JPS60178419A - Photographic lens - Google Patents

Abstract

PURPOSE:To use effectively an aspherical lens and to correct satisfactorily the aberration of a higher order, too, by forming at least one lens surface of the second lenses to an aspherical surface having such a shape as a negative refractive power increases toward the peripheral part of the lens. CONSTITUTION:A lens surface R3 and R4 is formed to the aspherical surface of such a shape as a negative refractive power increases toward the periphery of the lens, by which a light L3 of the lower part goes to a light L3' by increasing its refracting quantity, and a comatic aberration is corrected satisfactorily. As a result, a lateral aberration whose comatic aberration is corrected satisfactorily is obtained, and a resolving power can be improved. In case the lens surface R4 in the rear of the second lens is formed to an aspherical surface, the spherical surface of the lens surface is formed to an aspherical surface R4' having such a shape as a negative refractive power increases toward the periphery, by which light L4 of the lower part goes to light L4' by increasing its refracting quantity, and a comatic aberration is corrected. By using an aspherical surface to at least one lens surface of the second lens group, a lens shape to be worked easily to a aspherical shape can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photographic lens, and more particularly to a compact photographic lens with a short overall lens length (from the first surface of the lens to the focal plane) and a wide angle of view.

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 in 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 60° or more.

-This is because higher-order aberrations become more noticeable when the angle of view becomes wide.
This is because good aberration correction becomes difficult. In addition, in an optical system with such a refractive power arrangement, as the overall lens length is shortened, the aperture ratio is increased, and the angle of view is increased, distortion and astigmatism worsen, and coma and halo aberrations worsen. This is because it causes a significant increase.

For example, a lens system with such a refractive power arrangement is well known from Japanese Patent Publication No. 44-10831, but the angle of view of the lens described there is 46 degrees, which is about the same as a standard lens.
If an attempt is made to increase the angle of view, the astigmatism difference will increase. Further, later, Japanese Patent Publication No. 52-48011 has been known, but the F number is 1:4.5, so it cannot be said to be a bright lens.

In addition, when the lens is constructed using only spherical lenses without using an aspherical surface as proposed in Japanese Patent Publication No. 56-50248, the bulge of 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 when attempting 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 Japanese Patent No. 4317, the method of making the final meniscus lens with negative refractive power aspherical is effective as a means to remove coma aberration and improve resolution. The radius of curvature of a meniscus lens 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, there was a problem that processing and inspection were difficult.

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.
It has achieved a wide angle of view photographic lens.

In particular, the present invention effectively uses an aspherical lens to achieve a photographic lens that satisfactorily corrects even high-order aberrations.The main features of the photographic lens for achieving the object of the present invention are as follows: A meniscus-shaped lens with positive refractive power with its convex surface facing the side, a second lens with negative refractive power with both concave lens surfaces, a third lens with positive refractive power with both lens surfaces convex, and the object side. A meniscus-shaped fourth lens with a negative refractive power with a concave surface facing toward the periphery is arranged, and at least one lens surface of the second lens is shaped so that the negative refractive power becomes stronger toward the peripheral part of the lens. It is possible to obtain a lens shape that can be easily processed into an aspherical surface.

The degree to which the negative refractive power is strengthened by the aspherical surface is varied depending on the degree of aberration correction.

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 schematically depicts this.

The principal ray of the light beam reaching the middle part of the screen is represented by P8, the upper ray is represented by U3, the lower ray is represented by -, and the spherical surface of the lens is represented by a crow. A feature of this lens type is that the downward rays have too little refractive power and reach above the principal ray P3 on the film surface F. Therefore, this causes the occurrence of extroverted coma, resulting in deterioration of lateral aberration as shown in FIG. 3A.

In the present invention, by making the lens surface R4 aspherical so that the negative refractive power increases at the periphery of the lens, the amount of refraction of the downward light ray L3 is increased to become the light ray U, and coma aberration is reduced. It is well corrected. As a result, as shown in FIG. 3B, lateral aberrations with well-corrected comatic aberrations are obtained, making it possible to improve resolving power.

When the rear lens surface R4 of the second lens is made into an aspherical surface, as shown in FIG. This increases the amount of refraction of the lower light ray L4 to form light #1lL4 to correct coma aberration, and the principle is exactly the same.

In the present invention, by using an aspherical surface for at least one lens surface of the second lens group, it is possible to form the lens into an aspherical shape that is easy to process.

The aforementioned Japanese Patent Publication No. 44-10831, Japanese Patent Application Laid-Open No. 1987-
When the object-side lens surface of the fourth lens is made aspherical as proposed in Publication No. 94317, the angle at which the effective surface of this lens surface is viewed is 80° or more, and in extreme cases, the angle is 18°.
It is close to σ (hemisphere), and is extremely easy to process. According to the present invention, the same angle is approximately 6σ or less, and aspherical surface processing can be easily performed. In addition, inspection of machining accuracy can be performed more easily.

The aspherical lens of the present invention may be manufactured by polishing an ordinary lens, but it is preferable to use a glass mold or a plastic lens material because it can be easily manufactured by molding.

Although the photographing lens which is the object of the present invention can be achieved with the above configuration, it is preferable that K satisfy the following conditions in order to shorten the overall lens length and achieve good aberration correction. □ If the focal length of the taking lens is 11, the radius of curvature of the lens surface of the third lens is R11 + RIB in order from the object side, the refractive index of the glass is t-N, and the focal length of the fourth lens is 4, then 1, 7 (N , ・・・・・・・・・(3) In the above equation, conditional expression (1) sets the focal length of the fourth lens to an appropriate range and the lens This is to set the total length to an appropriate length.If a strong negative refractive power is applied beyond the upper limit, the total length of the lens will be shortened, but the change in aberration due to eccentricity in the lens assembly will increase, which is unfavorable for assembly. If it exceeds this, the lens will have a weak negative refractive power and the overall length of the lens will be long.)
At the same time as it lacks compactness, the Petzval sum increases in the positive direction, and the curvature of field at the middle of the screen increases in the negative direction, causing the flatness of the image surface to be lost.

Conditional expression (2) is used to regulate the lens shape of the third lens and to satisfactorily correct spherical aberration and Comis aberration. When the upper limit is exceeded, spherical aberration is over-corrected and introverted coma aberration occurs, whereas when the lower limit is exceeded, spherical aberration is under-corrected and extroverted coma aberration occurs.

Conditional expression (3) regulates the refractive index of the glass of the third lens, and is intended to satisfactorily correct field curvature and astigmatism. If the refractive index N3 is less than 1.7, the Petzval sum becomes large in the positive direction, which deteriorates the flatness of the image plane in the middle of the screen and increases the astigmatism difference, which is not preferable.

In addition, 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 T and focusing from the third lens to the third lens, or only the fourth lens. , or only the third lens a
It is also possible to take out the b.

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

Next, numerical examples of the present invention will be shown. In numerical examples R
, is the radius of curvature of the first lens surface on the object side υj,
Di is the thickness and air gap of the first lens in order from the object side, and N and ν are the refraction, refractive index, and Abbe number of the glass of the first lens, respectively, in order from the object side.

The aspherical shape has the X-axis in the optical axis direction, the Y-axis in the direction perpendicular to the optical axis, and the direction of light travel as positive, and the origin is the intersection of the apex of the lens and the X-axis, and R is the lens of the second group lens. When the paraxial radius of curvature of the surface is '1*%*le%, and as is the aspheric coefficient, it is expressed by the formula 10a4Y'10aJIG.

Numerical Example I F-100FNO-1:28 2ω-5aa′″R1”
3L73 D, -7,92N1-1.77250 y
@-49,6B, -8 & 31 D, -4,89 R, -120,64D, -R63N2-1.8466
6 Shi2-23.9R, -6R78D4- 6.70 Rs” 76.69 Ds” 7.40 N5-1.8
3400 C3-37.2R6--10105 D,-
21/114R0--20,63D,-3,16N,-
1,51633 shi, -64.1R, --32.43 R3 surface; aspheric coefficient al -of f4/ 7-1.208 a2-8.107 Example 2 F-100FNO-1: R82ω-59,3°R,-3
L82 D, = 9.84 N, -1,77250 shi1
-49.6Rt-8 & 62 D, -298 1 too, -126,76D, -2,63N2-1.805
18 ν2-25.4R, -5&37 D, -7,02 BI+-81,05DI," 6.51 N8-1.8
0610 v, -40,9R6--97,110,-2
3L49 R7--21,18D,-3,16N,-1,6031
1shi4-60.7R8maro -31,14 Number of surfaces; aspheric coefficient a, -Of4/ / = -1,246 a4-R617XIO-12 5-0 Numerical example 3 F-100FNO-1:R82ω"59 .3°R, -3
Z92 D, -10,55N1-1.77250
-49.6゜R2-8&99 D, -2,85 R3-124,73I) 3- R63Ntl, 8051
8 ν2=25.4R, -57,53D, -7,07 R11-80,340, -5,36N, -1,8061
0 shoe 40.9R6-100,61D, -23,77 R-20,85D7-3.16 N, -1,603
11 C-60, F R, -29,49 R3 surface; aspheric coefficient R4 surface; aspheric coefficient al -Oa
l-0 ~-4,160X10=a,-1,786X10-7
a3-1.318X10=IL,--1,588X1
0-'a4--3.859X10 R4-1,018×
10-1113 1! -Of'll Den O ” / / --1,369

[Brief explanation of drawings]

Fig. 1 is a lens sectional view showing the optical path of a photographing lens of the same type as that of the present invention, Fig. 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 Figs. 3A and B. is a lateral aberration diagram showing the correction of coma aberration when an aspherical surface is used, FIG. 4 is an explanatory diagram of the effect of the aspherical surface when an aspherical surface is used for the R4 surface of the second lens in the present invention, and FIG. is a cross-sectional view of the lens of Numerical Example 1 of the present invention, and FIGS. 6, 7, and 8 are various aberration diagrams of Numerical Examples 1 and 23 of the present invention, respectively. In the figure, R is a lens surface, D is a distance between lens surfaces, 6M is a meridional image surface, and ΔS is a sagittal surface. Figure S Figure 6 Figure 7

Claims (2)

[Claims] (1) In order from the object side: a meniscus-shaped lens with positive refractive power with its convex surface facing the object side, a second lens with negative refractive power with both concave lens surfaces, and a positive refracting lens with both lens surfaces convex. a third lens with a positive refractive power, and a meniscus-shaped fourth lens with a negative refractive power with a concave surface facing the object side, and at least one lens surface of the second lens has a negative refractive power toward the periphery of the lens. A photographic lens characterized by an aspherical surface shaped to increase force. (2) When the focal length of the photographing lens is R3°, the radius of curvature of the lens surface of the third lens is R3° in order from the object side, the refractive index of a bird or crow is N, and the focal length of the fourth lens is f4. 1, 7 (N3) The photographing lens according to claim 1, which satisfies the following condition: 1, 7 (N3).