Zoom lens system

Details Zoom lens system Zoom lens system

1998-10-16

2001-01-09

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

Reexamination Certificate

active

06172819

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system, having a short back focal length, of a compact camera; and in particular, a low cost zoom lens system having a zoom ratio ranging from about 2.5 to 3.
2. Description of the Related Art
A zoom lens system of a compact camera differs from that of a single-reflex camera in that the lens arrangement of a compact camera has more freedom of design due to fewer constraints on a back focal length (a short back focal length is acceptable). In a zoom lens system having a positive first lens group and a negative second lens group (two-lens-group type) in order from the object side, there are advantages, such as fewer lens elements, being small and compact, and the manufacture thereof being simple. For a zoom lens system, in the prior art, having a half-angle-of-view of about 30° at the short focal-length extremity (wide-angle extremity) and a zoom ratio ranging from about 2.5 to 3, a three-lens-group type has usually been employed in order to improve the optical performance thereof. In the three-lens-group type zoom lens system, the traveling distance of the third lens group can be reduced, so that the F-number at the long focal-length extremity (telephoto extremity) can be made smaller (brighter) , and the fluctuations of aberrations upon zooming between the wide extremity and the telephoto extremity can be reduced. However, when comparing the three-lens-group type with the two-lens-group type, the former is larger in size and more complicated in the lens arrangement than the latter, which means that the number of lens elements has to be increased, and the size of the front most lens group (the first lens group) is increased. All of these factors have resulted in an increase of the overall manufacturing costs.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a low cost two-lens-group type zoom lens system having a reduced traveling distances of the lens groups and reduced overall length of the lens system, and attaining a half-angle-of-view of about 30° and a zoom ratio in the range from about 2.5 to 3 while attaining satisfactory optical performance.
In order to achieve the above-mentioned object, there is provided a zoom lens system which includes a positive first lens group and a negative second lens group in this order from the object side; wherein zooming is performed by moving the first lens group and the second lens group, and by varying the distance between the first lens group and the second lens group; wherein the following condition is satisfied:
−7.0<f
T
/f
2G
<−5.1 . . .   (1)
wherein:
f
T
designates the focal length of the entire lens system at the long focal-length extremity and
f
2G
designates the focal length of the second lens group.
The following condition is also preferably satisfied:
4.2<f
T
/f
1G
<6.0 . . .   (2)
Wherein
f
1G
designates the focal length of the first lens group.
At least one lens element in the negative second lens group preferably has an aspherical surface satisfying the following condition:
0<&Dgr;I
ASP
<3 . . . (3)
Wherein
&Dgr;I
ASP
designates the variation of the spherical aberration coefficients due to the aspherical surface when the focal length at the short focal-length extremity is assumed to be 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No.09-288332 (filed on Oct. 21, 1997) which is expressly incorporated herein by reference in its entirety.


REFERENCES:
patent: 4720179 (1988-01-01), Ito
patent: 4906077 (1990-03-01), Ito
patent: 5257135 (1993-10-01), Kohno et al.
patent: 5278699 (1994-01-01), Ito
patent: 5570235 (1996-10-01), Yoneyama
patent: 5687028 (1997-11-01), Ito
patent: 5751498 (1998-05-01), Ito
patent: 5900991 (1999-05-01), Kang

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