JPH01188813A - Tessar type lens with diaphragm at the back - Google Patents

Abstract

PURPOSE:To sufficiently secure the end thickness, axial core thickness, etc., of a lens and attain the satisfactory aberration correction by being composed of three groups four sheets to stick a third lens and a fourth lens and satisfying special conditions. CONSTITUTION:The title lens is composed of the first lens of a positive lens to direct a concave surface from an object side to an object side, the second lens of both concave lenses, the third lens of a negative meniscus lens to direct a convex surface to the object side and the fourth lens of both convex lenses and composed of three groups four sheets to stick the third lens and the fourth lens. The conditions of expressions I-III are satisfied. In the expressions I-III, (f) means the synthesizing focus distance of the whole system, SIGMAd means the distance from the peak point of the object side lens surface of the first lens up to the peak point of the lens surface at, the image side of the fourth lens, (r) shows the curvature radius of respective lens surfaces and a subscript means the sequence from the object side. Thus, a respective lenses have a sufficient thickness, respective aberrations are balance-corrected and the easy-to-make and highly practical lens can be obtained while they are compact to the extent suitable to use for a small video camera.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a Tessar type lens, particularly a post-diaphragm type lens.

(Conventional technology) Generally used as a 35nn+ film camera lens.

As cameras became more compact, lenses were also required to be more compact, and in order to achieve this, a type in which the aperture was placed at the rear of the lens was often used.
There are many examples such as JP-A No. 102914, JP-A-59-18916, and JP-A-59-136712.

However, when this type of lens is used in a video camera or still video camera with a small screen size, the overall length of the lens system must be kept as short as possible to prevent a decrease in peripheral illumination, and to do so, the lens edge thickness and The axial core thickness becomes thinner.

Processing problems occur.

(Problem to be solved by this invention) This invention is F2
．． This relates to lenses for use with video cameras and still video cameras of about 800mm, and even when the overall length of the lens is shortened to prevent a decrease in peripheral light intensity, sufficient edge thickness and axial center thickness of the lens should be ensured. The objective is to obtain a Tesser-type lens placed after the aperture which has good aberration correction.

(Means for solving the problem) The Tetsusar type lens after the aperture of the present invention includes, from the object side, a first lens that is a positive lens with a convex surface facing the object side, a second lens that is a biconcave lens, and a second lens that has a convex surface facing the object side. It consists of a third lens that is a negative meniscus lens that is oriented toward the lens, and a fourth lens that is a biconvex lens. It is characterized by

(1) 0.6f<Σd (0,75f(2
) 0.1 < (r, -r4) / (r, +r
4) <0.25(3) 0.25f<r6<0
．． 35f Here, f is the composite focal length of the entire system, and Σd is the first
The distance from the apex of the object-side lens surface of the lens to the apex of the image-side lens surface of the fourth lens, r represents the radius of curvature of each lens surface, and the subscripts mean the order from the object side.

Furthermore, (4) 0.48 < r, /f (0,52(5)
0.07 < d, /f < 0.1 (6) 0
．． 6 < r2/f < 1.0 (7) 0.04
< d, /f < 0.07 (8) 1.8 <
r3/f<1.2(9) 0.28<
It is desirable to satisfy (ds+ds)/f<0-33. Here d2, d4. d, d6 represent the air distance between the first lens and the second lens, the air distance between the second lens and the third lens, the axial thickness of the third lens, and the axial thickness of the fourth lens, respectively. .

(Function) In a post-diaphragm Tetsusar type lens, the overall length of the lens system is generally shortened to prevent a decrease in peripheral light intensity.

However, in the case of a video camera with a small screen size, when reducing the overall length of the lens system, it becomes difficult to correct various aberrations due to processing limitations such as the edge thickness and axial center thickness of the lens.

In this invention, the above-mentioned problems are solved by making the shape of the air lens between the second lens and the third lens different from the usual one, and by making the curvature of the bonding surface of the third group lens tighter than that of the conventional one. .

Each condition will be explained below.

Conditional expression (1) is a condition that limits the total length of the lens system, and the lower limit of this expression is provided to ensure the edge thickness, axial center thickness, etc. of the lens. Moreover, if the upper limit of this formula is exceeded, the total length becomes long, which hinders the compactness of the lens system and causes a significant decrease in the amount of peripheral light.

Conditional expression (2) expresses the shape characteristics of the air lens between the second lens and the third lens. When the lower limit is exceeded,
When the curvature of field becomes under-corrected and exceeds the upper limit, the spherical aberration becomes under-corrected and the curvature of field becomes over-corrected.

Conditional expression (3) relates to the curvature of the bonded surface of the third group lens, and when the upper limit is exceeded, the Petzval sum increases, thereby increasing the curvature of field. Moreover, when the lower limit is exceeded, spherical aberration becomes insufficiently corrected.

Conditional expression (4) relates to the refractive power of the object-side lens surface of the first lens; if the upper limit is exceeded, the refractive power becomes too weak and the back focus becomes long, making it difficult to make the lens system compact. Moreover, when the lower limit is exceeded, spherical aberration becomes insufficiently corrected.

Conditional expression (5) relates to the air gap between the first lens and the second lens, and if the upper limit is exceeded, coma aberration will be insufficiently corrected and the lens diameter of the first lens will increase. Moreover, when the lower limit is exceeded, the intersection point between the image side surface of the first lens and the object side surface of the second lens becomes low, and as a result, the lens diameter must be made small, leading to a decrease in the amount of peripheral light.

Conditional expression (6) relates to the refractive power of the lens surface on the image side of the first lens; when the upper limit is exceeded, the field curvature increases, and when the lower limit is exceeded, the astigmatism increases in the positive direction.

Conditional expression (7) relates to the air gap between the second lens and the third lens, and if the upper limit is exceeded, distortion will be insufficiently corrected, and if the lower limit is exceeded, it will be difficult to properly correct coma aberration. The following six conditional expressions (8) are for correcting astigmatism, and when the upper limit is exceeded, the astigmatism increases in the positive direction.

When the lower limit is exceeded, astigmatism increases in the negative direction.

Conditional expression (9) relates to the thickness of the third group lens, and if the upper limit is exceeded, the distance from the first surface to the final surface of the lens becomes long, which is not preferable in terms of securing the required back focus. Moreover, when the lower limit is exceeded, negative astigmatism becomes large.

(Example) Examples of the lens system of the present invention will be shown below. Here, f is the focal length of the entire system, r is the radius of curvature of each lens surface, d is the lens thickness or lens spacing, n is the refractive index, and ν is the Abbe number. In addition, since a cover glass was attached to the image side of the lens system, the numerical value of the cover glass is also shown in the table.

Example 1 f=100 F No.=2.8Ha
r d n vl
50.034 18.26 1.79952 42.2
2 75.463 8.923
-144,876 s, 95 1.60342
38.04 35.955 4.2
55, 52.593 7.22 1
．． 75520 27.56 31.727
23.78 1.77250 49.67
-82.492 17.848 0
0 51.45 1.51633 64.1
9 Q Example 2 f=100 F No,=2.8Nα r
d n ν1 49
．． 799 18.69 1.79952 42.22
79.764 8.07 3 -160.065 5.95 1.6034
2 38.04 35.010 5.52 5 57.972 5.95 1.74077
27.86 33.147 23.78 1.
77250 49.67 -83.679 17.
84 8 C-) 51,45 1.5163
3 64.19 o.

Example 3 f=100 F No.=2.8Ha
r, d n ν1
50.806 18.69 1.83400 37.2
2 80.343 8.07 3 -150.639 5.95 1.6034
2 38.04 36.092 5.52 5 59.445 5.95 1.74077
27.86 30.690 23,78 1.
77250 49.67 -82.455 17.
84 8 (4) 51.45 1.5163
3 64.19 o.

Example 4 f=100 F No,=2.8Na
r d n ν1 5
1.348 18,69 1.83400 37.22
81.975 8.07 3 -156.404 5.95 +, 603
42 38.04 36.470 5.52 5 58.898 5.95 1.74077
27.86 30.655 23.78
1.77250 49.67 -84.0
10 17.848 oo51.45
1.51633 64.19 o
.

(Effects of the Invention) As seen from the numerical values and drawings of each embodiment, the lenses of the present invention are compact enough to be used in small video cameras, etc., yet each lens has sufficient thickness. , and the amount of ambient light is sufficient.

Each aberration can be corrected in a well-balanced manner, making it possible to obtain a lens that is easy to manufacture and highly practical.

[Brief explanation of the drawing]

1 to 4 are lens cross-sectional views of Examples 1 to 4 of the Tetsusar type lens of the present invention, and FIGS. 5 to 8 are aberration curve diagrams of Examples 1 to 4, respectively. .

Claims (1)

[Claims] In order from the object side, the first positive lens has a convex surface facing the object side.
lens, a second lens that is a biconcave lens, a third lens that is a negative meniscus lens with its convex surface facing the object side, and a fourth lens that is a biconvex lens.
Tessar-type lens after the aperture, characterized by having a configuration of 4 elements in 3 groups in which the third lens and the fourth lens are bonded together, and satisfying the following conditions (1) 0 .6f<Σd<0.75f (2) 0.1<(r_5-r_4)/(r_5+r_4
)<0.25 (3) 0.25f<r_6<0.35f However, f is the composite focal length of the entire system, and Σd is the distance from the apex of the object-side lens surface of the first lens to the image-side lens surface of the fourth lens. The distance to the apex, r, represents the radius of curvature of each lens surface, and the subscript indicates the order from the object side.