Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-07-06
2001-04-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S691000, C359S684000
Reexamination Certificate
active
06212018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system in which a back focal distance can be short.
2. Description of the Related Art
In a zoom lens system for a single lens reflex (SLR) camera, in order to have a space behind the photographing lens system, a retrofocus type lens systems, which includes negative first lens group (a front lens group) and a positive second lens groups (a rear lens group) in this order from an object, is generally employed so that a longer back focal distance can be provided. On the other hand, in a zoom lens system in which a back focal distance can be short (e.g., a zoom lens system in a compact camera), since there is no need to provide a longer back focal distance unlike a zoom lens system for a single lens reflex (SLR) camera, a telephoto type lens system, which includes a positive first lens group (a front lens group) and a negative lens group (a real lens group) in this order from the object, is generally employed. Because the two-lens-group zoom lens system is employed, the number of lens elements are fewer, simplification of the structure is attained, the manufacturing costs can be reduced, and the miniaturization of the lens system can be achieved.
Along with the popularization of compact cameras, not only the correcting of aberrations but also an increase of relative brightness (a smaller F-number) is required in the zoom lens system of a compact camera. In a two-lens-group zoom lens system, in order to satisfactorily correct aberrations from the short focal length extremity toward the long focal length extremity, it is necessary to correct aberrations in each lens group. However, the smaller the F-number is, the more difficult the correcting of aberrations in each lens group becomes. In particular, for correcting aberrations in the front lens group, the number of lens elements thereof have to be increased. Due to the increase of the lens elements, two major structural problems arise.
The first problem is that when the F-number is smaller the diameter of the shutter has to be made larger whereby the shutter unit itself has inevitably to be made larger. In a two-lens-group zoom lens system which performs zooming by varying the distance between the positive front lens group and the negative rear lens group, conventionally the diaphragm is provided between the front and rear lens groups from a structural point of view. However, if the above explained large-sized shutter unit as a result of a smaller F-number is positioned between the front and rear lens groups, there is a possibility that the large-sized shutter unit interferes with the rear lens group. In particular, in a collapsible barrel type zoom lens system with the large sized shutter unit, when the lens barrel is being collapsed, the front and rear lens groups cannot adequately approach each other, and the collapsed barrel length is increased. The overall thickness of the camera is then increased.
The second problem is that an increase of burden on the focusing mechanism due to an increase of the lens elements in the front lens group, which is attributable to a small F-number. Conventionally, in the two-lens-group zoom lens system which performs zooming by varying the distance between the positive front lens group and the negative rear lens group, focusing is generally performed by driving the entire front lens group. However, if the entire front lens group having an increased weight due to the increased lens elements as a result of a smaller F-number is to be driven, the focusing mechanism has to be enlarged, which hinders the miniaturization of a camera.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a two-lens-group zoom lens system for a compact camera, which can eliminate an adverse influence on the collapsed length of the lens system even when a relative brightness is increased (a smaller F-number), and can reduce the burden on the focusing mechanism.
It is noted that the present invention is created based on the following conceptions:
(1) In a two-lens-group zoom lens system, when the number of lens elements in the front lens group is increased for the purpose of obtaining a smaller F-number, it is possible to have a space, within the front lens group, for providing the diaphragm; and
(2) The burden on the focusing mechanism is reduced, if focusing is designed to be performed by the rear lens group which is not required to increase the number of lens elements for the purpose of obtaining a smaller F-number.
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, and in which zooming is performed by varying the distance between the positive first lens group and the negative second lens group; wherein a diaphragm is provided within the positive first lens group, and wherein focusing is performed by moving the negative second lens group along the optical axis.
The first lens group preferably includes more than seven lens elements, and the diaphragm is positioned between the two of the lens elements, which can form the longest distance therebetween.
When the present invention is applied to a zoom lens system having the half angle-of-view of more than 30° at the short focal length extremity, the zoom lens system preferably satisfies the following conditions:
0.7<&Sgr;d
s
/&Sgr;d
1G
<1 (1)
0.6<H
1
/H
s
<0.9 (2)
wherein:
&Sgr;d
1G
designates the distance from the most object-side surface of the most image-side surface of the first lens group;
&Sgr;d
s
designates the distance from the most object-side surface to the diaphragm of the first lens group;
h
1
designates the height of the paraxial ray which strikes the most object-side surface of the first lens group at the short focal length extremity; and
H
s
designates the height of the paraxial ray at the position of the diaphragm when the first lens group is at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
−1.5<f
w
/f
F
<−1.3 (3)
wherein:
f
w
designates the focal length of the entire zoom lens system at the short focal length extremity; and
f
F
designates the focal length of the second lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-191413 (filed on Jul. 7, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 5530589 (1996-06-01), Sato
patent: 6061186 (2000-05-01), Nishio
patent: 6-265827 (1994-09-01), None
Asahi Kogaku Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Thompson Tim
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