Image pickup lens system

Optical: systems and elements – Lens – Including a nonspherical surface

Reexamination Certificate

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Reexamination Certificate

active

06222682

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image pickup lens system and particularly, to an image pickup lens system which is comprised of two lenses and used in an image pick-up device, e.g., a CCD camera utilizing an image pickup element such as CCD, CMOS and the like mounted on a potable computer, a visual telephone and the like, and which can be reduced in size and weight.
2. Description of the Related Art
In recent years, multimedia have been developed remarkably, and for example, the demand for a camera utilizing an image pickup element such as CCD, CMOS and like, e.g., a CCD camera mounted on a portable computer, a visual telephone and the like is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight.
Therefore, it is also desired that a two-lens system using two lenses is conventionally used as such an image pickup lens system.
There are image-pickup lenses used in such a conventional two-lens system, which are disclosed, for example, in Japanese Patent Application Laid-open No.10-104511 and Japanese Patent Publication No.7-50246 and the like.
In the lens system using the image pickup lenses disclosed in the above Patents, the first and second lenses are disposed sequentially in the named order from the side of an object. The first lens is a meniscus concave lens having the same signs of radii of curvature at centers of first and second faces, and the second lens is a convex lens. Such a construction can ensure a back focal length, while shortening the focal length.
However, the conventional image pickup lens disclosed in Japanese Patent Application Laid-open No.10-104511 suffers from a problem that the radius of curvature of the center of the second face of the first lens is relatively small and for this reason, it is difficult to manufacture such lens. Another problem is that if a diaphragm is disposed at a location displaced from the second lens toward an image surface in order to correct the magnification chromatic aberration, the diameter of the first lens is increased and hence, it is further difficult to form the second face of the first lens. Such conventional image pickup lens suffers from a further problem that because it is difficult to form the second face, it is little possible to accommodate the angle-widening of the image pickup lens, i.e., the shortening of the focal length.
The image pickup lens disclosed in Japanese Patent Publication No.7-50246 has a fatal problem that the entire length of the optical system of the lens is increased by reason of the structure of the lens. The optical system of such an arrangement also suffers from the following problem: If stray light enters the lens system of the image pickup lens from outside an angle of view, a ghost is generated. Therefore, it is required that the shape of the first face of the first lens in the vicinity of an optical axis approximates to a planar shape. However, an increase in diameter of the first lens leads to a convex shape of the first face of the first lens, resulting in a reduction in performance of the image pickup lens. Moreover, the power of the first lens is largely associated with the generation of each aberration such as a distortion and the like and hence, there is also a problem that if the shape of the first face of the first lens is convex, the distortion and the like cannot be removed.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an image pickup lens system in which a back focal length can be ensured, while shortening the focal length and moreover, which can be produced easily.
To achieve the above object, according to the present invention, there is provided an image pickup lens system, comprising a first lens which is a concave lens having those radii of curvature of faces at the center thereof, which are not of the same sign, and a second lens which is a convex lens, the first face of at least the first lens being formed into an aspherical shape, so that the following conditions (1) and (2) are satisfied:
(1) 0.67Bf<f<1.45Bf
(2) 0.25|f
1
<|f
2
|<0.8|f
1
|
wherein Bf is a back focal length; f is a focal length of the entire lens system; f
1
is a focal length of the first lens; and f
2
is a focal length of the second lens.
In the above feature of the present invention, the expressions (1) and (2) represent conditions ensuring that in a state in which the curvatures of the first and second lenses have been maintained at small values, the focal length of the entire lens system can be shortened, and a large back focal length can be ensured. If the focal length of the entire lens system exceeds the lower limit value in the expression (1) it is impossible to provide a reduction in size of the lens system, and if the lens focal length of the entire lens system exceeds the upper limit value, it is impossible to insert any of various types of filters. On the other hand, if the focal length of the second lens exceeds the lower limit value in the second expression (2), it is impossible to ensure a large back focal length, while maintaining a desired optical performance, and to insert any of various types of filters. If the focal length of the second lens in the second expression (2) exceeds the upper limit value, the power of the first lens is too strong and hence, it is difficult to manufacture the first lens. Moreover, each of aberrations such as distortion and the like is liable to be generated. According to the present invention, however, the back focal length can be increased relative to the focal length of the entire lens system, while maintaining the desired optical performance, by satisfying the conditions represented by the expressions. Moreover, the radii of curvature of the first and second faces of the first lens are not of the same sign and hence, the radius of curvature of the second face of the first lens at the center thereof can be increased and thus, the first lens can be produced easily. Particularly, even when a diaphragm for correcting the chromatic aberration is disposed at a location displaced from the second lens toward an image, it is possible to easily produce the first lens. In addition, it is possible to easily accommodate the angle-widening.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.


REFERENCES:
patent: Re. 34455 (1993-11-01), Arai et al.
patent: 5917661 (1999-06-01), Tochigi et al.

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