JPS5934508A - Photographic lens having short overall lens length - Google Patents

Abstract

PURPOSE:To decrease the number of lenses and to shorten the overall lens length, by disposing the 1st meniscus lens having positive refracting power, the 2nd lens having negative refracting power, the 3rd lens having positive refracting power, and the 4th meniscus lens having negative refracting power, and forming the lens face on the object side of the 3rd lens to an aspherical face. CONSTITUTION:A lens face R5 is made into such an aspherical face by which the positive refracting power is decreased on the periphery of the lens, thereby decreasing the refraction quantity of a lower ray L to L' and correcting satisfactorily comatic aberration. The transverse aberration corrected well of comatic aberration is thus obtd. and the resolving power is considerably improved. If the lens face on the object side of the 3rd lens group is used, the lens shape that makes finishing t an aspherical shape easy is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a compact wide-angle photographic lens having a short overall lens length (from the first surface of the lens to the focal plane).

In recent years, with the miniaturization of cameras, there has been a demand for compact lenses with short overall lens lengths. In particular, in order to make the total lens length of the lens system less than or equal to one time the focal length, it is desirable to configure the front group of the lens system to have positive refractive power and the rear group to have negative refractive power. Such a refractive power arrangement is often used for long focal length lenses with a narrow angle of view, but it is rarely applied to wide-angle lenses with a short overall lens length and a large aperture ratio of 600 or more.

The reason for this is that with this type of refractive power arrangement, as the overall length of the lens is shortened, as the aperture ratio is increased, and as the angle of view is increased, distortion and astigmatism worsen; The lens system is well known, but the angle of view of the lens described there is 46 degrees, which is about the same as a standard lens.
: 4.5, so it cannot be said that it is a bright l/lens.

In addition, when the lens is constructed from only spherical lenses without using an aspherical surface as proposed in Japanese Patent Publication No. 56-50248, the curvature of field is large in the middle of the screen, and this It has the disadvantage that a large amount of coma aberration occurs if an attempt is made to correct it. Comatic aberration is extremely undesirable because it significantly reduces resolution.

On the other hand, Japanese Patent Publication No. 44-10831 and Japanese Unexamined Patent Publication No. 56-9
As proposed in Publication No. 4317, the method of making the final meniscus-shaped lens with negative refractive power aspherical is effective as a means for removing coma aberration and improving resolution.
However, the radius of curvature of a meniscus-shaped lens with negative refractive power is extremely small, and in extreme cases, it may be necessary to process the nearly hemispherical lens surface into an aspherical surface. For this reason, it has the disadvantage of being difficult to process and inspect.

The purpose of the present invention is to provide a compact photographic lens with a small number of lenses and a short overall lens length. Achieved.

The features of the lens configuration for achieving the purpose of the present invention are, in order from the object side, a meniscus-shaped lens with a positive refractive power with a convex surface facing the object, and a second lens with a negative refractive power with both lens surfaces concave. , a third lens with positive refractive power with both convex lens surfaces, and a meniscus-shaped fourth lens with negative refractive power with the concave surface facing the object, and the object-side lens surface of the M3 lens is an aspherical surface. I'm sorry for what I did. In such a lens configuration, by making the object-side lens surface of the third lens aspherical, coma aberration in the middle part of the screen can be well corrected, and the lens shape can be easily processed by aspherical processing. can.

As mentioned above, in the lens type of the present invention, if all the lens surfaces are composed of only spherical surfaces, a large amount of comatic aberration will occur in the middle part of the screen. Figure 2 is a schematic depiction of this.

The principal ray of the light beam reaching the middle part of the screen is Pl, the ray on the upper side is U, the lower ray is Ll, and the spherical surface of the lens is R, 5, R6.
It is expressed as. A feature of this lens type is that the downward light ray has too little refractive power and reaches above the principal ray P on the film surface F. Therefore, the cause of extroverted coma is the deterioration of transverse aberration, as shown in FIG. 3A.

In the present invention, by making the lens surface R5 aspherical so that the positive refractive power decreases at the periphery of the lens, the amount of refraction of the downward light ray is reduced to L', thereby eliminating coma aberration. is well corrected. As a result, as shown in FIG. 3B, lateral aberrations with well-corrected comatic aberrations are obtained, making it possible to greatly improve resolving power.

In addition, in the present invention, since an aspherical surface is used for the object side lens surface of the third lens group, it is preferable to use a lens shape that can be easily processed into an aspherical shape. In this case, the angle looking into the effective surface of this lens surface is 80
degree or more, and in extreme cases it approaches 180 degrees (hemisphere), making it extremely difficult to process. According to the present invention, the same angle is about 20° or less, and aspherical processing can be easily performed.

In addition, inspection of machining accuracy can be made easier.

Note that if the aspherical lens of the third lens group is manufactured from a synthetic resin material, an inexpensive photographic lens can be provided.

Further, in the photographic lens according to the present invention, focusing may be performed by extending the entire lens as is generally done, or by fixing the fourth lens and extending from the lens to the third lens. be. It is also possible to extend only the third lens or the third lens.

By determining the lens configuration as described above, it is possible to achieve a compact photographic lens with a short overall lens length and excellent aberration correction.

Next, examples of the present invention will be shown. In the example, Ri#- is the radius of curvature of the first lens surface in order from the object side, Di is the i-th 1/lens thickness and air gap in order from the object side, and N
i and νi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side.

The aspherical shape has the Y-axis in the optical axis direction, the Y-axis in the direction perpendicular to the optical axis, and the direction of light propagation as positive.The origin is the intersection of the apex of the lens and the Y-axis, and R is the third group lens The paraxial radius of curvature of the [/lens surface on the object side, at is the even coefficient of the aspherical surface, and bi is the odd coefficient of the aspherical surface, +bIY8+b2Y5+b3Y7+・
It is expressed by the following expansion formula.

Numerical example 1°F=100 FNO=1: 2.8 2ω=5
9.31%1=33.15 D1=10.44
N1=1.77250 Si1=49.6R2=1
02.55 D2= 2.63R3=-125,6(
ID3=2.63 N2=1.80518
ν2=25.4R4=56.37 D4=7.5
4R5'= 95.48 D5= 5.61 N
5=1.80610 C3-40.9R6=-90.
67 D6=22.98R7= -21,20D7=
3.16 N4=1.51633 C4=64.
IR8= -32,85 Aspheric coefficient 4 a, = 1.893 X 10 b, = 1
．． 649 x 10-'a2 = 1.190 x 1
0' bz = 1.758 x 10-"10 a! = -2,507x 10 bs = 2.
127 x 10-”14 a4 := -7,929X 10 b4 =
1.046 X 10-” Numerical Example 2°F=100. FNO=1: 3.5 2ω=
63.4R1=34.23 DI=10.53
N1=1.77250 Si1=49.6R22 1
18.16 D2=3.43R3--116,98D
3=2.86 N2=1.80518 ν2=2
5.4R4 = 55.07 D4 = 9.09R5 pieces 59.57 D5 = 5.64 N5 = 1.605
62 Shi3=43.7R6=-69,04D6=2
2.17R7=-21,36D7=3.43N
4=1.51633 ν4brown4.1R8= -37,
63 Aspheric coefficient a, = 1.697 x 10 b, = 1
．． 774 x 10'as = 5.786 x 10
bz = 1.721 x 10'10 am = 9.483 x 10 b, = -
1,831X 10"13 a4 = -4,197X 10 b4 = -
2,684X 1O-14a, == 3.450X
10-”

[Brief Description of the Drawings] Figure 1 is a cross-sectional view of a lens showing the optical path of the same lens type as the present invention, Figure 2 is an explanatory diagram of a light beam that passes through an aspherical surface of a light beam that forms an image in the middle of the screen, and Figure 3. Figures A and B are lateral aberration diagrams showing the correction of coma aberration when an aspherical surface is used. Figure 4 is a cross-sectional view of the lens of Numerical Example 1 of the present invention. Figures 5 and 6 are each of the lenses of the present invention. FIG. 6 is a diagram of various aberrations of Numerical Example 1.2. In the figure, R is a lens surface, D is a distance between lens surfaces, ΔM is a meridian image surface, and ΔS is a sagittal image surface. Applicant Canon Co., Ltd.

Claims (2)

[Claims] (1) In order from the object side: a meniscus-shaped lens with positive refractive power with a convex surface facing the object, a second lens with negative refractive power with both concave lens surfaces, and a positive refractive power with both lens surfaces convex. and a meniscus-shaped fourth lens with a negative refractive power with a concave surface facing the object side, 2, and the object-side lens surface of the third lens is an aspherical lens. A photographic lens with a short overall length. (2) The aspherical surface of the third lens has a shape such that the positive refractive power becomes weaker toward the periphery of the lens. photographic lens.