Zoom lens system

Details Zoom lens system Zoom lens system

1999-03-23

2001-03-27

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

Reexamination Certificate

active

06208471

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system, in particular, to a zoom lens system for a medium-format camera in which the diagonal length (
2
y
) of the format is more than 60 mm, and having a finder optical system and a photographing optical system being separately provided.
2. Description of the Related Art
In a zoom lens system for a compact camera in which a finder optical system and a photographing optical system are separately provided, there is no need to have a long back focal distance unlike in a zoom lens system for a single lens reflex (SLR) camera. 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 side, while a retrofocus type lens system 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 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 1.7, and the F-number at the long focal length extremity is 6.7 or less.
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 1.7, and an F-number of 6.7 or less at the long focal length extremity, while the zoom lens system well corrects aberration fluctuations upon zooming by correcting aberrations at each lens group, wherein the entire lens system is made compact.
In order to achieve the above-mentioned object, according to the present invention, there is provided a zoom lens system having a positive first lens group and a negative second lens group in this order from the object side, and zooming is performed by varying the distance between the first and the second lens groups. The zoom lens system satisfies the following conditions (1) and (2):
1.4
<f
w
/f
1G
<1.7  (1)
0.85<&Sgr;
d1G
/f
w
<1.1  (2)
wherein:
f
w
designates the focal length of the zoom lens system at the short focal length extremity;
f
1G
designates the focal length of the positive first lens group;
&Sgr;
d1G
designates the thickness of the positive first lens group.
In the first positive lens group, the first lens element closest to the object is preferably formed as a negative lens element, and the positive first lens group satisfies the following condition (3):
−1
<f
1G
/f
1P
<−0.6  (3)
wherein:
f
1P
designates the focal length of the negative first lens element of the positive first lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-107966 (filed on Apr. 17, 1998) which is expressly incorporated herein by reference in its entirety.


REFERENCES:
patent: 5126884 (1992-06-01), Sato
patent: 5844725 (1998-12-01), Itoh
patent: 5986821 (1999-11-01), Yoon

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