Zoom lens system

Details Zoom lens system Zoom lens system

1999-12-22

2001-05-22

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S717000, C359S740000

Reexamination Certificate

active

06236518

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, in particular, relates to achieving high zoom ratio and miniaturization of the zoom lens system.
2. Description of the Related Art
In a zoom lens system for a compact camera, there is no need to provide a long 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 system. 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 SLR camera generally employs a retrofocus type lens system in which negative and positive lens groups are provided in this order from the object.
If a zoom lens system is constituted by a two-lens-group arrangement, the number of lens elements therein can be reduced, and the structure thereof can be simplified, so that a small and light-weight zoom lens system can be achieved, and further, such a zoom lens system is advantageous in regard to production costs. Therefore a two-lens-group arrangement has frequently been employed in a zoom lens system for a compact camera. However, in a two-lens-group zoom lens system, if attempts are made to obtain a higher zoom ratio and further miniaturization, field curvature becomes larger at intermediate focal lengths in a zooming range, and thereby it is difficult to achieve a higher zoom ratio and further miniaturization.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a miniaturized two-lens-group zoom lens system of a telephoto type, which can reduce the occurrence of field curvature at intermediate focal lengths, and can obtain higher zoom ratio (i.e., the focal length at the long focal length extremity/the focal length at the short focal length extremity).
In order to achieve the above mentioned object, 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 varying the distance between the first lens group and the second lens group. The positive first lens group includes a negative 1-a sub lens group (a negative first sub lens group) and a positive 1-b sub lens group (a positive second sub lens group), in this order from the object, and the zoom lens system satisfies the following conditions:
0.4
<f
W
/|f
1-a
|<0.7  (1)
 0.07
<d
a-b
/f
W
<0.14  (2)
4
<f
T
/f
1-b
<6  (3)
wherein
f
W
designates the entire focal length at the short focal length extremity;
f
1-a
designates the focal length of the 1-a sub lens group (f
1-a
<0);
d
a-b
designates the distance between the 1-a sub lens group and the 1-b sub lens group;
f
T
designates the focal length of the entire the zoom lens system at the long focal length extremity; and
f
1-b
designates the focal length of the 1-b sub lens group.
The zoom lens system according to the present invention preferably satisfies the following condition:
0.70
<TL
(
L
)/
f
T
<0.86  (4)
wherein
TL(L) designates the distance from the most object-side surface of the first lens group to the image plane.
In the zoom lens system according to the present invention, it is preferable for the negative second lens group to include a lens element having an aspherical surface which satisfies the following condition:
0.2
<&Dgr;V
ASP
<0.4  (5)
wherein
&Dgr;V
ASP
designates the amount of change in the distortion coefficient due to the aspherical surface when the focal length at the short focal-length extremity is assumed to be 1.0.
Furthermore, according to the zoom lens system of the present invention, a diaphragm is provided between the positive first lens group and the negative second lens group. The diaphragm is preferably controlled so that the mechanical full-aperture diameter of the diaphragm at the short focal length extremity is smaller than the mechanical full-aperture diameter thereof at the long focal length extremity.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-10-368175 (filed on Dec. 24, 1998) which is expressly incorporated herein by reference in its entirety.


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