Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-05-30
2001-10-16
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S679000
Reexamination Certificate
active
06304388
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on Patent Application No. 11-153569 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system, for embodiment, a zoom lens system suited for use as an image reading optical system used in a reading devices such as document copiers and film scanners.
2. Description of the Related Art
When reading a finite distance object, Koeler illumination is often used to uniformly illuminate the object surface. In order to maintain illumination uniformity when Koeler illumination is used, the image forming position of the Koeler illumination system and the entrance pupil position of the finite distance system must be at virtually the same position. Such an optical arrangement is not very complex when the finite distance system is a simple focal lens.
When a zoom lens system is the finite distance system, it is difficult to ensure illumination uniformity when conventional Koeler illumination is used since the entrance pupil position moves during zooming operation. If a well-known front stop zoom lens system is used as a finite distance system, illumination uniformity can be ensured by maintaining a constant entrance pupil position during zooming operation. However, distortion and lateral chromatic aberration correction is difficult because the system has a positive optical power behind the stop in a front stop zoom lens system. Moreover, if these corrections are made during zooming operation, the increase in the zooming ratio becomes complicated, and the zoom lens system is greatly enlarged.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved zoom lens system.
A further object of the present invention is to provide a zoom lens system which maintains a constant entrance pupil position during zooming.
A still further object of the present invention is to provide a compact, high zoom ratio zoom lens system which provides excellent correction of various aberrations such as distortion and lateral chromatic aberration.
These object is attained by a zoom lens system for focusing a finite distance object on an image plane as an image, comprising, in order from one conjugate side, a first lens unit having a positive optical power, the first lens unit being stationary during a zooming operation, a stop stationary during a zooming operation, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power, wherein the zooming operation is performed by varying distances between the first through fourth lens units, wherein an approximate afocal condition exists between the first lens unit and the second lens unit.
REFERENCES:
patent: 5062695 (1991-11-01), Iwasaki
patent: 5101299 (1992-03-01), Iwasaki
patent: 5138493 (1992-08-01), Iwasaki
patent: 5157552 (1992-10-01), Hagimori
patent: 5257134 (1993-10-01), Sugawara
patent: 5313330 (1994-05-01), Betensky
patent: 5805347 (1998-09-01), Mizutani et al.
Burns Doane , Swecker, Mathis LLP
Minolta Co. , Ltd.
Schwartz Jordan M.
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