JPH09288234A - Photographic lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a retrofocus type photographic lens which is excellent in an image forming function, and which is made compact and suitable to a visual telephone and a monitor camera. SOLUTION: The photographic lens is constituted of a 1st lens group G1 constituted of one negative lens, a 2nd positive lens group G2 constituted of one positive lens and one negative lens and a diaphragm 1 for limiting at least a part of an off-axis luminous flux in this order from an object side, and the lens is constituted so as to satisfy the following conditional expressing; 50<=υ2<=150, in this case, a value of υ2 satisfies (ϕ2P+ϕ2N)/υ2=(ϕ2P/υ2P)+(ϕ2 N/υ2N), provided that ϕ2P denotes the power of the positive lens of the 2nd lens group G2, υ2P denotes Abbe's number, ϕ2N denotes the power of the negative lens of and 2nd lens group G2 and υ2N denotes Abbe's number.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type photographing lens, and more particularly to a videophone, a surveillance camera,
The present invention relates to a photographing lens suitable for a camera for a personal computer video conference.

[0002]

2. Description of the Related Art Photographing lenses used for videophones, surveillance cameras, etc. have a wide angle of view, have a good telecentricity because a CCD is used as a light receiving element, have a good imaging performance, and are inexpensive and compact. Etc. are required.

In order to meet these requirements, a retrofocus type lens having a negative first lens having a strong refractive power is suitable, and an inexpensive lens having a three-lens structure is disclosed in Japanese Patent Laid-Open No. 7-333499. Lenses described in JP-A-6-148518 and the like are known.

[0004]

However, the above-mentioned lens includes the first lens group composed of a negative lens having a strong refractive power, and the second lens group composed of a positive lens and a negative lens.
It is inevitable that a negative first lens, which is composed of a diaphragm located in the vicinity of the second lens group, causes lateral chromatic aberration in which the image height of a short wavelength becomes small.

It is an object of the present invention to propose a retrofocus type photographing lens which satisfies the above-mentioned requirements and is particularly inexpensive and has excellent lateral chromatic aberration.

[0006]

SUMMARY OF THE INVENTION The above-mentioned problem is solved by the second lens element of the first lens group consisting of one negative lens element, one positive lens element and one negative lens element in order from the object side. This is solved by a photographing lens that includes a lens group and a diaphragm that limits at least a part of off-axis light flux, and that satisfies the following conditional expression.

50 ≦ ν2 ≦ 150 where ν2 is the power of the positive lens of the second lens group ψ2
P, Abbe number is ν2P, the negative lens power of the second lens group is ψ2N, and Abbe number is ν2N, then (ψ2P + ψ2N) / ν2 = (ψ2P / ν2P) + (ψ
2N / ν2N).

According to the invention of the present application, a chromatic aberration of magnification caused in the first lens group having a negative refractive power is provided by providing a diaphragm for limiting at least a part of off-axis light flux on the image side of the second lens group.
It is reduced by the second lens group having a positive refractive power and an equivalent ν value in an appropriate positive range. Of course, this diaphragm may be a so-called post diaphragm that restricts not only the off-axis light beam but also the on-axis light beam.

The conditional expression is a conditional expression relating to the equivalent ν value of the second lens group. If the lower limit of the conditional expression is not satisfied, the axial chromatic aberration will be largely under. If the upper limit is deviated, the effect on the lateral chromatic aberration is small.

Further, if the following conditional expression is satisfied, a greater effect can be obtained.

60 ≦ ν2 ≦ 90 The aperture diameter of the diaphragm is invariable, and it is desirable that the following conditional expression is satisfied.

1.0 ≦ rs / hs ≦ 1.3 1.0 ≦ d / f ≦ 3.0 where rs: aperture radius of diaphragm hs: radius of axial light flux at diaphragm d: first lens group and first lens group On-axis distance between two lens groups f: focal length of the entire system The conditional expression is a desirable conditional expression that determines an appropriate aperture radius when the aperture diameter of the diaphragm is unchanged, and the lower limit is obvious and the upper limit is If it is out of the range, the effect on the chromatic aberration of magnification is small.

If this value is other than 1, a separate diaphragm for determining the F number is required in front of this diaphragm.

The conditional expression defines a desirable distance between the first lens group and the second lens group. If the lower limit is not satisfied, the refracting power of the first lens group becomes strong and the distortion is deteriorated, or the back focus is reduced. Becomes shorter and the telecentricity deteriorates. Also, if the upper limit is exceeded, compactness is impaired.

Further, if the following conditional expression is satisfied, a greater effect can be obtained.

1.4 ≦ d / f ≦ 2.5

[0017]

EXAMPLES The following will describe four examples of photographing lenses according to the present invention.

The meanings of the reference numerals in each embodiment are as follows.

F: Focal length F: F number ω: Half angle of view r: Radius of curvature d: Distance between surfaces n: Refractive index for d line ν: Abbe number for d line Further, in the lens data table, "*" The lens surface shown by is an aspherical surface, where the optical axis direction is the x axis, the direction orthogonal to the optical axis is the y axis, and K, Ai, and Pi are the aspherical surface coefficients, which can be expressed by the following equation.

[0020]

[Equation 1]

In the sectional view of the lens, G1 is the first
A lens group, G2 is a second lens group, 1 is a diaphragm, 2 is a filter, and 3 is a cover glass.

Example 1 f = 4.11, F = 2.8, 2ω = 61.4 ° Lens data are shown in Table 1.

[0023]

[Table 1]

However, the diaphragm is located 0.50 on the image side of the fifth surface.

Table 2 shows the aspherical surface coefficients.

[0026]

[Table 2]

Further, a sectional view of the lens is shown in FIG. 1, and an aberration diagram is shown in FIG.

Example 2 f = 4.04, F = 2.8, 2ω = 61.8 ° Table 3 shows lens data.

[0029]

[Table 3]

However, the diaphragm is located 0.50 on the image side of the fifth surface.

Table 4 shows the aspherical surface coefficients.

[0032]

[Table 4]

Further, a sectional view of the lens is shown in FIG. 3, and an aberration diagram is shown in FIG.

Example 3 f = 4.10, F = 2.8, 2ω = 62.0 ° Lens data are shown in Table 5.

[0035]

[Table 5]

However, the stop is at the position of 0.20 on the image side of the sixth surface.

Table 6 shows the aspherical surface coefficients.

[0038]

[Table 6]

Further, a sectional view of the lens is shown in FIG. 5, and an aberration diagram is shown in FIG.

Example 4 f = 4.09, F = 2.8, 2ω = 61.2 ° Lens data are shown in Table 7.

[0041]

[Table 7]

However, the diaphragm is located at a position of 0.50 on the image side of the fifth surface.

Table 8 shows the aspherical coefficients.

[0044]

[Table 8]

Further, a sectional view of the lens is shown in FIG. 7, and an aberration diagram is shown in FIG.

In each of the above embodiments, the values corresponding to the conditional expressions 1 to 5 are as shown in Table 9 below.

[0047]

[Table 9]

[0048]

According to the first to third aspects of the present invention, it is possible to obtain a retrofocus type taking lens which is excellent in image forming performance such as lateral chromatic aberration, compact, and suitable for videophones and surveillance cameras.

[Brief description of drawings]

FIG. 1 is a sectional view of a lens according to a first embodiment.

FIG. 2 is an aberration diagram of the first embodiment.

FIG. 3 is a sectional view of a lens according to a second embodiment.

FIG. 4 is an aberration diagram of the second embodiment.

FIG. 5 is a sectional view of a lens according to a third embodiment.

FIG. 6 is an aberration diagram for Example 3.

FIG. 7 is a lens cross-sectional view of Example 4.

FIG. 8 is an aberration diagram for Example 4.

[Explanation of symbols]

 G1 First lens group G2 Second lens group 1 Aperture 2 Filter 3 Cover glass

Claims (3)

[Claims] 1. A first lens group consisting of one negative lens in order from the object side, a second lens group of positive lens consisting of one positive lens and one negative lens, and at least an off-axis light beam. And a diaphragm that restricts a part of the above, and satisfies the following conditional expression. 50 ≦ ν2 ≦ 150 where ν2 is the power of the positive lens of the second lens group ψ2
If P and Abbe number are ν2P, the power of the negative lens of the second lens group is ψ2N, and Abbe number is ν2N, then (ψ2P + ψ2N) / ν2 = (ψ2P / ν2P) + (ψ
2N / ν2N). 2. The first lens group and the second lens group are made of synthetic resin.
Shooting lens described in. 3. The diaphragm according to claim 1, wherein an aperture diameter of the diaphragm is invariable and the following conditional expression is satisfied.
Shooting lens described in. 1.0 ≦ rs / hs ≦ 1.3 1.0 ≦ d / f ≦ 3.0 where rs: aperture radius of diaphragm hs: radius of axial light flux at diaphragm d: first lens group and second lens group On-axis distance f: focal length of the entire system