Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-04-21
2001-10-02
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S714000
Reexamination Certificate
active
06297914
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera.
2. Description of the Related Art
In a zoom lens system for a compact camera, there is no need to have a longer back focal distance unlike a zoom lens system for a single lens reflex (SLR) camera, which requires a space for providing a mirror behind the photographing lens. Accordingly, a compact camera generally employs a telephoto type lens system in which positive and negative lens groups are provided in this order from the object while a retrofocus type lens system, which includes negative and positive lens groups in this order from the object, is generally employed in a SLR camera.
In a telephoto type two-lens-group zoom lens system, distribution of power over the front and the rear lens groups is an important factor in order to reduce aberrations and to make a camera compact. Furthermore, in order to reduce aberration fluctuations upon zooming, correction of aberrations at each lens group is required. However, it has been difficult to correct aberrations in a zoom lens system in which the half angle-of-view at the short focal length extremity is about 35°, and the zoom ratio is about 2.5. Therefore the number of lens elements and cemented lens elements have been increased, and consequently, a three-lens-group zoom lens system has to be employed. These factors have caused an increase in production cost.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a zoom lens system which attains a half-angle-of-view of about 35° at the short focal length extremity, a zoom ratio of about 2.5, and makes the entire lens system compact, while the zoom lens system sufficiently corrects aberrations at each lens group, and reduces aberration fluctuations upon zooming.
Another object of the present invention is to provide a less expensive two-lens-group zoom lens system with fewer number of lens elements.
In order to achieve the above-mentioned object, according to the present invention, there is provided a two-lens-group zoom lens system having a positive first lens group and a negative second lens group in this order from the object, and zooming is performed by varying the distance between the first and the second lens groups. The positive first lens group includes a negative first lens element, a plastic-made second lens element having at least one aspherical surface, and a positive third lens element in this order from the object. The zoom lens system satisfies the following conditions:
3.5<f
T
/f
1G
<4.5 (1)
−4.7<f
T
/f
2G
<−3.7 (2)
63<&ngr;
dL3
(3)
wherein:
f
T
designates the focal length of the entire zoom lens system at the long focal length extremity;
f
1G
designates the focal length of the positive first lens group;
f
2G
designates the focal length of the negative second lens group;
&ngr;
dL3
designates the Abbe number with respect to the positive third lens element in the positive first lens group.
The zoom lens system according to the present invention preferably includes, in the negative second lens group, a lens element having at least one aspherical surface, and the zoom lens system satisfies the following condition:
0<&Dgr;V
ASP
<0.2 (4)
wherein
&Dgr;V
ASP
designates an amount of change of a distortion coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is assumed to be 1.0.
The negative second lens group preferably includes a plastic-made fourth lens element having at least one aspherical surface, and a negative fifth lens element in this order from the object. Furthermore, the fourth lens element is preferably a positive lens element.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-112317 (filed on Apr. 22, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 5280390 (1994-01-01), Ito
patent: 5570235 (1996-10-01), Yoneyama
patent: 5663838 (1997-09-01), Hasushita et al.
patent: 5687028 (1997-11-01), Ito
patent: 5777800 (1998-07-01), Yamaguchi et al.
patent: 5796527 (1998-08-01), Ohtake
patent: 5825557 (1998-10-01), Ohno
patent: 6108137 (2000-08-01), Ohtake
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Schwartz Jordan M.
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