Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-11-02
2001-07-31
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S686000, C359S689000, C359S782000, C359S784000, C359S795000
Reexamination Certificate
active
06268965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and in particular, relates to a zoom lens system having a wider angle-of-view at the short focal length extremity.
2. Description of the Related Art
Unlike a zoom lens system for a single lens reflex camera which requires space to be provided behind the photographing lens system in order to place a mirror, a zoom lens system of a compact camera does not require a long back focal distance. Accordingly, a zoom lens system for a compact camera generally employs a telephoto type lens system having a positive lens element and a negative lens element, in this order from the object; whereas a zoom lens system for a single lens reflex camera employs a retrofocus type lens system having a negative lens element and a positive lens element, in this order from the object.
When a zoom lens system is a two-lens-group arrangement, the number of lens elements can be reduced, and whereby size and weight reduction of a camera can be achieved, which is also advantageous for reducing production costs of the camera. Accordingly, it is common for a zoom lens system of a compact camera to employ a two-lens-group arrangement. On the other hand, in the case where the half angle-of-view is required to be more than 35° and the zoom ratio is required to be about 2.8, a three-lens-group arrangement is predominantly employed in order to maintain optimum optical performance. However, if a three-lens-group zoom lens system is employed, the diameter of the front-most lens element in the front lens group has to be increased, and the number of lens elements is also increased, both of which adversely influence miniaturization of the camera. Furthermore, due to the complexity of a three-lens-group arrangement, adjustment work thereof is difficult.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a telephoto type two-lens-group zoom lens system with a smaller number of lens elements, in particular, to provide a zoom lens system which is compact and inexpensive, and has a half angle-of-view of more than 35° at the short focal length extremity, and a zoom ratio of about 2.8.
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, wherein 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 first sub-lens-group, a positive second sub-lens-group, and a positive third sub-lens-group, in this order from the object, wherein the zoom lens system satisfies the following condition:
0.7<f
T
/f
1
-
2
<1.5 (1)
wherein
f
T
designates the focal length of the entire zoom lens system at the long focal length extremity; and
f
1
-
2
designates the focal length of the positive second sub-lens-group in the first lens group.
The positive second sub-lens-group in the first lens group preferably includes a cemented sub-lens-group having a positive lens element and a negative lens element. The order of the positive lens element and a negative lens element is not necessarily specified. The cemented sub-lens-group satisfies the following conditions:
0.6<f
W
/|f
(
1
-
2
)c
|<1.2 (2)
30<&ngr;
p
−&ngr;
n
(3)
wherein
n′ designates the refractive index of the image-side lens element of the cemented sub-lens-group;
n designates the refractive index of the object-side lens element of the cemented sub-lens-group;
r
c
designates the radius of curvature of the cemented surface of the cemented sub-lens-group;
&ngr;
p
designates the Abbe number of the positive lens element of the cemented sub-lens-group;
&ngr;
n
designates- the Abbe number of the negative lens element of the cemented sub-lens-group; and
f
(
1
-
2
)c
designates r
c
/(n′−n).
The zoom lens system according to the present invention preferably satisfies the following conditions:
−1.0<r
1
/f
W
<−0.2 (4)
−1.2<f
1
G
/f
1
-
1
<−0.5 (5)
40<&ngr;
1
-
1
(6)
wherein
r
1
designates the radius of curvature of the first surface of the negative first sub-lens-group of the first lens group;
f
W
designates the focal length of the entire zoom lens system at the short focal length extremity;
f
1
G
designates the focal length of the positive first lens group;
f
1
-
1
designates the focal length of the negative first sub-lens-group in the first lens group; and
&ngr;
1
-
1
designates the average of the Abbe number of the negative first sub-lens-group in the first lens group.
The negative first sub-lens-group can be composed of a negative single lens element.
The positive first lens group preferably includes a lens element having an aspherical surface which satisfies the following condition:
−30<&Dgr;I
ASP
<−18 (7)
wherein &Dgr;I
ASP
designates the amount of change in the spherical aberration coefficient due to the aspherical surface when the focal length at the short focal length extremity is converted to 1.0.
The negative second lens group preferably includes a lens element having an aspherical surface which satisfies the following condition:
0<&Dgr;V
ASP
<0.3 (8)
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 converted to 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 at the long focal length extremity.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-10-313658 (filed on Nov. 4, 1998) which is expressly incorporated herein by reference in its entirety.
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Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Lester Evelyn A
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