JPS62105112A - Compact photographic lens - Google Patents

Abstract

PURPOSE:To easily execute working, and also to reduce a focal movement caused by a temperature variation, by providing a positive meniscus lens whose convex surface has been turned to an object side, a biconcave lens, a positive meniscus lens whose convex surface has been turned to an image side, a positive lens having a convex surface to the object side, and a diaphragm, and specifying various conditions. CONSTITUTION:The titled lens is constituted of four groups and four pieces consisting of the first lens of a positive meniscus lens whose convex surface has been turned to an object side, the second lens of a biconcave lens, the third lens of a positive meniscus lens whose convex surface has been turned to an image side, the fourth lens of a positive lens having a convex surface to the object side, and a diaphragm, in order from the object side, and conditions shown by inequalities (1)-(2) are set. In this way, a partial charge of power of the third lens is reduced, becomes a strong meniscus, and a Z coefficient becomes small, but since a refractive index is low, plastic can be used. In this way, working is easily executed, and a focal shift caused by a temperature change can also be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a behind-the-aperture photographic lens with a brightness of approximately F3.5, an angle of view of 60° or more, and a telephoto ratio close to 1.0.

[Conventional technology]

Conventionally, the mainstream of compact camera lenses has been the triplet or Tetsusar type, or the telephoto type with four elements in four groups as shown in Japanese Patent Application Laid-open No. 73014/1983. However, the triplet or Tetsusar types generally have a telephoto ratio as high as about 1.1, and the latter has a cost problem because it has an internal aperture. To solve these problems, we developed a behind-the-scenes aperture lens with a telephoto ratio close to 1.0.
A triplet type as shown in Japanese Patent Laid-Open No. 152413/1982 and a four-element type in four groups in which the third lens of the triplet is divided into two are considered. ing.

[Problem that the invention seeks to solve]

However, if the total length of a triplet type lens is shortened, it is difficult to properly correct the image plane, and since the power of each group is strong, the lens system becomes vulnerable to eccentricity, where a slight eccentricity can cause a large change in aberration. , In order to further reduce the Penotsuvial sum, a convex lens with an expensive high refractive index is used.
A problem arises in that it becomes necessary to use a glass with low dispersion. Also, with the 4-group, 4-element type, the 3rd and 4th lenses will have a strong meniscus, resulting in poor workability.
Although processing can be easily done using plastic lenses, a new problem arises in that the focus shifts significantly due to temperature changes.

The present invention has been made by focusing on these problems, and although it is a behind-the-aperture lens with a telephoto ratio close to 1.0, it is easy to process, inexpensive, and has low temperature when made of plastic. It is an object of the present invention to provide a lens whose focus is less likely to shift due to changes.

[Means for solving problems]

As shown in FIG. 1, the lens system based on the present invention includes, in order from the object side, the first lens, the second lens of a positive meniscus lens with a convex surface facing the object side, and the second lens of a positive meniscus lens with a convex surface facing the image side. This lens system consists of a third lens, a fourth lens which is a positive lens having a convex surface on the object side, and four lenses in four groups in which an aperture is arranged, and satisfies the following conditions.

(1) ni > 1.65 (21nt < 1.7 (3) ns < 1.65 (4) na > 1.65 (5) ν1>40 (6) ν2<45 (7) 0.3<β4 < 1.7 (s) Z4 > 0.06 (9) 2f < f, < 5f where: ni + refractive index ν of the i-th lens; Abbe number β of the i-th lens; the fourth lens expressed by the following formula Shape rB -r7 r?+Radius of curvature of the object side of the fourth lens r8; Radius of curvature of the image side of the fourth lens Z4; Z coefficient ER of the fourth lens expressed by the following formula;
The effective radius of the fourth lens is "3; the focal length of the third lens is the focal length of the entire system.

[Effect]

Conditions (1), (21, (41 to (6)) are the conditions for correcting chromatic aberration and keeping the image quality by keeping the Peso-Tsuvual sum to a small value.Conditions (11, (21, (4)
), it becomes difficult to maintain good image plane properties, and when conditions (5) and (61) are exceeded, it becomes difficult to correct chromatic aberration.

Conditions (31, (41) are conditions for keeping the lens shape and power distribution of the third and fourth lenses favorable. If this range is exceeded, the lens shape and power distribution described below become difficult.

In order to realize an inexpensive lens, it is very important to have a lens shape that is easy to process. Here, a lens shape that is easy to process is one that has a gentle curvature and is easy to center.
A larger coefficient is desirable. Therefore, the condition that defines the shape of the fourth lens is (7). If the upper limit of the 0 condition (7), which is (81), is exceeded, the polishing cost will increase, and the distortion accommodation portion will increase in the positive direction, which is undesirable. If the lower limit is exceeded, the fourth lens will have a large power, and due to power distribution requirements, this fourth lens will move closer to the object side, making it difficult to maintain a distance from the third lens. Even if this is the case, the distortion aberration worsens in the negative direction, which is not desirable.

If the range of condition (8) is exceeded, centering of the fourth lens becomes extremely difficult.

In order to satisfy these conditions and obtain a lens system with good field properties and well corrected coma aberration, it is necessary to satisfy conditions (31, (4)) as described above. 3. If you use glass with a low refractive index for both the 4th lens,
The Penzval phase takes on a large positive value, making it difficult to maintain good image plane properties, and conversely, the 3rd phase takes on a large positive value. If glass with a high refractive index is used for both the fourth lens, both lenses will have a strong meniscus, resulting in a lens shape with a small Z coefficient and poor workability. Also, ns > 1.65.

If n = < 1.65, the image plane property can be maintained well, but the third lens will have a large power, the distance between the second lens and the third lens will increase, and the fourth lens will have a large power. This is undesirable because it becomes a strong meniscus with the convex surface facing the object side, and the distance from the aperture becomes wide, making it impossible to obtain a sufficient amount of peripheral light.

It is necessary that the focal length of the third lens falls within the range shown in condition (9); if the lower limit is exceeded, the influence of temperature changes will become greater and defocus will occur. Furthermore, if the upper limit is exceeded, it becomes difficult to maintain a sufficient distance from the second lens, making it difficult to maintain the brightness of F/3.5.

Even if the distance can be maintained, distortion and coma aberration will worsen.

〔Example〕

By doing so, the power shared by the third lens will be reduced, resulting in a strong meniscus, and the Z coefficient will be reduced, but since the refractive index is low, plastic can be used. Therefore, processing problems can be avoided, and since the power is shared less, it is possible to reduce out-of-focus caused by temperature changes in the plastic lens. Furthermore, since the thickness at the center and peripheral parts of the lens is approximately equal, molding of plastic is easy.

An example in which the third lens is made of plastic will be shown below.

Example 1 f-100F/3.5 26+=64”ER*=
11.7 βa-1,3Z4-0.09f3 =
2.8f Telephoto ratio 1.04r+” 28.18
0 d, = 7.396 ni-1,77250vl-
49,66 r t ” 115.148 d, -1,598 r s ” -260,701 ds -5,421 FI2 - 1.64769
yt -33,8Or, -24,839 da "" 4.344 rs -43,377 ds = 2.959 13 = 1.49
216 7/3 - 57.5Or, -33,628 d, -0,444 rt" 59.206 dy -3,254na -1,69350pg
= 50.81ra'' 443.787 Example 2 FIG. 2 is a sectional view showing the lens configuration of the second example.

f-100F/3.6 2ω-646ER,,=1
1.9 da = 0.7 Za = 0.
1f Yu = 3.9f Telephoto ratio 1.06 r I -26,896 d, -7,396J = 1.72000 ν1
= 50.25r t = 109.373 dt ” 1.657 r 3 = 205.918 ds -3,550nt -1,625881/i
-35,70r4- 24.953 d4- 4.793 r, = -34,916 ds = 2.663 13 - 1.4921
6 Wz -57,5Or, = -30,23
8 db depth 0.444 r, = 69.930 d7 - 3.254 na = 1.696
80 ya -55,52rs = 419.
178 Example 3 f-100F/3.5 2ω-64″ER* -1
1,7β4-1.6 Z4-0.08fs-2,3
f Telephoto ratio 1.04 ri -27,871 d+ -7,1011+ -1,78800ν+
= 47.38r, = 117.875 d2 - 1.598 r = ” 270.812 d3 - 5.274 nt -1,67270
Wt -32,1Or, -24,806 aa -4,409 r, = -46,944 ds = 2.959 1s = 1.50
430Vs = 57.80rb =
34.254 a, -0,444 rt -58,225 a7- 2.959 na -1,72000v
a = 43.70 r m ”245.562 However, f ; Focal length of the entire system f, ; Focal length of the third lens F ; F number 2ω; Angle of view ER, , ; Effective radius β4 of the fourth lens; The shapes of the fourth lens are represented by r, -r.

r,; Radius of curvature of the object side surface of the fourth lens r8: Radius of curvature of the image side surface of the fourth lens Z4; Z coefficient ERt of the fourth lens expressed by the following formula
; Effective radius rl of the fourth lens ; Radius of curvature di of each surface sequentially from the object side; Thickness and air gap nii of each lens sequentially from the object side; Refractive index ν1 of each lens sequentially from the object side; The Atsube number of each lens is shown in sequence.

〔Effect of the invention〕

3 to 5 are aberration curve diagrams of Examples 1 to 3, respectively. As is clear from these aberration curve diagrams, the present invention provides a compact and inexpensive lens in which each aberration is well corrected. In particular, it is possible to obtain photographs with high contrast over a high image height, and the amount of light at the upper periphery is sufficient, with almost no noticeable distortion. Furthermore, the third
．． The fourth lens can be made resistant to eccentricity. Also, when plastic is used for the third lens, it is less affected by temperature changes.

[Brief explanation of drawings]

FIG. 1 shows the first aspect of the present invention. FIG. 2 is a cross-sectional view of the lens structure of the second embodiment of the present invention, and FIGS. 3 to 5 are aberration curves of the first to third embodiments of the present invention, respectively. It is a diagram.

Claims (1)

[Claims] In order from the object side, the first lens is a positive meniscus lens with a convex surface facing the object side, the second lens is a biconcave lens, the third lens is a positive meniscus lens with a convex surface facing the image side, and the third lens is a positive meniscus lens with a convex surface facing the image side. A compact photographic lens comprising four lenses in four groups in which a fourth lens is a positive lens having a convex surface and an aperture, and satisfies the following conditions. (1) n_1>1.65 (2) n_2<1.7 (3) n_3<1.65 (4) n_4>1.65 (5) ν_1>40 (6) ν_2<45 (7) 0.3 <β_4<1.7 (8) Z_4>0.06 (9) 2f<f_3<5f where, n_i; refractive index ν_i of the i-th lens; Abbe number β_4 of the i-th lens; fourth lens expressed by the following formula Shape factor β_4=(r_8+r_7)/(r_8-r_7)r
_7; Radius of curvature of the object side of the fourth lens r_8; Radius of curvature of the image side of the fourth lens Z_4; Z coefficient of the fourth lens expressed by the following formula Z_4=
(ER_4｜1/r_7-1/r_8｜)/2ER_4
; Effective radius f_3 of the fourth lens; Focal length f of the third lens; Focal length of the entire system.