Photography – Having variable focal length of camera objective
Reexamination Certificate
2002-08-22
2003-09-09
Perkey, W. B. (Department: 2851)
Photography
Having variable focal length of camera objective
C359S692000
Reexamination Certificate
active
06618558
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a compact and two-lens-group zoom lens system having a short back focal distance, which is suitable for a lens shutter camera.
2. Description of the Prior Art
Unlike a zoom lens system for a single lens reflex 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. Accordingly, as a zoom lens system for a lens shutter camera in which there is no need to prepare a space for a quick-return mirror, a telephoto-type lens system, which (i) includes a positive front lens group and a negative rear lens group, in this order from the object, and (ii) performs zooming by varying the distance between the front lens group and the rear lens group, is suitable for miniaturization of a compact camera.
One solution for increasing the zoom ratio of such a telephoto-type zoom lens system is to (i) secure the distance between the front lens group and the rear lens group as long as possible at the short focal length extremity, and (ii) increase the amount of change in the distance between the front lens group and the rear lens group.
However, a simple increase of the distance therebetween results in an increase of the diameter of each lens group. This is because the diameter of each lens group has to be made larger in order to secure peripheral illumination. Consequently, demands for miniaturization and weight reduction on the zoom lens system cannot be satisfied.
SUMMARY OF THE INVENTION
The present invention provides a miniaturized and low-cost zoom lens system having a zoom ratio of 3 or more; and more specifically, such a zoom lens system is materialized in a two-lens-group zoom lens system which is suitable for miniaturization and simplification of the structure thereof.
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 the positive first lens group and the negative second lens group in an optical axis direction.
The positive first lens group includes a negative first lens element
1
a
, a second lens element
1
b
having an aspherical surface on which the curvature is made smaller toward the periphery thereof, and a positive third lens element
1
c
, in this order from the object.
The negative second lens group includes a positive fourth lens group
2
a
and a negative fifth lens group
2
b,
in this order from the object.
The zoom lens system satisfies the following conditions:
−7
<f
T
/f
2G
<−4.5 (1)
−9
<f
T
/f
2GN
<−7 (2)
wherein
f
T
designates the focal length of the entire zoom lens system at the long focal length extremity;
f
2G
designates the focal length of the negative second lens group, and
f
2GN
designates the focal length of the negative fifth lens group
2
b
in the negative second lens group.
Note that in the negative second lens group, each of the positive fourth lens group
2
a
and a negative fifth lens group
2
b
can be constituted by a single lens element or more than two lens elements, while the positive first lens group includes the three lens elements. However, from the viewpoint of costs, it is preferable that each of the positive fourth lens group
2
a
and a negative fifth lens group
2
b
be constituted by a single lens element.
The zoom lens system preferably satisfies the following condition:
4
<f
T
/f
1G
<6 (3)
wherein
f
1G
designates the focal length of the positive first lens group.
The zoom lens system can satisfy the following condition:
2
<f
T
/f
2GP
<4 (4)
wherein
f
2GP
designates the focal length of the positive fourth lens group
2
a
in the negative second lens group.
The zoom lens system can further satisfy the following condition:
−5
<f
W
/r
1
<0 (5)
wherein
f
W
designates the focal length of the entire zoom lens system at the short focal length extremity, and
r
1
designates the radius of curvature of the object-side surface of the negative first lens element la in the positive first lens group.
The zoom lens system preferably satisfies the following condition:
1.75
<Nd
L1
(6)
wherein
Nd
L1
designates the refractive index, with respect to the d-line, of the negative first lens element la in the positive first lens group.
The zoom lens system preferably satisfies the following condition:
64
<&ngr;d
(7)
wherein
&ngr;d designates the Abbe number of the positive third lens element
1
c
in the positive first lens group.
In the positive first lens group, a lens element on which at least one aspherical surface is formed is provided, and the aspherical surface preferably satisfies the following condition:
−40
<&Dgr;I
ASP
<−10 (8)
wherein
&Dgr;I
ASP
designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
In the negative second lens group, a lens element on which at least one aspherical surface is formed is provided, and the aspherical surface preferably satisfies the following condition:
0
<&Dgr;V
ASP
<0.5 (9)
wherein
&Dgr;V
ASP
designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length at the short focal length extremity is converted to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-262753 (filed on Aug. 31, 2001) which is expressly incorporated herein in its entirety.
REFERENCES:
patent: 5663838 (1997-09-01), Hasushita et al.
patent: 5687028 (1997-11-01), Ito
patent: 5930052 (1999-07-01), Enomoto et al.
patent: 6072638 (2000-06-01), Enomoto
patent: 6268965 (2001-07-01), Enomoto
patent: 6297914 (2001-10-01), Enomoto
patent: 6-347696 (1994-12-01), None
patent: 11-84236 (1999-03-01), None
Greenblum & Bernstein P.L.C.
Pentax Corporation
Perkey W. B.
LandOfFree
Zoom lens system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Zoom lens system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Zoom lens system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3068241