Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-05-01
2003-12-02
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000
Reexamination Certificate
active
06657792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system for camera, and in particular, to a zoom lens system for a lens-shutter-type compact camera.
2. Description of the Prior Art
Unlike a zoom lens system for a single lens reflex (SLR) camera which requires a space for a quick-return mirror behind the photographing lens system, a zoom lens system for a compact camera does not require a long back focal distance.
As a zoom lens system of this type having no specific requirement on the back focal distance, the three-lens-group arrangement, i.e., a positive lens group, a positive lens group and a negative lens group in this order from the object, have been proposed. More specifically, a zoom lens system of the three-lens-group arrangement has been disclosed in, e.g., U.S. Pat. No. 4,978,204. However, in the above zoom lens system, if the zoom ratio thereof is increased, the overall length thereof at the long focal length extremity becomes longer.
Furthermore, in order to miniaturize the zoom lens system, and to obtain a higher zoom ratio, a zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, has been proposed, as disclosed in, e.g., JUPP6-265788 and JUPP2000-180725. Even by such a four-lens-group arrangement, the overall length of the zoom lens system at the long focal length extremity becomes longer due to the long traveling distances of the lens groups for zooming; and miniaturization of the zoom lens system cannot be attained, since the entrance pupil is distant from the front (most object-side) lens group at the short focal length extremity, so that the diameter thereof becomes larger.
Still further, if an attempt is made to obtain both a higher zoom ratio and the shorter overall length of the zoom lens system at the long focal length extremity, the focusing sensitivity thereat increases, so that the focal point moves largely even with respect to a slight positional error caused by the movement of each lens group. Consequently, the image-forming performance deteriorates.
Particularly, in the case where a higher zoom ratio is attained by a zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, such a four-lens-group arrangement is advantageous to shorten the overall length of the zoom lens system at the long focal length extremity, in comparison with a zoom lens system of the three-lens-group arrangement; however, in the prior art, the focusing sensitivity of internal lens groups (the second and the third lens groups) cannot be reduced.
SUMMARY OF THE INVENTION
The present invention is to provide a miniaturized zoom lens system with a higher zoom ratio, which can achieve suitable optical performance over the entire focal length ranges, in spite of a small number of lens elements in the zoom lens system. These features can be attained by making the overall length of the zoom lens system at the long focal length extremity shorter, and by making the diameter of the front (most object-side) lens group smaller.
Furthermore, the present invention is to provide a miniaturized zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, a positive lens group and a negative lens group in this order from the object, which is advantageously small for manufacturing and has a higher zoom ratio, while the focusing sensitivity which tends to become larger in the internal lens groups (the second and the third lens groups) can be reduced.
Still further, the present invention is to provide a zoom lens system which (i) is suitable for a retractable lens-shutter-type compact camera, and (ii) has the smaller diameter of the front (most object-side) lens group.
More specifically, a zoom ratio of the zoom lens system can be set to Z(=f
T
/f
W
)>3, and the overall length TL
T
thereof can be made shorter to the extent that TL
T
/f
T
<1.0;
wherein
TL
T
designates the distance, at the long focal length extremity, between the most object-side surface of the zoom lens system and the image plane;
f
T
designates the focal length of the entire zoom lens system at the long focal length extremity; and
f
W
designates the focal length of the entire zoom lens system at the short focal length extremity.
In addition to the above, a zoom ratio of the zoom lens system according to the present invention can be made higher, such as Z(=f
T
/f
W
)>4, under the same condition of TL
T
/f
T
<1.0.
According to a first aspect of the present invention, there is provided a zoom lens system including a positive first lens group (hereinafter, the first lens group), a negative second lens group (hereinafter, the second lens group), a positive third lens group (hereinafter, the third lens group), and a negative fourth lens group (hereinafter, the fourth lens group), in this order from the object. zooming is performed by moving each of the first through fourth lens groups in the optical axis direction, and the zoom lens system satisfies the following conditions:
1.1
<f
23T
/f
23W
<1.8 (1)
0.2
<LD
23W
/f
W
<0.45 (2)
0.01<(
D
23W
−D
23T
)/
f
T
<0.05 (3)
wherein
f
23W
designates the combined focal length of the second and third lens groups at the short focal length extremity;
f
23T
designates the combined focal length of the second and third lens groups at the long focal length extremity;
LD
23W
designates the distance between the most object-side surface of the second lens group and the most image-side surface of the third lens group at the short focal length extremity;
f
W
designates the focal length of the entire zoom lens system at the short focal length extremity;
D
23W
designates the axial air-distance between the second lens group and the third lens group at the short focal length extremity; and
D
23T
designates the axial air-distance between the second lens group and the third lens group at the long focal length extremity.
The zoom lens system according to the first aspect of the present invention further satisfies the following conditions:
0.1
<|r
1
/f
T
|<0.25(
r
1
<0) (4)
6.0
<|f
T/
f
2
|<12.0 (
f
2
<0) (5)
wherein
r
1
designates the radius of curvature of the most object-side surface of the first lens group; and
f
2
designates the focal length of the second lens group.
In the zoom lens system according to the first aspect of the present invention, the traveling distances of the first and fourth lens groups are the same upon zooming. In other words, these lens groups are integrally moved. According to this arrangement, the structure of the lens barrels can be simplified, and the size of the zoom lens system in the radial direction can be made smaller. As a result, a lens-shutter-type compact camera having a retractable zoom lens system can be miniaturized, and further, the structure of the lens barrels can also be simplified.
Focusing can be performed by integrally moving the second lens group and the third lens group in the optical axis direction. In this regard, for miniaturizing a camera, it is preferable that a lens group to perform focusing (hereinafter, focusing lens group) be small and light (not heavy), and that the traveling distance thereof be shorter. The second lens group or the third lens group, in which the optical-effective diameter is relatively smaller than other lens groups, can be a focusing lens group. However, if an attempt is made to perform focusing by the second or third lens group only, the traveling distance thereof becomes longer, so that miniaturization of the zoom lens system is difficult. On the other hand, by performing focusing with the integrally-moved second and third lens groups, miniaturization of the zoom lens system can be attained, and optical deterioration can be preven
Eguchi Masaru
Ito Takayuki
Greenblum & Bernstein P.L.C.
Pentax Corporation
Raizen Deborah A.
Sugarman Scott J.
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