JPS6051686B2 - Tetsusar type lens for behind aperture - Google Patents

Description

Detailed Description of the Invention The present invention is a modified Tetsusar type for behind aperture that is used in small cameras, etc., has an angle of view of about 600, a brightness FNo. of 2.8, and has various aberrations well corrected. This relates to improvements to lenses.

As is well known, the behind aperture lens used for small cameras has an FN of 0.2.8 to 3 and an angle of view of approximately 600 mm.
There are conventional Tetsusar type lenses of about 100 degrees, and modified Tetsusar type lenses in which the position of the third group lens is changed to positive or negative. Since it is necessary to correct the aberration of the light beam at a fairly large angle of view, coma flare may not be removed sufficiently, the field curvature is large, or the peripheral image, especially the sagittal image, is extremely curved. The shortcomings of the current situation are unavoidable.

In view of the above-mentioned drawbacks, the present invention aims to provide a compact lens system in which various aberrations are well corrected while particularly reducing field curvature. The objective is to eliminate field curvature by making the air distance between the first, second, and third group lenses as small as possible relative to the overall length, and reducing the Petzval sum. be.

A lens system that follows the above concept and is suitable for the above purpose is, as shown in Fig. 1, a biconcave lens with a first group consisting of a positive meniscus lens element when viewed from the object world side! and a third group consisting of a double convex lens L and a meniscus negative lens L4.
It is composed of 4 lenses in 3 groups and has an aperture ratio of F2.
& It is a modified Tetsusar type lens with an angle of view of 600°, where f is the focal length of the entire system, Σd is the axial thickness of the lens,
and the total air distance, 4 is the air distance between the first group lens and the second group lens, the mountain is the air distance between the second group lens and the third group lens, and R3 is the surface of the second group lens facing the object world. radius of curvature, R
5 is the radius of curvature of the surface of the third group lens facing the object world, R6 is the radius of curvature of the cemented surface of lenses L3 and L4 in the third group lens, and F. (!: The river is L3 and L of the 3rd group lens.
It is the curvature index of the d-line at 4.

Hereinafter, the technical background of the present invention and the significance of the above-mentioned conditions will be explained in detail regarding a Tesser-type lens for behind aperture, which is characterized by having the following conditions.

Conventionally, it has been well known that for behind-the-scenes aperture lenses that require an angle of view of 600 degrees, the overall length of the lens is kept as short as possible compared to a case with a normal aperture position in order to prevent insufficient peripheral light, but the present invention Even in
This equation is established as an indispensable precondition.

As mentioned above, we attempted to reduce the Petzval sum by keeping the air spacing as small as possible relative to the entire length of the lens, and also to prevent insufficient light intensity caused by the bending of the optical path of the peripheral light intensity relative to the central ray beam. This is equation (2).

That is, equation (1) is FNO. 2.8 regulates the total lens length required to correct various aberrations, and if the lower limit is exceeded, FNO. It becomes impossible to correct spherical aberration as 2.8, and if the upper limit is exceeded, an insufficient amount of light for the behind aperture becomes inevitable.

(2) Formula 4! The lower limit of U is a mechanical limit value for narrowing the air gap, and the upper limit is a limit value for measuring the reduction of the Petzval sum and suppressing insufficient light quantity.

In order to further promote the above-mentioned purpose and to make it effective, the following conditions were established.

This means that in order to reduce the air gap between lenses L1 and L2, it is desirable that R3 be made loose, and furthermore, this will suppress the occurrence of coma flare of the outer peripheral rays on this surface with a large angle of view, and will also reduce the sagittal image. It helps prevent falling over.

That is, the lower limit of equation (3) is a limit value for achieving the above-mentioned purpose, and the upper limit is for regulating negative spherical aberration, field curvature, and the like.

Also, in order to reduce the air gap between the lens L and
It is convenient for R5 to become tighter, and furthermore, by making R7 looser, the spherical aberration generated by R7 is reduced, thereby reducing the spherical aberration that is insufficiently corrected by R3. This contributes to the positive correction of . Furthermore, the upper limit of equation (4) is the limit value for achieving the above objective, and the lower limit is the negative coma of the outer peripheral luminous flux on this surface due to the large angle of view caused by extremely tight R5. This is to suppress the occurrence of aberrations. Furthermore, in order to further promote the purpose of reducing the curvature of field, the condition of equation (5) was provided, and in order to make this effective, the condition of equation (6) was provided. That is, the lower limit of equation (5) is a limit value for achieving the above objective, and the upper limit is a limit value resulting from the chromatic aberration correction conditions for the entire system.
When N3 and n4 are under such conditions, R6 is (
6) It is desirable that the range is within the range of the formula; if the lower limit is exceeded, the spherical aberration will become too negative and cannot be corrected in the outer part, and the upper limit is the opposite and the correction of higher-order spherical aberrations that act positively is possible. It becomes impossible. Among other conditions mentioned above, (3)
, (4), and (6) are useful for shortening the lens back, and the total length of the lens can be made relatively short.

Next, several specific embodiments based on the present invention will be described. However, the above Seidel aberration coefficients are calculated when the object distance is set to infinity and the aperture is set at a position of 0.024 behind the seventh surface of the lens. In the table, S is a coefficient representing spherical aberration, C is comatic aberration, A is astigmatism, P is a Petzval term, D is a coefficient representing distortion, and Σ represents the sum of each aberration coefficient.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of a Tessa type lens for behind aperture according to the present invention, and FIG. 2 is a diagram of each aberration curve. Ll, L2, L3, L4: Lens, Rl, R2,...
R7: radius of curvature, Dl, d3, d5, d6: thickness on the axis of the lens, D2, d4: air distance on the axis of the lens.

Claims (1)

[Claims] 1. Viewed in order from the object world side, the first lens group consists of a positive meniscus lens L_1, and the second group consists of a biconcave single lens L_2.
It is composed of 4 lenses in 3 groups of the 3rd group lens, which is made up of a cemented double-convex lens L_3 and a meniscus negative lens L_4, and has an aperture ratio of F2.8, an angle of view of 60°, and is a modified Tetsusar type lens. , (1) 0.34f<Σd
<0.38f (2) 0.05f<d_2+d_4<0.07f0.1
4<(d_2+d_4/Σd)<0.20(3)1.4
f<|R_3|<3.0f (however, R_3<0) (4) 0
．． 5f<R_5<0.73f(5)0.16<n_3-
n_4<0.25(6)0.3f<|R_6|<0.3
7f (However, R_6<0) "However, f is the focal length of the entire system,
Σd is the sum of the axial thickness of the lens and the air spacing, d
_2 is the first group lens and second group lens, d_4 is the second group lens.
The air distance between the group lens and the third group lens, R_3 is the radius of curvature of the surface of the second group lens facing the object world, R_5 is the radius of curvature of the surface of the third group lens facing the object world, and R_6 is the radius of curvature of the surface of the third group lens facing the object world. This is the radius of curvature of the cemented surface of the lenses L_3 and L_4 in the lens group, and n_3 and n_4 are the d-line curvature of the lenses L_3 and L_4 of the third lens group. A Tetsusar type lens for behind aperture, which is characterized by meeting the following conditions.