JPH0621897B2 - Shooting lens - Google Patents

Description

The present invention relates to a taking lens having a three-lens structure, and further to 3
The present invention relates to a photographing lens having a diaphragm behind a single lens.

The taking lens consisting of three lenses, positive, negative and positive, is one of the best known photographic lenses.

The rear diaphragm type lens does not have to be moved when the lens is extended and the rear diaphragm type lens does not have to be moved. Can be a camera.

However, compared to a lens that has a diaphragm between the lenses, the off-axis light flux passes around the front lens, which causes coma aberration.
In particular, the underlined coma flare is large, and the image is often flared from the middle of both sides to the periphery.
This tendency is particularly significant when the length is shortened and compactness is promoted.

(Object) An object of the present invention is to remove flare generated from the middle of the screen to the periphery of the rear diaphragm lens to obtain an image with good contrast. In order to achieve this objective, a biconvex positive lens and a diaphragm are arranged in this order on the image side of the multiple lenses, and the positive lens is composed of a gradient index lens whose refractive index changes in the optical axis direction. The gradient dN (x) / dx of the refractive index distribution in the optical axis direction at the image-side surface apex of the positive lens when the focal length is normalized to 1 is determined so as to satisfy the following formula.

-3 <dN (x) / dx <-0.2 (Explanation of Example) The optical operation of the example will be described with reference to FIG. (A) shows the behavior of off-axis rays passing through the lens, and (B) shows the refractive index distribution in the third lens. N (N) is the refractive index, x is the coordinate on the optical axis, and the traveling direction of the light ray is positive. In FIG. 1 (A), 1 is a positive first lens having a surface having a strong curvature (compared to the other surface) to the object side, and 2 is a negative first lens having a surface having a strong curvature to the image side. The second lens 3 is a biconvex third lens.

In the case of the rear diaphragm lens, the underline of the off-axis ray is largely bent downward on the object-side curved surface of the second lens 2 and causes coma flare. Although slightly corrected by the image side curved surface of the third lens 3, due to insufficient correction, the result remains as flare like the ray 4 '. In this example, the positive first lens 1 having a strong curvature convex from the object side to the object side, the second negative lens 2 having a strong curvature concave to the image side, and the third lens 3 having a positive biconvex shape The underlined coma flare is corrected by using, as the third lens 3, a gradient index lens in which the refractive index decreases as it advances toward the image side in the optical axis direction.

That is, when the image side surface of a material whose refractive index becomes smaller as it goes to the image side in the optical axis direction is given a curvature, the refractive index of the curved surface is larger than that near the optical axis, and the refractive index of the outer peripheral portion is the same. Even a light beam incident at an angle can be bent strongly. Therefore, the underlined coma flare like the ray 5'is corrected.

Therefore, if the inclination of the refractive index distribution in the optical axis direction at the image-side surface apex of the third lens 3 is made negative, the above-mentioned correction effect is enhanced, but if it is made too large, spherical aberration occurs underneath. Can not be corrected in terms of.

Therefore, when the end point distance of the entire system is normalized to 1, the inclination of the refractive index distribution in the optical axis direction at the image-side surface vertex of the third lens It is decided to satisfy the condition of.

If the inclination of N (X) becomes larger than the lower limit value, there is a correction effect for the underline flare correction of the off-axis light beam, but spherical aberration occurs largely in the under direction and correction becomes impossible.
When the inclination of N (X) becomes smaller than the upper limit, the underline flare of the off-axis light beam is uncorrected or promoted.

Although the object of the present application is realized by the above configuration, it is preferable to satisfy the following condition 1 or 2 in order to further refine the image quality.

Here, R6 is the radius of curvature of the image side surface of the third lens, R3 is the radius of curvature of the object side surface of the second lens, and R4 is the radius of curvature of the image side surface of the second lens.

(1) defines the curvature of the image side surface of the third lens, and if it exceeds the upper limit, spherical aberration cannot be completely corrected and the total length is increased. If it goes below the lower limit and becomes loose, the coma flare off axis will not be corrected sufficiently.

The condition (2) relates to correction of coma flare, spherical aberration, and astigmatism of the off-axis light beam generated by the second lens,
When the R3 surface exceeds the upper limit and becomes negative with a negative value, coma flare occurs on the R3 surface. On the other hand, if the lower limit is exceeded and the R4 surface becomes tight, astigmatism occurs excessively.

Examples of lens data will be shown below, but the third lens of the first to fourth examples has a characteristic that the refractive index from the object side lens surface to the image side lens surface continuously decreases. The fifth example is 1.17 from the object side surface of the third lens.
The refractive index is 1.834 up to mm, but between 1.17 mm and 4.27 mm, the refractive index distribution is such that the refractive index decreases toward the image side.

For example, when an ion exchange method is used to obtain a refractive index distribution, such a distribution may be obtained by performing ion exchange from one surface and stopping the ion exchange operation when the distribution reaches a desired depth. Also in this example, the same effects as those of the first to fourth examples can be obtained.

Example 1 (Figs. 2 and 3) Example 2 (FIGS. 4 and 5) Example 3 (FIGS. 6 and 7) Example 4 (Figs. 8 and 9) Example 5 (Figs. 10 and 11) (Effect) As described above, the biconvex positive lens just before the diaphragm of the rear diaphragm type lens uses the gradient index lens whose refractive index changes in the optical axis direction, and the negative refraction at the vertex of the image side surface of the positive lens. By providing the rate gradient, it is possible to correct the coma flare of the off-axis light beam underline peculiar to the rear diaphragm lens due to the divergence surface immediately before the positive lens, and it is possible to obtain a compact and high-contrast image.

[Brief description of drawings]

FIG. 1 is an optical sectional view according to the present invention. 2 is a lens cross-sectional view of Example 1, and FIG. 3 is an aberration diagram. FIG. 4 is a lens cross-sectional view of Example 2, and FIG. 5 is an aberration diagram. FIG. 6 is a lens cross-sectional view of Example 3, and FIG. 7 is an aberration diagram. FIG. 8 is a lens cross-sectional view of Example 4, and FIG. 9 is an aberration diagram. FIG. 10 shows Example 5
11 is an aberration diagram of the lens cross section of FIG.

Claims (1)

[Claims] 1. A positive first lens having a surface having a strong curvature convex from the object side to the object side, a second negative lens having a surface having a strong curvature concave to the image side, a third lens biconvex, and a diaphragm. And the third lens is a gradient index lens in which the refractive index changes in the optical axis direction, and the focal length of the entire system is normalized to 1, the image side of the third lens A photographic lens characterized in that the aberration is corrected by setting the inclination dN (x) / dx of the refractive index distribution in the optical axis direction at the apex of -3 to satisfy -3 <dN (x) / dx <-0.2.