JP2001330771A - Retrofocus type wide-angle lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens whose aperture ratio is small such as 1:1.8 and whose photographing magnification in the case of short distance is about 1:3 by appropriate power arrangement. SOLUTION: This lens is constituted of a 1st group having positive or negative refractive power, a diaphragm, and a 2nd group having positive refractive power in order from an object side, and the 2nd group has a 2a-th group having positive refractive power, a 2b-th group having negative refractive power, and a 2c-th group having positive refractive power. Then, in the case of focusing on a short-distance object from infinity, the 1st group and the 2nd group move to the object side along an optical axis so that a distance between the 1st group and the 2nd group may be reduced, and the lens satisfies a fixed condition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
m It is used for an interchangeable lens for a single-lens reflex camera, a retrofocus wide-angle lens for a still video camera, and a video camera.

[0002]

2. Description of the Related Art Various types of retrofocus (reverse telephoto) lenses, which are preceded by a lens unit having a negative refractive power, have been proposed as a wide-angle shooting lens having a long back focus. However, since the retrofocus lens has an asymmetric lens configuration with a negative refractive power in the front group and a positive refractive power in the rear group, focusing from an object at infinity to an object at a short distance is possible. Since the change in the ray height of the off-axis light beam to the lens system increases, the amount of occurrence of various aberrations tends to increase. For this reason, in a retrofocus type lens, various focus methods are used in order to correct aberration fluctuation at the time of focus. For example,
In Japanese Patent Application Laid-Open No. 61-90116, when focusing from infinity to a close distance with an angle of view of about 95 ° and an aperture ratio of about 1: 2.8, the distance between the first lens group and the second lens group is changed while changing both distances. A retrofocus lens that moves a lens group to the object side is disclosed. Also, JP-A-7-359
In Japanese Patent No. 74, when focusing from infinity to a short distance with an angle of view of about 84 ° and an aperture ratio of about 1: 1.4, a retrofocus lens that fixes the first lens group and moves only the second group to the object side. Is disclosed.

[0003]

In the prior art,
JP-A-61-90116 discloses that the aperture ratio is 1: 2.8.
And big. Further, Japanese Patent Application Laid-Open No. 7-35974 has a small aperture ratio of 1: 1.4, but has problems such as low photographing magnification at a short distance.

According to the present invention, the aperture ratio is as small as 1: 1.8,
It is an object of the present invention to provide a lens having a photographing magnification at a short distance of about 1: 3 by appropriate power arrangement.

[0005]

According to the present invention, a first lens unit having a positive or negative refractive power, a stop, and a second lens unit having a positive refractive power are arranged in this order from the object side. The second lens unit has a positive refractive power. Power 2a
A group, a second group b having a negative refractive power, and a second group c having a positive refractive power. When focusing on an object at a short distance from infinity, the distance between the first and second groups is reduced. The first and second lens units move toward the object side along the optical axis, and the above object is achieved by satisfying the following conditions.

(1) 0.0 <| f / f ₁ | <0.25 (2) 0.5 <Δd ₁ / Δd ₂ <0.75 where f ₁ is the focal length f of the first lens unit. The focal length Δd ₁ of the entire lens system is a moving amount Δd ₂ of the first group when focusing is performed at a short distance from infinity to a photographing magnification of about 1: 3. The amount of movement of the second group when focusing

The first lens group has a first refractive power 1a.
It is preferable that the zoom lens includes a first lens unit, a first lens unit having a negative refractive power, and a first lens unit having a positive refractive power. (3) 0.6 <D _1c /f<0.8 (4) 1.15 <f ₂ / f _2a <1.40 where D _1c is the sum of the center thicknesses of the first group c and f ₂ is The focal length f _2a of the second group is the focal length of the second group.

Further, the first and second lens units have an aspheric surface, and preferably satisfy the following conditions. (5) 0.020 <Δx _H / H _max <0.040 where Δx _H is the amount of deviation from the aspherical paraxial spherical surface in the direction of the optical axis around the effective diameter H _max is the aspherical ray height

[0009]

The conditional expression (1) defines the refractive power of the first lens unit, and is a condition for correcting aberration fluctuation during focusing, mainly spherical aberration. If the lower limit is exceeded, the spherical aberration will be undercorrected. If the upper limit is exceeded, spherical aberration will be overcorrected.

Conditional expression (2) defines the moving ratio between the first and second lens units, and is a condition for correcting astigmatism in a close range. Generally, for retrofocus lenses,
When taking a picture at a short distance, a method of simultaneously moving all the optical systems to perform focusing greatly corrects astigmatism. If focusing is performed to reduce the distance between the first and second groups, astigmatism that has been overcorrected can be satisfactorily corrected. Here, when the value goes below the lower limit, astigmatism is insufficiently corrected, and the amount of movement of the lens unit increases, which is not preferable. If the upper limit is exceeded, astigmatism will be overcorrected, and a good image cannot be obtained at a short distance.

Conditional expression (3) defines the sum of the center thicknesses of the first lens subunit, and a thick positive lens is necessary for correcting negative distortion and field curvature. Here, if the lower limit is exceeded, a sufficient aberration correction effect cannot be obtained. Exceeding the upper limit is not preferable because the overall length of the lens becomes long.

Conditional expression (4) defines the refractive power of the second lens unit. If the lower limit of the conditional expression (4) is exceeded, the effective diameter of the second lens unit becomes large, and the lens barrel becomes large. If the ratio exceeds the upper limit, the sensitivity of the lens increases, which is not preferable in manufacturing. Also, since the effective diameter of the lens close to the image plane increases,
This will hinder the lens barrel design.

Conditional expression (5) defines the aspherical shape. If the aspherical shape of the first lens unit falls below the lower limit, the effect of the aspherical surface is weakened, and the astigmatism is sufficiently corrected. Can not do. If the upper limit is exceeded, the astigmatism will be overcorrected, and the productivity will deteriorate due to the drastic change in the aspherical shape. If the aspherical shape of the second lens subunit is below the lower limit, the effect of the aspherical surface is weakened, and spherical aberration cannot be sufficiently corrected. If the upper limit is exceeded, the marginal spherical aberration falls in the positive direction, so that the performance deteriorates. Further, the sensitivity is increased, which is not preferable in manufacturing.

More preferably, by satisfying the following conditions, the amount of movement of the first lens group when focusing on a short-distance object can be limited, and the burden on the lens barrel design can be reduced. (6) −0.25 <f / f ₁ <0.0 where f ₁ is the focal length of the first lens unit f is the focal length of the entire lens system

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Numerical examples 1, 2, and 3 of the retrofocus wide-angle lens according to the present invention will be described below. 1 is a sectional view of a lens of Numerical Embodiment 1, FIG. 2 is a sectional view of a lens of Numerical Embodiment 2, FIG. 3 is a sectional view of a lens of Numerical Embodiment 3, and FIG. FIG. 5 is an aberration diagram at the time of focusing, and FIG. 5 is an aberration diagram at the time of short-distance focusing with a photographing magnification of 1: 2.6 in Numerical Example 1 of the present invention. FIG. FIG. 7 is an aberration diagram at the time of close-up focusing at a photographing magnification of 1: 2.6 in Numerical Example 2 of the present invention, and FIG. 8 is an aberration diagram at infinity focusing in Numerical Example 3 of the present invention. FIGS. 9A and 9B are aberration diagrams at the time of close-up focusing at a photographing magnification of 1: 2.6 in Numerical Example 3 of the present invention.

In the numerical examples, f is the focal length, Fn
o is the F number, 2ω is the angle of view, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing, n is the d-line refractive index of each lens, and ν is the Abbe number.

The shape of the aspherical surface is as follows: R is the radius of curvature at the center of the lens surface, H is the height from the optical axis, A is the cone coefficient, and A ₂ , A ₄ , A ₆ , A ₈ , It denotes the a ₁₀ by the following formula when the respective aspherical coefficients.

(Equation 1)

Numerical Example 1 f = 27.10 Fno = 1.86 2ω = 77.2

[0019]

Aspherical surface coefficients of five surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.39371E-05 A6 = 0.10779E-07 A8 = -0.97950E-11 A10 = 0.52670E-13

Aspherical surface coefficients of 15 surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.13651E-04 A6 = -0.38380E-08 A8 = 0.60900E-11 A10 = -0.58850E-14

[0022]

Conditional expression (1) 0.00 (2) 0.64 (3) 0.64 (4) 1.31 (5) 5 planes = 0.033 15 planes = 0.031 (6) 0.00

Numerical Example 2 f = 24.69 Fno = 1.85 2ω = 82.44

[0025]

Aspherical surface coefficients of five surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.38440E-05 A6 = 0.10581E-07 A8 = -0.99037E-11 A10 = 0.50550E-13

Aspherical surface coefficients of 15 surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.13754E-04 A6 = -0.36849E-08 A8 = 0.26087E-11 A10 = 0.39670E-14

[0028]

Conditional expression (1) 0.08 (2) 0.62 (3) 0.72 (4) 1.25 (5) 5 planes = 0.027 15 planes = 0.033 (6) -0. 08

Numerical Example 3 f = 27.27 Fno = 1.87 2ω = 76.8

[0031]

Aspherical surface coefficients of five surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.13080E-05 A6 = 0.97820E-08 A8 = -0.97950E-11 A10 = 0.52670E-13

Aspherical surface coefficients of 15 surfaces A = 1.0 A2 = 0.00000E + 00 A4 = 0.13581E-04 A6 = -0.38380E-08 A8 = 0.60900E-11 A10 = 0.10920E-13

[0034]

Conditional expression (1) 0.05 (2) 0.72 (3) 0.71 (4) 1.37 (5) 5 planes = 0.018 15 planes = 0.032 (6) Not satisfied

[0036]

As described above, according to the structure of the present invention, it is possible to obtain a lens having a bright aperture ratio of 1: 1.8 and a photographing magnification at close range of about 1: 3.

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 of the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 2 of the present invention.

FIG. 3 is a lens cross-sectional view of Numerical Example 3 of the present invention.

FIG. 4 is an aberration diagram at the time of focusing on infinity in Numerical Example 1 of the present invention.

FIG. 5 is a photographing magnification 1: in Numerical Embodiment 1 of the present invention.
It is an aberration figure at the time of short-distance focusing of 2.6.

FIG. 6 is an aberration diagram at the time of focusing on infinity in Numerical Example 2 of the present invention.

FIG. 7 shows a photographing magnification of 1: in Numerical Embodiment 2 of the present invention.
It is an aberration figure at the time of short-distance focusing of 2.6.

FIG. 8 is an aberration diagram at the time of focusing on infinity in Numerical Example 3 of the present invention.

FIG. 9 shows a photographing magnification of 1: in Numerical Embodiment 3 of the present invention.
It is an aberration figure at the time of short-distance focusing of 2.6.

Claims (3)

[Claims] 1. A first positive or negative refractive power in order from the object side.
A second group having a positive refractive power, a second group having a positive refractive power, a second group having a negative refractive power, and a second group having a positive refractive power. When focusing on an object at a short distance from infinity, the distance between the first unit and the second unit is reduced,
A first lens group and a second group move to the object side along the optical axis and satisfy the following conditions. (1) 0.0 <| f / f ₁ | <0.25 (2) 0.5 <Δd ₁ / Δd ₂ <0.75 where f ₁ is the focal length of the first lens unit f is all lenses The focal length Δd _{1 of the} system is the moving distance Δd ₂ of the first lens group when focusing at a short distance from infinity to about 1: 3 magnification, and the focusing distance Δd ₂ is focusing at a short distance from about infinity to about 1: 3 magnification. Movement amount of the second group at the time 2. The first group includes a first group having a positive refractive power, a first group having a negative refractive power, and a first group having a positive refractive power.
The retrofocus wide-angle lens according to claim 1, wherein the following condition is satisfied. (3) 0.6 <D _1c /f<0.8 (4) 1.15 <f ₂ / f _2a <1.40 where D _1c is the sum of the center thicknesses of the first group c and f ₂ is The focal length f _2a of the second group is the focal length of the second group. 3. The first lens group and the second lens group have an aspheric surface, and satisfy the following conditions.
Or a retrofocus wide-angle lens according to 2. (5) 0.020 <Δx _H / H _max <0.040 where Δx _H is the amount of deviation from the aspherical paraxial spherical surface in the direction of the optical axis around the effective diameter H _max is the aspherical ray height