JPH01185507A - Photographic lens of inner focus type - Google Patents

Abstract

PURPOSE:To obtain a high optical performance by constituting each lens group under a specific condition and executing the focusing by moving the second group among these groups on an optical axis. CONSTITUTION:Focusing is executed by moving the second group having negative refracting power on an optical axis, and when a radius of curvature of the lens surface of an image face side of a lens 1A being the nearest to the image face side of the first group having positive refractive power, a refractive index of a material of the lens 1A, radiuses of curvature of lens surfaces being the nearest to an object side and being the nearest to the image face side of the second group, a refractive index of a material of a lens 2A being the nearest to the object side of the second group, a radius of curvature of the lens surface of the object side of a lens 3A being the nearest to the object side of the third group having positive refracting power, each refracting power of the second group and the whole system, and an air interval at the time of having been focused to an infinity object of the second group and the third group are denoted as R1A, N1A, R2A and R2B, N2A, R3A, phi2 and phi, and D, respectively, this lens is allowed to satisfy conditions of expressions I-VI. In such a way, an aberration fluctuation at the time of executing the focusing by moving the second group decreases, and a high optical performance is obtained extending from a wide object distance range from an infinity object to the closest object.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an inner focus type photographic lens suitable for photo cameras, video cameras, etc., and in particular to an inner focus type photographic lens suitable for photographic cameras, video cameras, etc. This relates to an inner focus type photographing lens with an aperture ratio of 1.8 and a relatively wide angle of view of approximately 46 degrees.

(Prior Art) Recently, photographic lenses for photographic cameras, video cameras, and the like are required to have high optical performance and a lens system that can focus easily and quickly.

If the focusing lens group is small and lightweight, it has the advantage of being able to reduce the torque of the motor for driving the focusing lens group when used in an autofocus photographing system, for example. For this reason, various so-called inner focus type photographic lenses have been proposed in which focusing is performed by moving a part of a relatively small and lightweight lens group inside the lens system.

For example, JP-A-59-17519 and JP-A-59-6
In Publication No. 5821, etc., the first lens with positive refractive power is
Discloses an inner focus type photographing lens which is composed of three lens groups: a second group having a negative refractive power, and a third group having a positive refractive power, and in which focusing is performed by moving the second group. There is.

However, when trying to achieve an angle of view of 45 degrees or more, which is the so-called standard lens, with an inner focus type photographic lens, there are many aberration fluctuations during focusing.
Generally, it has been very difficult to obtain high optical performance.

(Problems to be Solved by the Invention) The present invention aims to reduce the weight and size of a focusing lens group with an F number of 1.8 and a shooting angle of view of about 46 degrees, and to achieve wide shooting with less aberration fluctuation during focusing. The purpose of the present invention is to provide an inner focus type photographic lens that has high optical performance over a distance range and is particularly suitable for photographic cameras, video cameras, and the like.

(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 three lens groups: a group and a third group with positive refractive power;
Focusing is performed by moving the second group on the optical axis, and the first group
The radius of curvature of the lens surface on the image side of the lens 1A closest to the image side of the group is R1A, the refractive index of the material of the lens 1A is N1A,
The radius of curvature of the lens surface closest to the object side and the closest to the image plane of the second group are R2A and R2B, respectively. The refractive index of the material of the lens 2A closest to the object side of the second group is N2A. The radius of curvature of the object side lens surface of the object side lens 3A is R3A.
, the refractive powers of the second group and the entire prefecture are respectively φ2. φ, and when the air distance when the second and third groups are focused on an object at infinity is D, 1.8<-φ2/φ<2. t---(1)
0.2<D·φ<0.3 -----(2) is to be satisfied.

(Example) 1st. Second. FIG. 3 is a cross-sectional view of lenses of numerical examples 1 to 3 of the present invention, respectively. In the figure, I is the first group with positive refractive power, ■
is the second group with negative refractive power, and m is the third group with positive refractive power.

This embodiment employs an inner focus type in which focusing is performed by moving the second group on the optical axis.

The lens 1A closest to the image plane of the first group has a concave surface facing the image side, and the lens surfaces of the second group closest to the object and closest to the image plane are both convex toward the object side. In addition, the lens 3A closest to the object side of the third group has a concave surface facing the object side, and the radius of curvature of the lens surface on the image side of the lens 1A is the closest to the object side of the second group. The radius of curvature of the lens surface on the surface side, the radius of curvature of the lens surface on the object side of lens 3A, the refractive index of the material of lens 1A and lens 2A closest to the object in the second group, and the refraction of the second group and the entire system. By setting the forces etc. as in the above conditional expressions (1) to (7), aberration fluctuations when focusing by moving the second group can be reduced, and a wide range of focus can be achieved, from an object at infinity to a close object. High optical performance is obtained over the object distance range.

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

Conditional expression (1) restricts the range of refractive power of the second group for focusing, and if the refractive power of the second group for focusing becomes weaker by exceeding the lower limit of conditional expression (1), the second group for focusing becomes weaker. The amount of extension when focusing is increased, and the second and third groups
This is not a good idea because it requires a large air gap between the lens group and the lens group, which increases the size of the lens system. Also, when focusing on an object at infinity, the air distance between the second and third groups becomes wider, and when focusing on a short distance object, the air distance between the first and second groups becomes wider. In this case, it becomes difficult to correct aberrations of off-axis rays. Conversely, if the upper limit of conditional expression (1) is exceeded and the refractive power of the second group for focusing becomes strong, many higher-order aberrations will occur, and in order to properly correct these aberrations,
This is not a good idea because it becomes necessary to increase the number of lenses in the second group, which increases the weight of the second group.

Conditional expression (2) appropriately determines the air gap between the second and third focusing groups when focusing at infinity. If the air gap becomes short beyond the lower limit, the refractive power of the second group set by conditional expression (1) cannot be extended to a close distance, so the close distance becomes long. Further, it causes deterioration of high-order astigmatism and deterioration of off-axis under-axis. On the other hand, if the air gap becomes longer than the upper limit of conditional expression (2), the height of incidence of off-axis rays on the third group increases, and as a result, the image plane becomes significantly under-centered off-axis.
Also, extraversion coma occurs, making it difficult to correct. Furthermore, the amount of peripheral light decreases, the total length of the lens system becomes longer, and the back focus becomes shorter, which is not good.

Conditional expressions (3) and (4) apply to the most image-side surface of the second group and the third group.
This is to set the range of curvature of the surface closest to the object side of the lens group, and is mainly used to correct spherical aberrations occurring on other lens surfaces, image surface characteristics that tend to be underexposed, and coma aberration in a well-balanced manner. .

If the lower limits of conditional expressions (3) and (4) are exceeded, the spherical aberration and image surface characteristics will be insufficiently corrected, and conversely, conditional expressions (3) and (4)
) is not good because off-axis halo and coma aberration will increase.

Conditional expression (5) appropriately adjusts the refractive power of the air lens generated by the lens surface closest to the image plane of the first group and the lens surface closest to the object side of the second group in order to properly correct the tilt of the image plane characteristics. . If the lower limit value is exceeded, it will not be possible to satisfactorily correct the tilt of the image plane characteristics that occurs on other lens surfaces, and conversely, if the upper limit value is exceeded, higher-order aberrations will occur frequently.
Furthermore, fluctuations in aberrations increase when focusing is performed by moving the second lens group, which is not good.

Conditional expression (6) is used to appropriately set the radius of curvature of the lens surface closest to the object side of the second group and to reduce aberration fluctuations during focusing. If the lower limit value is exceeded, higher-order aberrations will occur, and conversely, if the upper limit value is exceeded, aberration fluctuations will increase, which is not good.

In this embodiment, focusing from an object at infinity to a close object is performed by moving the second group toward the image plane. For this reason, the height of incidence of the axial ray that enters the second group is lower for a close object than for an object at infinity, and spherical aberration tends to be undercorrected.

Therefore, by specifying the lens shape and material of the lens 1A closest to the image plane in the first group as described above, the spherical aberration, which tends to be under-corrected, can be corrected in the direction of over-correction while reducing the Petzval sum. Corrected in a well-balanced manner.

Furthermore, by specifying the lens shape of the lens 3A closest to the object side of the third group as described above, the negative refractive power in the third group can be positioned toward the object side, and the positive refractive power can be positioned toward the image plane side. I try to do that. As a result, the angle of incidence of off-axis rays entering the third group is made gentler, and while off-axis aberrations are well corrected, the aperture diameter can be prevented from increasing due to a large aperture ratio, and the overall lens system can be made more compact. ing.

In this example, the negative second group is configured to have one positive lens and one negative lens, so that the second group reduces aberration fluctuations when focusing, and provides high accuracy over a wide object distance range. Obtaining optical performance.

In order to further improve the optical performance of the entire screen in the present invention, the first group is configured to have at least two lenses: a positive lens and one meniscus-shaped positive lens with a convex surface facing the object side. It is preferable to configure the third group to include a cemented lens made by bonding negative and positive lenses together and at least one positive lens.

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 υi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side.

Furthermore, Table 1 shows the relationship between each of the above-mentioned conditional expressions and each numerical example.

Numerical Example IF=IFNo-1:1.8 2ω-
45° Back 7 Orca X-0,8081-1,27
35D I-0,08598l-1,77250v
1-49.6R2--5,832102-0,0029
R3-0,5025D 3-0.0831N
? 1.77250 v 2-49.6R4 auto
1.1346 D 4- 0.0400R5-2
2,7419D 5-0.0382 N 3 Tortoise 1.88
300ν3-40.8R6= 1.3112D
6-0.0286 N 4-1.74077 v 4-
27.8R7 Proverbs 0.3775 D 7-0.221
2R8--0,3392D 8-0.038285-1
．． 78472υ5−25.71R9= −0,812
3D 9” 0.0802 N 6-1.81
600 v 6 46.6RIO--0,4065
DIO rumor 0.0029R11-2,4885Dll-0
,057387-1,77250v 7-49.6R1
2--0,9342 Numerical Example 2 F=l FNO-1:1.8 2(&
l-45° Back focus-〇, 74R1-1,
4901D I-0,0770N 1-1.77250
v 1-49.6R2--5,2812D 2-0
．． 0029R3=0.5114D3-
0.0770 N 2-1.88300 v
2-40.8R4-1,450904-0,0289
N 3 = 1.76182 v 3-26.6R5-0,
9189D 5- 0.0347R6-8,9906
D 6- 0.0289 N 4-183400
ν 4-37.2R7-0, 7513D 7-
0.0385 N 5-1.78472 ν 5
-25.71R8-0,4093D 8- 0.22
33R9--0,3658D 9-0.0231
N 6-1.72825 ν 6-28.5RI
O--0,9052010-0,077087=1.8
3481 v 7”42.7R11--0,439
5Dll-0,0029R12・2.3397D
12=0.0577 8 8-1.77250
v 8-49.6R13--1,0016 Numerical Example 3 F-I FNo=l:1.8 2(JJ-
47° back focus -〇, 82R1=-39,
6724D I-0,1001N l-1,61800
v 1-63.4R2--2, 3248D 2-0.
0030R3-1, 105703-0, 0801N 2
J, 77250 v 2-49.6R4-2, 3794
D4-0.0030R5=0.6294 0
5 = 0.0801 N 3-1.77250
v 3-49.8R6= 4.3345D
6-0.040084-1.62588 v 4-3
5.7R7-1,7665D 7-0.0145R8-
7,3516D 8-0.056185-1.8830
0 v 5-40.8R9= 0.4343 D
9-0.0300 N B-1,61800v 6-6
3.4RIO-0,4511010-0,2602R1
1=-0,3121Dll-0,040087-1,
69895v 7-30. lR12--8,5326
Di2-0.100188-1.77250 v 8-
49.6R13--0,4539Di3-0.0030
RI4--1.7784 DI4=0.0901
N 9-1.77250 v 9-49.8R15--
0,7608Di5-0.0030R16-6,161
1Di6-0.1001 Nl0-1.77250 1
710-49.6RI7- -1.3643 Table 1 (Effects of the invention) According to the present invention, each lens group is configured as described above, and focusing is performed by moving the second group on the optical axis. A photographic camera with an aperture ratio of 1.8 and a relatively wide angle of view of 46 degrees, which enables easy and quick focusing and provides high optical performance over a wide range of object distances, from objects at infinity to close objects. It is possible to achieve an inner focus type photographic lens suitable for cameras, video cameras, and the like.

[Brief explanation of the drawing]

1 to 3 are cross-sectional views of lenses of numerical examples 1 to 3 of the present invention, respectively, and Figures 4 to 6 are lens sectional views of numerical examples 1 to 3 of the present invention, respectively.
3 is a diagram of various aberrations when the object is at infinity. In the cross-sectional view of the lens, I, II, and m indicate the first degree, second degree, and second degree, respectively.
In the aberration diagram of the third group, S is a sagittal image plane, and M is a meridional image plane. No. 1 Gun No. 5 Zuto 4 Monki 6 En

Claims (2)

[Claims] (1) It has three lens groups in order from the object side: a first group with positive refractive power, a second group with negative refractive power, and a third group with positive refractive power, and the second group is located on the optical axis. Move and focus,
The radius of curvature of the lens surface on the image side of the lens 1A closest to the image plane in the first group is R1A, and the refractive index of the material of the lens 1A is N.
1A, the radius of curvature of the lens surface closest to the object side and the closest to the image plane of the second group are R2A and R2B, respectively, the refractive index of the material of the lens 2A closest to the object side of the second group is N2A, the third group The radius of curvature of the lens surface on the object side of the lens 3A closest to the object side is R
3A, the refractive power of the second group and the entire system are φ2, φ, the second group, respectively.
The air distance between the group and the third group when focusing on an object at infinity is D
When 1.8<-φ2/φ<2.1 0.2<D・φ<0.3 2<1/(R2B・φ)<3 2.5<-{1/(R3A・φ)}<3.50<-(1
/φ) {(1-N1A)/(R1A)+(N2A-1)
/(R2A)}<10<1/(R2A·φ)<0.2 An inner focus type photographic lens is characterized in that it satisfies the following condition.