JPS6330609B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a compact photographic lens, and more particularly to a compact photographic lens with a relatively short close distance and with small fluctuations in aberrations when focusing from an object at infinity to a close object. 2. Description of the Related Art Various compact photographic lenses have been proposed in the past that are composed of about four to five lenses whose distance from the first surface of the lens system to the image plane is relatively short. In such a photographic lens, when photographing with a close object distance, it is necessary to increase the amount of extension of the lens group for focusing as the object distance approaches. However, if the amount of extension of the lens group is increased, the off-axis light beam may not pass through the center of the pupil of the photographic lens, and when the aperture is set to a small aperture, the amount of vignetting of the light beam at the periphery of the screen increases. For this reason, in many photographic lenses, the diameter of the front lens of the photographic lens is made relatively large in order to prevent vignetting of the light beam. Also, when focusing from an object at infinity to a close object, aberration fluctuations were relatively large, partly because the number of lenses was small. For this reason, it was difficult to approach them at close range. An object of the present invention is to provide a compact photographic lens with little variation in aberrations due to focusing from an object at infinity to a close object. The main features of the lens configuration for achieving the object of the present invention are: starting from the object side, the first lens has a positive refractive power with a convex surface facing the object side and has a positive refractive power; 2 lenses, a third lens whose both lens surfaces are convex, a fourth lens with negative refractive power, and a meniscus-shaped fifth lens. Focusing is performed from an object at infinity to a close object by extending the front group integrally and extending the rear group of the fifth lens by a smaller amount than the front group. As mentioned above, the front group is composed of four lenses with positive, negative, positive, and negative refractive powers, and by extending them for focusing, it is possible to reduce fluctuations in aberration during focusing, and furthermore, the diameter of the front lens can be reduced. The aim is to make the photographic lens more compact while preventing an increase in the size of the lens. In this embodiment, focusing may be performed by extending both the front group and the rear group consisting of the fifth lens. In this case, by changing the amount of extension of the front and rear groups, the off-axis light beam passes through the center of the pupil of the photographing lens without making the diameter of the front lens too large, thereby making the camera more compact overall. . In the present invention, the refractive power of the rear group may be positive or negative. Also, by making the rear group a meniscus-shaped lens, it is possible to keep the rear group fixed during focusing, but by extending it by a smaller amount than the front group, it is possible to prevent aberration fluctuations from an object at infinity to a close object. It becomes possible to reduce the amount of Although the object of the present invention is achieved with the above lens configuration, it is more preferable that the focal length of the entire system of the front group and the rear group be f, the focal length of the front group be _f1 , and the focal length of the rear group be _f2.
When 0.9< ₁ /<1.1...(1) −0.025< _1/2 <0.025...( ₂ ) It is preferable to satisfy the following conditions. Conditional expressions (1) and (2) define the basic refractive power arrangement of the photographic lens, and by this, various aberrations of off-axis light beams such as halo, coma aberration, and image curvature can be well corrected. There is. If the lower limit of conditional expression (1) is exceeded, the refractive power of the front group increases, and aberration fluctuations, particularly fluctuations in image curvature, increase when focusing from an object at infinity to a close distance. On the other hand, if the upper limit is exceeded, the focal length of the front group becomes longer and the overall length of the lens becomes longer, making it difficult to make the photographic lens more compact. Conditional expression (2) shows the relationship between the refractive powers of the front group and the rear group, and when the lower limit is exceeded, a halo becomes noticeable in the lower part of the luminous flux at intermediate angles of view, and an extrovert phenomenon occurs at peripheral angles of view. Comatic aberration increases, and field curvature further increases in the positive direction. If the upper limit is exceeded, extroverted coma aberration at intermediate angles of view and halo at peripheral angles of view will increase, and curvature of field will further increase in the negative direction. As described above, in the present invention, aberration fluctuations are reduced by making the refractive power of the rear group smaller than that of the front group and performing focusing by changing the distance between the front group and the rear group. In particular, as in the numerical examples described below, it is preferable to set the range of 0.2<S ₂ /S ₁ <0.7 (3) where S ₁ is the extension amount of the front group and S ₂ is the extension amount of the rear group. If the lower limit of conditional expression (3) is exceeded, the amount of extension of the rear group will be small and it will be difficult to reduce aberration fluctuations due to focusing, and if the upper limit is exceeded, the amount of extension of the rear group will be large and the amount of extension will be equal to that of the entire front lens. The diameter increases, which is not desirable. In general, in order to properly correct various basic aberrations in a photographic lens, it is necessary to particularly well correct rays of off-axis light that pass through a position high from the optical axis. In the present invention, an aspherical surface is introduced into the lens surface where off-axis rays pass through a high position from the optical axis, for example, the object-side lens surface of the first lens and the second lens, or the image-side lens surface, and The object of the present invention can be better achieved by correcting aberrations of the light beam. In the present invention, it is preferable that the aspherical surface has a shape that bends the off-axis light beam passing through the periphery of the screen downward in order to satisfactorily correct halo and coma aberrations. Specifically, on the object side lens surface of the first lens and the object side lens surface of the second lens, the amount of deviation from the paraxial radius increases toward the object side around the lens, and the deviation amount on the image side of the first lens increases. The lens surface, the image side lens surface of the second lens, is preferably one in which the amount of deviation from the paraxial radius toward the image side increases around the lens periphery. As mentioned above, when an aspherical surface is used in the present invention,
It is possible to satisfactorily correct the deterioration of aberrations caused by light rays passing through a point high from the optical axis of the off-axis light beam, and to satisfactorily correct various basic aberrations. As described above, according to the present invention, it is possible to obtain a compact photographic lens that achieves good aberration correction and has small aberration fluctuations due to focusing. Next, numerical examples of the present invention will be shown. In the numerical examples, R _i is the radius of curvature of the i-th lens surface in order from the object side, D _i is the thickness and air gap of the i-th lens surface in order from the object side, and N _i and ν _i are the radius of curvature of the i-th lens surface in order from the object side. These are the refractive index and Atsube number of the glass of the i-th lens. a _i and b _i are the aspheric even coefficient and the aspheric odd coefficient of the aspheric surface according to the following formula. The symbol "D-04" means "10 ^-4 ". The shape of the aspherical surface is defined by the X-axis in the direction of the optical axis, the Y-axis in the direction perpendicular to the optical axis, and the direction of movement as positive, with the origin being the intersection of the apex of the lens surface and the X-axis, which contributes to determining the aspherical surface and focal length. The difference in the X-axis direction from the extended spherical surface is ΔX, R _i is the paraxial radius of curvature of the i-th surface in order from the object side, and R _i ^* is R _i ^* = 1/1/R _i + 2a _1. Let the radius of curvature of the defined lens reference plane be Let it be expressed by the expansion formula. The ratio of the extension amount of the front group and the rear group in Numerical Examples 1 and 2 is 2:1. In Numerical Example 3, the ratio of the extension amounts of the front group and the rear group is 10:3. In either case, when the focal length of the entire system is set, the distance from the image plane to the object is from infinity to 17
Aberration fluctuations are small and aberrations are well corrected up to close distances of about 19 to 19 degrees.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the lens of Numerical Example 1, Figure 2-
Figures a, 2-b, and 2-c are aberration diagrams when the object distance is 55.6 and 16.7, respectively, from the image plane to infinity when the focal length of the entire system is taken as the focal length of the entire system in Numerical Example 1. be. Figure 3 is a cross-sectional view of the lens in Numerical Example 2, and Figures 4-a, 4-b, and 4-c are for Numerical Example 2 with object distances of infinity, 55.6, and 16.7 from the image plane, respectively. FIG. Figure 5 is a cross-sectional view of the lens of Numerical Example 3;
Figure -a, Figure 6-b, and Figure 6-c are numerical example 3.
, the object distance is infinitely far from the image plane,
It is an aberration diagram at the time of 55.6 and 19.4. ΔM is a meridional image plane, ΔS is a sagittal image plane, and S and C are sine conditions.

Claims (1)

[Scope of Claims] 1. In order from the object side, a first lens group having a positive refractive power and a positive refractive power with a convex surface facing the object side, a second lens group having a negative refractive power and having both lens surfaces concave, and both lens surfaces. a third lens with a convex surface, a fourth lens with a negative refractive power,
The front group consisting of the first, second, third, and fourth lenses is integrally extended forward, and the rear group of the fifth lens is Focusing is performed from an object at infinity to a close object by extending forward an amount smaller than the amount of extension of the front group, and the focal length of the entire system is f, the focal length of the front group is f1, and the focal length of the rear group is is f2, and the amount of advance of the front group is
A compact photographic lens that satisfies the following conditions: S1 and S2, the amount of extension of the rear group.