Optical: systems and elements – Compound lens system – Telescope
Reexamination Certificate
2000-09-07
2003-11-04
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
Telescope
C359S432000, C359S831000
Reexamination Certificate
active
06643062
ABSTRACT:
This application claims benefit of Japanese Application No. Hei 11-252535 filed in Japan on Sep. 7, 1999, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates to a finder optical system having an image-inverting optical system. More particularly, the present invention relates to a finder optical system for use in a camera, a video camera, etc. which uses an image-inverting optical system whereby an inverted image of an object formed by an objective optical system is observed as an erect image. The present invention also relates to an image pickup apparatus using the finder optical system.
A finder optical system used in a compact camera or the like is constructed separately from a photographic optical system and generally placed above the photographic optical system. Among finder optical systems of this type, real-image finders are well known, in which a real image formed as a first image by an objective optical system is inverted by an image-inverting optical system and observed as an erect image through an ocular optical system.
Recently, it has been demanded that compact cameras should be further reduced in size, particularly in thickness, i.e. size in the direction of the optical axis. In a zoom lens, zooming is performed by varying the spacing between lens units. Therefore, if the zoom lens is arranged to attain a higher zoom ratio, the zoom lens basically becomes large in size in the direction of the entering optical axis. In the case of taking lenses, therefore, schemes have been devised for the lens mount structure to attain a reduction in thickness. That is, when not used for photography, the taking lens is stored with the zooming spaces reduced (this system will hereinafter be referred to as the “collapsible barrel”).
In the case of finder optical systems, however, the collapsible barrel system as used in taking lenses is unfavorable from the viewpoint of camera design. Therefore, it is very difficult to reduce the thickness of finder optical systems. This is an obstacle in attaining a reduction in size of cameras.
Cameras have also been demanded to provide a higher zoom ratio. Therefore, it is also necessary to form a finder optical system from an increased number of optical units in order to ensure the required performance. However, if the number of optical units is increased, the sum total of the thicknesses of lenses increases. Therefore, such a finder optical system arrangement is not always favorable for attaining a reduction in thickness.
As a conventional technique, Japanese Patent Application Unexamined Publication Number [hereinafter referred to as “JP(A)”] Hei 5-53054 has, in order from the object side, a negative first unit, a positive second unit, and a negative third unit. The first and second units move for zooming. The negative third unit, which is stationary, ensures the negative power by refraction through a refracting lens or a prism entrance surface. The optical system attains a reduction in overall length by disposing a negative unit closest to the pupil.
Meanwhile, it has heretofore been common practice to use a plane, powerless surface as a reflecting surface of an image-inverting optical system. Accordingly, there have been made some propositions that a reflecting surface of a prism or a mirror constituting an image-inverting optical system is given a power so as to have the function of an objective optical system or the function of an ocular optical system, thereby attaining a reduction in size.
JP(A) Hei 8-248481 used a rotationally symmetric curved surface as a reflecting surface of a prism in a real-image finder. Although it is stated that an aspherical surface or a toric surface is applicable to the curved surface, the curved surface disclosed in the specification is a rotationally symmetric aspherical surface. In general, a toric surface is symmetric with respect to two coordinate axes. Therefore, it is not an asymmetric surface. No numerical values are mentioned in regard to the amounts of displacement of reflecting surfaces in Examples.
JP(A) Hei 10-197796-uses a rotationally asymmetric curved surface in an image-inverting optical system of a real-image finder optical system. However, almost all Examples of this finder optical system fail to disclose design examples. Therefore, the performance, size, etc. of the finder optical system are unclear. Numerical Example 5 of this finder optical system uses rotationally asymmetric curved surfaces as refracting and reflecting surfaces of a prism in the finder optical system. In addition, the image-inverting optical system is arranged to have the function of an ocular lens, thereby reducing the number of lenses used.
JP(A) Hei 11-38472 and 11-38473 use a rotationally asymmetric surface as a reflecting surface of one of Porro prisms of a real-image finder that is disposed on the object side of the intermediate image formation plane, thereby attaining a reduction in the thickness of the finder optical system.
However, these prior art optical systems suffer from various problems as stated below.
In JP(A) Hei 5-53054, if a strong power is given to the negative third unit, the positive power of the second unit unavoidably needs to be increased in order to ensure the required power, which is unfavorable from the viewpoint of performance. For this reason, a very strong power cannot be given to the third unit. Therefore, the effect of reducing the overall length is limited. Moreover, an optical system having a power cannot be placed on the pupil side of the third unit, and it is therefore necessary to ensure a long back focus. For this reason, the basic structure of the optical system is the retrofocus type. Accordingly, it is still difficult to reduce the overall length of the optical system.
In JP(A) Hei 8-248481, a power is given to a reflecting surface of a prism. However, because this reflecting surface is tilted so as to be decentered with respect to the axial principal ray, rotationally asymmetric decentration aberrations are produced. The aberrations cannot be corrected by the rotationally symmetric aspherical surface configuration. At the toric surface also, aberration correction with respect to skew rays cannot satisfactorily be performed. In this regard, no solution means is disclosed for any of the arrangements of this optical system. Thus, the disclosed optical system is unsatisfactory in terms of performance. Furthermore, the size of the optical system is unclear from the numerical values mentioned in the specification.
JP(A) Hei 10-197796 states a layout and arrangement of prisms, etc. that seem to allow the finder optical system to be reduced in size. However, no consideration is given in terms of performance. Therefore, the disclosed technique lacks feasibility. In Numerical Example 5, a reduction in thickness is attained by reducing the number of lenses constituting the ocular optical system. Accordingly, the objective optical system itself cannot be reduced in thickness. When the optical system is arranged to attain a higher zoom ratio, it is difficult to reduce the thickness satisfactorily.
In JP(A) Hei 11-38472 and 11-38473, the prisms are large in size although the spacing between the movable lens units is reduced. Accordingly, the overall size of the finder optical system, including the prism arrangement, has not yet been reduced satisfactorily.
Thus, all the prior art optical systems have problems to be solved in terms of performance or size. A compact and high-performance finder that simultaneously satisfies the demands for high performance and size reduction has not yet been attained.
SUMMARY OF THE INVENTION
In view of the above-described problems with the prior art, an object of the present invention is to provide a high-performance real-image finder optical system reduced in size, particularly reduced in thickness.
To attain the above-described object, the present invention provides a first finder optical system that includes, in order from the object side thereof: an objective optical system having a
Nguyen Thong
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
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