Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-11-08
2003-03-18
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000, C359S686000
Reexamination Certificate
active
06535339
ABSTRACT:
This application claims benefit of Japanese Application(s) No. Hei 11-316827 filed in Japan on Nov. 8, 1999, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens system and an image pickup system using the same, and more particularly to a compact yet low-cost zoom lens system for cameras using an electronic image pickup means, for instance, camcorders, digital cameras, surveillance monitor cameras and cameras incorporated in portable telephones or PCs.
SUMMARY OF THE INVENTION
For zoom lens systems which belong to this field and are reduced in size and cost for consumer-oriented purposes, there has been proposed a four-group zoom lens system of +−++ construction in order from its object side, as shown in JP-A's 4-43311 and 4-78806. In this zoom lens system, the first and third lens groups are fixed during zooming, and the second lens group having negative power moves on an optical axis for zooming while the fourth lens group moves on the optical axis for correction of fluctuations of an image plane position with zooming. In zoom lens systems as set forth in JP-A's 6-94997 and 6-194572, on the other hand, the third lens group is moved from the image plane side to the object side for zooming from the wide-angle end to the telephoto end for the purpose of aiding in zoom action, thereby achieving further size reductions. These publications show zoom lenses having a relatively high zoom ratio of the order of 8 to 12. For a zoom lens system reduced exclusively in size and cost at the expense of zoom ratios, however, such prior art systems are still less than satisfactory because no sufficient size reductions are achievable thanks to an increased number of lenses.
In the zoom lenses shown in the aforesaid JP-A's 6-94997 and 6-194572, a substantial portion of their zooming action is assigned to the second lens group. To keep a substantially constant image point in this case, the transverse magnification of the second lens group must be in the neighborhood of −1 in the range from the wide-angle end to the telephoto end of the system. When further size reductions are intended by making the zoom ratio smaller than this, however, the amount of movement of the second lens group can be so reduced that the space margin between the first and second lens groups can be cut to the bone, thereby achieving efficient size reductions.
To perform zooming while the second lens group has a transverse magnification in the neighborhood of −1 with a narrower spacing between the first and second lens groups, however, it is required to increase the power of the first lens group with respect to the second lens group. This in turn causes an entrance pupil to be located at a farther position and so the height of off-axis rays passing through the first lens group to increase, resulting unavoidably in an increase in the size and, hence, the thickness of the first lens group. It is also required to increase the curvature of each lens in the first lens group. To ensure each lens of sufficient edge thickness, it is then necessary to increase the thickness of each lens in the first lens group.
SUMMARY OF THE INVENTION
In view of such states of the prior art as explained above, an object of the present invention is to provide a zoom lens system much more reduced in size and cost than ever before, and an image pickup system using the same.
One specific object of the present invention is to provide a four-group zoom lens system which can have the desired zoom ratio while its size is reduced without increasing the power ratio of the first lens group with respect to the second lens group.
Another specific object of the present invention is to achieve a compact zoom lens system suitable for use on digital cameras, and cameras added to portable telephones and PCs, which is designed in such a way as to provide a nearly telecentric exit beam with image pickup devices such as CCDs and CMOSs in mind. This zoom lens system ensures the desired back focus enough to receive a low-pass filter, a beam splitter, etc. if required, and achieves improved image-formation capability with a reduced number of lenses.
According to one aspect of the present invention, these objects are achievable by the provision of a zoom lens system characterized by comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and designed to be fixed during zooming, a second lens group having negative refracting power and designed to move from the object side to an image plane side of the zoom lens system for zooming from a wide-angle end to a telephoto end of the zoom lens system, a third lens group having positive refracting power and designed to move from the image plane side to the object side for zooming from the wide-angle end to the telephoto end, and a fourth lens group having positive refracting power and designed to be movable for zooming, wherein the following conditions are satisfied:
0.5
<|F
2
/F
3
|<1.2 (1)
2.5 mm<
f
B
(min)<4.8 mm (10)
where F
i
is the focal length of an i-th lens group and f
B(min)
is the length, as calculated on an air basis, of the final surface of a lens having power in said zoom lens system to an image plane of said zoom lens system, representing a figure at which said zoom lens system becomes shortest in a whole zooming space.
According to another aspect of the present invention, there is provided a zoom lens system characterized by comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and designed to be fixed during zooming, a second lens group having negative refracting power and designed to move from the object side to an image plane side of the zoom lens system for zooming from a wide-angle end to a telephoto end of the zoom lens system, a third lens group having positive refracting power and designed to move from the image plane side to the object side for zooming from the wide-angle end to the telephoto end, and a fourth lens group having positive refracting power and designed to be movable for zooming, wherein the following conditions are satisfied:
0.49
<|L
3
/L
2
|<1 (2)
2.5 mm<
f
B
(min)<4.8 mm (10)
where L
i
is the amount of movement of an i-th lens group from the wide-angle end to the telephoto end and f
B(min)
is the length, as calculated on an air basis, of the final surface of a lens having power in said zoom lens system to an image plane of said zoom lens system, representing a figure at which said zoom lens system becomes shortest in a whole zooming space.
According to yet another aspect of the present invention, there is provided a zoom lens system characterized by comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and designed to be fixed during zooming, a second lens group having negative refracting power and designed to move from the object side to an image plane side of the zoom lens system for zooming from a wide-angle end to a telephoto end of the zoom lens system, a third lens group having positive refracting power and designed to move from the object side to the image plane side for zooming from the wide-angle end to the telephoto end, and a fourth lens group having positive refracting power and designed to be movable for zooming, wherein the following conditions are satisfied:
2<(
F
3.4
w
)/
IH<
3.3 (3)
2.5 mm<
f
B(min)
<4.8 mm (10)
where (F
3.4W
) is the composite focal length of the third and forth lens groups at the wide-angle end, IH is the radius of an image circle, and f
B(min)
is the length, as calculated on an air basis, of the final surface of a lens having power in said zoom lens system to an image plane of said zoom lens system, representing a figure at which said zoom lens system becomes shortest in a whole zooming space.
Accordin
Epps Georgia
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
Spector David N.
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