Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-03-15
2003-06-17
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S682000
Reexamination Certificate
active
06580564
ABSTRACT:
BACKGROUND OF THE INVENTION
A conventional zoom lens is described in Japanese Laid-Open Application H5-297276 that includes, in sequential order from the object side, a first lens group of negative refractive power that is fixed during zooming and moves for focusing, a second lens group of positive refractive power that moves for zooming, a third lens group of negative refractive power that moves for correction of field curvature caused by zooming, and a fourth lens group of positive refractive power that is fixed in position.
Many prior art zoom lenses are designed for use with small imaging devices such as a CCD array. In order to use a zoom lens in a projection-type display unit that uses a liquid crystal, a zoom lens that forms a projection image should have a large conjugate distance on the reducing side. This increases the lens size required. In addition, conventional techniques do not satisfactorily correct distortion when zoom lenses are used as projection lenses.
For a projection lens in a unit that uses a liquid crystal, it is desirable that the optical system be nearly telecentric on the reducing side of the projection lens. However, this is not a common feature of conventional zoom lenses. In addition, few conventional zoom lenses have a sufficiently large back focus that allows for the insertion of an optical system, between the zoom lens and the image surface, for dividing or combining color components, when necessary.
To overcome the above problems, a known zoom lens is described in Japanese Laid-Open Application H10-268193 that includes, in order from the object side, a first lens group of negative refractive power that is fixed during zooming and moves for focusing, a second lens group of positive refractive power that moves during zooming, a third lens group of positive refractive power that moves during zooming, a fourth lens group of negative refractive power that moves during zooming for correction of curvature of field, and a fifth lens group of positive refractive power that is fixed during zooming, wherein the zoom lens satisfies certain conditions.
Recently, there has been a demand for a projection-type display unit to project images onto a large screen from a short distance using a wider-angle projection lens. The lens described in the above-mentioned Japanese Laid-Open Application H10-268193 is compact but with a sufficient conjugate distance on the reducing side for the insertion of a beam-combining prism for projecting color images, and this lens is also nearly telecentric on the reducing side. However, this lens provides an image angle of only 23-25 degrees, and thus hardly satisfies the demand that the lens be able to project images onto a large screen from a short distance. Downsizing of the projection-type display unit is also a recent demand which requires the downsizing of the projection lens used in such a display unit, as well.
In view of the above, the present applicant disclosed a five-group zoom lens in Japanese Patent Application 2000-339955. The zoom lens includes second and third lens groups having positive refractive power and a fourth, movable, lens group having negative refractive power. The focal length of each lens group is appropriately arranged in order to reduce fluctuation in aberrations due to zooming while providing larger image angles than conventionally available. This also allows the lens system to have a compact structure on the reducing side. Rays in the tangential plane on the reducing side are nearly parallel to the optical axis and an appropriate length of back focus is available for inserting, for example, a beam combiner for projecting color images. Therefore, a wide-angle projection-type display unit using this zoom lens can be compact and project high-resolution images.
However, recent projection-type display units are supposed to be used in different circumstances. Larger projected images are often desired even in a smaller projection space. To satisfy this demand, a zoom lens that projects with wider-angle images is required. The five-group zoom lens described in the above specification has an image angle of approximately 60° at the wide-angle end, which is relatively large. However, when an attempt is made to obtain still wider-angle images, for instance, an image angle of approximately 70°, a problem may arise in that it is difficult to correct the curvature of field.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a wide-angle zoom lens having an image angle of approximately 70°, a large back focus, a nearly uniform distribution of rays relative to the optical axis in the tangential plane on the reducing side, a compact structure on the reducing side, and satisfactorily corrected aberrations, particularly curvature of field. The present invention also aims to provide a projection-type display unit that uses the zoom lens described above.
REFERENCES:
patent: 5847875 (1998-12-01), Kodama et al.
patent: 6008952 (1999-12-01), Yamamoto
patent: 6137638 (2000-10-01), Yamagishi et al.
patent: 5-297276 (1993-11-01), None
patent: 10-268193 (1998-10-01), None
Arnold Bruce Y.
Arnold International
Choi William
Epps Georgia
Fuji Photo Optical Co., Ltd.
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