Zoom lens system

Details Zoom lens system Zoom lens system

2002-10-03

2004-08-31

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

Reexamination Certificate

active

06785058

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system which is suitable for a lens-shutter camera and the like.
2. Description of the Prior Art
Various zoom lens systems for lens-shutter cameras have been proposed. For example, in a zoom lens system of the prior art as disclosed in Japanese Unexamined Patent Publication No. Hei-5-11181, the lens arrangement is simple; however, the zoom ratio is low. Furthermore, in the case where the number of lens elements is small, as disclosed in U.S. Pat. Nos. 5,434,711; 5,973,849, 6,236,515 and 6,067,196, a three-lens-group zoom lens system is employed, so that the zooming mechanism thereof becomes complicated, and low production cost thereof cannot be achieved. In the case of a large zoom ratio as disclosed in Japanese Unexamined Patent Publication No.Hei-11-84236, the amount of change in the overall length thereof upon zooming is too large, so that it is difficult to constitute the lens barrels therefor. Moreover, in a lens system disclosed in U.S. Pat. No. 6,154,323, the number of lens is small; however, expensive diffraction optical elements are employed therein, so that the production thereof is difficult, and higher-order diffracted light could become flare.
SUMMARY OF THE INVENTION
The present invention provides a low-cost zoom lens system having a smaller number of lens elements and the zoom ratio of approximately 3.1.
As an aspect of the present invention, there is provided a zoom lens system including a positive first lens group and a negative second lens group, in this order from the object. Zooming is performed by moving each of the positive first and negative second lens groups in an optical axis direction.
The positive first lens group includes a positive first lens element having a convex surface facing toward the object, a negative second lens element, and a positive third lens element, in this order from the object.
The negative second lens group includes a fourth lens element which is formed as a positive meniscus lens element having the concave surface facing toward the object, and a fifth lens element which is formed as a negative meniscus lens element having the concave surface facing toward the object, in this order from the object.
The zoom lens system satisfies following conditions:
 1.2
<ft/f
1<1.8  (1)
3.8
<ft/f
1
G<
4.2  (2)
1
≦SF
1<6  (3)
wherein
ft designates the focal length of the entire zoom lens system at the long focal length extremity;
f1 designates the focal length of the positive first lens element;
f1G designates the focal length of the positive first lens group, and
SF1 designates the shape factor of the positive first lens element, defined as SF1=(r2+r1)/(r2−r1),
r1 designates the radius of curvature of the first surface of the positive first lens element; and
r2 designates the radius of curvature of the second surface of the positive first lens element.
The zoom lens system preferably satisfies the following conditions:
1.50
<n
1
p<
1.68  (4)
−81.2×
n
1
p+
172.7<&ngr;1
p<−
250×
n
1
p+
468  (5)
wherein
n1p designates the refractive index of the positive lens elements in the positive first lens group; and
&ngr;1p designates the Abbe number of the positive lens elements in the positive first lens group.
The zoom lens system can satisfy the following condition:
0.45
<&Dgr;X/ft<
0.50  (6)
wherein
&Dgr;X designates the traveling distance of the positive first lens group upon zooming from the short focal length extremity to the long focal length extremity.
Furthermore, the zoom lens system satisfies the following condition:
−3.0
<&Dgr;ASP
2×1000/
fs<−
1.0  (7)
wherein
&Dgr;ASP2 designates the amount of asphericity on the object-side surface of the negative second lens element.
fs designates the focal length of the entire zoom lens at the short focal length extremity.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2001-309656 (filed on Oct. 5, 2001) which is expressly incorporated herein in its entirety.


REFERENCES:
patent: 5434711 (1995-07-01), Tetsuya et al.
patent: 5751498 (1998-05-01), Ito
patent: 5900990 (1999-05-01), Konno et al.
patent: 5973849 (1999-10-01), Hashimura
patent: 5986821 (1999-11-01), Yoon
patent: 6067196 (2000-05-01), Yamamoto et al.
patent: 6154323 (2000-11-01), Kamo
patent: 6215600 (2001-04-01), Nishimura et al.
patent: 6236515 (2001-05-01), Yamamoto et al.
patent: 6236518 (2001-05-01), Enomoto
patent: 6268966 (2001-07-01), Ishii et al.
patent: 5-11181 (1993-01-01), None
patent: 11-84236 (1999-03-01), None

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