Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-10-26
2001-06-12
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S681000, C359S685000, C359S689000
Reexamination Certificate
active
06246529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wide-angle zoom lens system, for single lens reflex cameras, which has an angle-of-view (=2&ohgr;) of more than 90° at the short focal length extremity.
2. Description of the Related Art
In a single lens reflex camera, it is necessary to maintain a long back focal distance because a quick-return mirror is positioned behind the photographing lens system. For this reason, a retrofocus lens system having a negative lens element and a positive lens element, in this order from the object, is generally employed for a wide-angle lens system. However, in a retrofocus lens system, it is extremely difficult to correct field curvature in the vicinity of the short focal length extremity. Generally, in a zoom lens system where a negative lens group precedes a positive lens group, each lens group has to be provided with stronger power in order to make an overall length of the lens system shorter while securing a long back focal distance and maintaining the diameter of lens elements smaller.
However, if the power of each lens group is increased, optical deterioration on the wide-angle side occurs at the periphery of the image forming area. In other words, the occurrence of field curvature, astigmatic difference and distortion becomes noticeable. More concretely, in order to prevent field curvature on the wide-angle side which tends to become larger, at a periphery of an image, in the positive direction, field curvature at a lower image height has to be largely bent in the negative direction. Field curvature at a periphery of an image on the telephoto side corresponds to the tendency of field curvature at the lower image height on the wide-angle side, that is, the field curvature at a periphery of a image on the telephoto side is largely bent in the negative direction. As a result, in such a zoom lens system, field curvature extremely changes from the wide-angle towards the telephoto-angle.
Due to the above-mentioned problems, in a zoom lens system, particularly, in a wide-angle zoom lens system in which the angle-of-view at the short focal length extremity is more than 90°, for the purpose of miniaturization and in order to correct aberrations other than field curvature, field curvature has to be allowed to remain to some extent.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a zoom lens system, for single lens reflex cameras, which has a superwide angle-of-view of more than 90° at the short focal length extremity, and which is compact, and can suitably correct filed curvature over the entire focal length range including the vicinity of the short focal length extremity.
In order to achieve the above mentioned object, there is provided a wide-angle zoom lens system including a negative first lens group, a positive second lens group, a diaphragm, and a positive third lens group, in this order from the object; whereby upon zooming from the short focal length extremity towards the long focal length extremity, each lens group is arranged to move so that the space between the first and second lens groups and the space between the second and third lens groups are narrowed; and the wide-angle zoom lens system preferably satisfies the following condition:
0.36<(D2S−D2L)/(fL−fS)<0.7 (1)
wherein:
D2S designates the distance between the second lens group and the third lens group at the short focal length extremity;
D2L designates the distance between the second lens group and the third lens group at the long focal length extremity;
fL designates the focal length of the entire lens system at the long focal length extremity; and
fS designates the focal length of the entire lens system at the short focal length extremity.
The wide-angle zoom lens system preferably satisfies the following condition:
0.25<f23S/f2<0.5 (2)
wherein:
f23S designates the combined focal length of the second lens group and the third lens group at the short focal length extremity; and
f2 designates the focal length of the second lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-10-304424 (filed on Oct. 26, 1999) which is expressly incorporated herein by reference in its entirety.
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patent: 4735493 (1988-04-01), Tachihara
patent: 5164859 (1992-11-01), Ito
patent: 5325236 (1994-06-01), Tanaka
patent: 5329401 (1994-07-01), Sato
patent: 5434710 (1995-07-01), Zozawa
patent: 5748384 (1998-05-01), Sensui
patent: 5786942 (1998-07-01), Komori et al.
patent: 5808808 (1998-09-01), Yamanashi
patent: 5877901 (1999-03-01), Enomoto et al.
patent: 5910860 (1999-06-01), Ozaki et al.
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Spector David N.
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