24= -32,90 FNO=I: 4.5~8 2ω=58″
~18.7”DI=1.5Nl=
1.80518 v l〒25.402=0.
3 D 3 = 4.8 N 2 = 1.48749
ν2=70.2D4・Variable D5=1.0 N5=1.77250 C3=49.606=0.88 D7=3.0 N4=1.805+8
ν4=25.408=1.0 N5=1.
5]633 5=64. ID9= Variable DIO= 1.2 D11= 2.7 N 6=1.51633
v 6= 64.1D12= 0.2 DI3= 3.5 N7=1.51633
Schiff=64.1D14=1.1 D15=1.6 Ng=1.83400
Sig = 37.2016 = 1.5 DI7 = 4.55 N 9 = 1.51835
v9=60.3D18=variable D19=3. ONl0=1.68893 ν1o=
31.1D20=4.25 D21=1.2 Nl]=1.69680 ν
ll= 55.5D22= 2.5 D23=1.2 N+2=1.69680 ν
12 = 55.5 Numerical Examples F-39 to 131 Rl = 50.48 R2 = 34.87 R3 = 31.60 R4 = -173,21 R5 = -106,95 R6 = 26.60 R7 = 31.26 88= -20,82 R9= 66.72 R1O= (Aperture) RI+= 312.37 RI2= -30,00 R13= 12.38 R14= -125,70 RI5= -31,85 R16= 15.46 R17 = 46.84 RI8= -18,27 RI9= -21,51 R20= -14,69 R21= -14,24 R22= -40,40 R23= -20,92 R24= -42,81 FNO=I: 4.5~8 2ω=58″~l87
6D l=1.7 N I=1.72+51
v I= 29.202=0.3 D 3= 6.3 N 2=1.51633
v2=64. ID4=Variable D5=1.0 N3=1.69680
C3=55.506=0.25 D7=6.0 N4=1.59270
ν4=35.308=1.0 N5=1.7
1299 ν5= 53.809・Variable DIO= 1.2 DII= 2.7 N6=1.60311
White = 60.7012 = 0.2 D13 = 3.5 N 7 = 1.51633
v7=64.1D14=1.1 DI5=1.6 Ng=1.83400
C8=31.2DI6=2.1DI? = 4.55 N9=1.60311
DI9=60.7D18: Variable DI9=3. ONl0=1.68893 ν1O=
31. l020=4.34 D21=1.2 N11=1.69680 ν
11=55.5022=2.03 D23=1.2 N12=1.69680 ν
12 = 55.5 Numerical Examples (Table 1) F-39 to 131 Rl = 40.54 R2 = 30.48 R3 = 31.35 R4 = -286,32 R5 = -58,88 R6 = 60.67 R7=58.69 R8=12.24 R9=44. +4 RlO- (aperture) RI+= 312.37 R12= -24,71 R13= 11.62 RI4= -122,03 R15= -30,34 R16= 14.31 RI7= 26.55 R18= -17,92 R19= -25,00 R20= -14,94 R21= -12,79 R22= -48,54 R23= -19,77 R24= -31,58 FNO=I :4.5~8 2ω=58''~ 18
．． 7”DI=1.5N]=1.846
66 ν I=23.902=0.3 (] 3= 4.4 82=1.48749
ν 2-70.2D4 = Variable D 5 = 1.0 N 3 = 1.77250
C3=49.606=0.9 D7=1.0 N4=1.69680 ν
4=55.5D B=5.0N S=1.6
8893 ν5=31.1D9・Variable DlO=1.2 DI+=2.7 N6=1.48749
White = 70.2 DI2 = 0.2 D13 = 3.5 N7 = 1.51633
Schif=64.1DI4=1.1 DI5=1.6 88=1.11340[1shi8
=31.21116=2.1 DI7=4.55 N9=1.51835
DI9=60.3D18=Variable DI9=3. ONl0=1.68893 νl0=
31. l020=4.08 D21=1.2 Nl1=1.69680 ν
ll= 55.5022= 2.11 D23=1.2 N+2=1.59680 ν
12=55.5 (Effect of the Invention) According to the present invention, as described above, the refractive power of the four lens groups, the movement conditions of each lens group during zooming, the zoom ratio, the refractive power of the entire system at the wide-angle end and the telephoto end, etc. By setting the 3rd group to the object side and focusing from an object at infinity to a close object, a high zoom ratio of around 35x is achieved with less variation in aberrations during focusing. Moreover, it is possible to achieve a rear focus type zoom lens that has high optical performance over the entire zoom range.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the paraxial refractive power arrangement of a rear focus type zoom lens of the present invention, FIGS. 2 to 4 are cross-sectional views of lenses of numerical examples 1 to 3 of the present invention, and FIG. 7 is a diagram showing various aberrations of numerical examples 1 to 3 of the present invention. In the aberration diagrams, (A), (B), and (C) are the aberrations at the wide-angle end, middle, and telephoto end when the object is at infinity, respectively, and (D), (E), and (F) are the aberrations when the object is at the close distance (50 f). Shows the aberrations at the wide-angle end, intermediate, and telephoto end. Also, 1 in the figure. I■, ■, ■ are the first, second, third, and fourth in order.
groups, the arrows indicate the movement direction of each lens group during zooming from the wide-angle end to the telephoto end or during focusing, S is the sagittal image plane,
M is the meridional image surface, d is the d-line, g is the g-line, and y is the image height. Patent Applicant: Canon Co., Ltd. Figure (E) Figure (F)

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 negative refractive power.
It has four lens groups, and when changing magnification from the wide-angle end to the telephoto end, all four lens groups from the first group to the fourth group move toward the object side, and the distance between the first and second groups increases. and the distance between the second group and the third group is decreased, and the distance between the third group and the fourth group is decreased, and the focus is performed by moving the third group. A rear focus zoom lens featuring (2) The focal length of the first group is f1, the focal lengths of the entire system at the wide-angle end and the telephoto end are fW and fT, respectively, and the total lens length and back focus at the wide-angle end are LW and SKW, respectively.
When the zoom ratio is Z and the amount of movement of the i-th group due to zooming from the wide-angle end to the telephoto end is Mi, 0.17<LW/(Z・fT)<0.23 0.25<SKW/fW< 0.45 1.9fW<f1<1.5fT 0.06<|M1|/(Z・fT)<0.130.01
<M2|/(Z・fT)<0.080.06<|M4|
2. The rear focus type zoom lens according to claim 1, wherein the rear focus type zoom lens satisfies the following condition: /(Z·fT)<0.13.