Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-12-21
2004-02-03
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S680000, C359S684000
Reexamination Certificate
active
06687059
ABSTRACT:
This application claims benefits of Japanese Application(s) No. Hei 11-362681 filed in Japan on Dec. 12, 1999, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens, and more particularly to a zoom lens system that lends itself to a camera using a CCD or other electronic image pickup device.
In recent years, digital cameras (electronic cameras) have attracted attention as the next generation of cameras taking the place of silver halide 35 mm film (usually called Leica size) cameras. In consumer applications in particular, zoom lenses from a single-focus lens having a diagonal field angle of about 60° to a wide-angle zoom lens having a zoom ratio of the order of 3 have gone mainstream. For current higher-class zoom lenses, much is desired on their wide-angle side or telephoto side and, at the same time, higher-grade cameras of the single-lens reflex type are in demand. As a matter of course, higher image quality, too, is needed. Zoom lenses suitable for use on single-lens reflex cameras having a diagonal field angle of the order of 75°, for instance, are disclosed in JP-A's 4-163415 and 5-27175.
These publications disclose a zoom lens system comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power, a third lens group having negative refracting power and a fourth lens group having positive refracting power, wherein for zooming from the wide-angle end to the telephoto end of the zoom lens system, the respective lens groups move in such a way that the space between the first lens group and the second lens group, and the space between the third lens group and the fourth lens group becomes narrow while the space between the second lens group and the third lens group, and the space between the fourth lens group and an image-formation plane becomes wide. This zoom lens system has a reduced F-number of the order of 2 to 2.8.
However, the number of pixels then suggested for the single-lens reflex type was at most about 1,000,000; the aforesaid publications do no give any suggestion about the achievement of a zoom lens capable of taking full advantage of the performance of an electronic image pickup device expected to have 6,000,000 or 10,000,000 pixels at some future time.
Maintaining performance possible leads to a size increase. This point, too, is the problem to be solved.
In consideration of loads on, and the layout of, a driving system and the effective diameter of the first lens group, a rear focusing mode is preferable for focusing. However, when rear focusing is carried out in the aforesaid zoom lens system wherein the first lens group has negative refracting power, the second lens group has positive refracting power, the third lens group has negative refracting power and the fourth lens group has positive refracting power, some problems arise. For instance, one problem is a fluctuation of aberrations, and another problem is that focusing becomes impossible or the amount of movement of the focusing group should be increased because of the presence of a zooming zone where the magnification of the fourth lens group is equal to or close to life-size.
JP-A 4-264412 or JP-A 9-203861 discloses a zoom lens system comprising a first lens group having negative refracting power, a second lens group having positive refracting power, a third lens group having negative refracting power, a fourth lens group having positive refracting power, and a fifth lens group having positive refracting power and designed to be fixed during zooming. The zooming action is allocated to four lens groups located on the object side of the zoom lens system. However, these publications give no particular suggestion about focusing.
JP-A 6-102455 discloses a rear focusing zoom lens system comprising a first lens group having negative refracting power, a second lens group having positive refracting power, a third lens group having negative refracting power, a fourth lens group having positive refracting power, and a fifth lens group having negative refracting power and designed to move during zooming, wherein rear focusing is carried out with the fifth lens group. The publication shows that the fifth lens group is also movable toward the image side for focusing from an infinite distance to a nearby distance. For an optical system that can be used with a CCD or other electronic image pickup device and so is required to be of substantially telecentric construction, it is not preferable that the lens group located nearest to the image side has negative power, because the whole optical system becomes thick.
SUMMARY OF THE INVENTION
In view of such problems with the prior art as explained above, an object of the present invention is to provide a high-performance, large-aperture yet wide-angle zoom lens system best suited for use with an electronic image pickup device in particular. Another object of the invention is to provide a method for focusing a high-performance, large-aperture yet wide-angle zoom lens system best suited with an electronic image pickup device. Yet another object of the invention is to provide a high-performance, large-aperture yet wide-angle zoom lens system which has a zoom ratio of the order of 3 at a diagonal field angle of 75° (focal length 28 mm class as calculated on a 35 mm size basis) at its wide-angle end, so that it can be used with a single-lens reflex camera using a miniature electronic image pickup device with the number of pixels being of the order of 6,000,000.
According to the present invention, the aforesaid objects are achieved by the provision of a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having negative refracting power, a second lens group having positive refracting power, a third lens group having negative refracting power, a fourth lens group having positive refracting power and a fifth lens group having positive refracting power, wherein upon movement of an object point, focusing is carried out with said fifth lens group.
Why the aforesaid arrangement is used in the present invention, and how it work is now explained.
As already pointed out with reference to the prior art, the zoom lens arrangement of −+−+ construction is suitable for a wide-angle arrangement. In this arrangement, the fourth lens group is divided to a fourth lens subgroup having positive refracting power and a fifth lens subgroup having positive refracting power to reduce fluctuations of aberrations with zooming, and ensure telecentric performance without increasing the thickness of the whole optical system. In addition, it is possible to carry out rear focusing (with the lens group located nearest to the image side of the zoom lens system) while the amount of movement of the lens group is reduced with a limited deterioration of the image formation capability upon focusing all over the zooming area.
FIGS. 11
to
13
are geometries illustrative of the actions of the present invention.
FIG. 11
illustrates one conventional case where the first lens group G
1
is used in the form of a focusing group. As can be seen from the state of FIG.
11
(
b
) where the first lens group G
1
is moved out of the state of FIG.
11
(
a
), the effective diameter of the first lens group G
1
must be increased for focusing.
FIG. 12
illustrates another conventional case where a zoom lens arrangement of −+−+− construction is achieved by locating a negative lens group G
1
to the image side; the first lens group G
1
is used in the form of a focusing group, and shows that for a telecentric optical system it is necessary to increase the effective diameter of the fourth lens group G
4
.
FIG. 13
illustrates one zoom lens arrangement of the present invention, and shows that it provides an efficient optical system. Although not understood from
FIG. 13
, the spacing layout between the fourth lens group G
4
and the fifth lens group G
5
Olympus Corporation
Pillsbury & Winthrop LLP
Raizen Deborah
Sugarman Scott J.
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