Photography – With viewfinder – With prism
Reexamination Certificate
2000-01-18
2001-08-07
Mathews, Alan A. (Department: 2851)
Photography
With viewfinder
With prism
C396S386000, C359S834000
Reexamination Certificate
active
06272294
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates a real-image finder optical system.
2. Description of the Related Art
In a lens-shutter type camera, a real-image finder optical system, which is provided independently from the photographing optical system, generally includes a positive objective optical system, an erecting optical system, and a positive eyepiece optical system. In the real-image finder optical system, the objective optical system forms an inverted image which is upside down and reversed from left to right, and the erecting optical system erects the inverted image to the proper orientation. The erected image is viewed through the eyepiece optical system. In such an erecting optical system, a Porro prism having four reflection surfaces is generally used. Further, an erecting optical system is usually provided between the inverted image formed by the objective optical system and the eyepiece optical system.
In the above explained prior art, since an erecting optical system is provided between the inverted image formed by the objective optical system and the eyepiece optical system, the optical path from the inverted image to the eyepiece optical system is long. Therefore the focal length of the eyepiece optical system has to be made longer. However, if the focal length of the eyepiece optical system is long, the finder magnification (f
o
/f
E
; f
o
: the focal length of the objective optical system; f
E
: the focal length of the eyepiece optical system) becomes lower, which results in a decrease of the apparent visual angle. Due to this decreased the apparent visual angle, an operator feels difficulties in observing an image through the eyepiece optical system, and further the quality the image is deteriorated.
The size of the eyepiece optical system in the radial direction is substantially determined by the apparent visual angle and the eye relief. More specifically, the larger the apparent visual angle and the eye relief become, the larger the eyepiece optical system is in the radial direction. Furthermore, the size of the light emitting surface of the erecting optical system is substantially the same as the final surface of the eyepiece optical system. In addition, it is understood that the optical path length of the erecting optical system is approximately three times as long as the diameter of the light emitting surface of the erecting optical system. Therefore if the eye relief and the apparent visual angle are made larger, the optical path length of the erecting optical system is inevitably made longer, and thereby the finder magnification decreases as explained. Subsequently, the erecting optical system is made larger, and finally the entire finder optical system has to be made larger.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a miniaturized real-image finder optical system in which the focal length of the eyepiece optical system can be made shorter, and the finder magnification and the apparent visual angle can be increased.
The present invention is based on the following conceptions:
(i) the optical path length between the image (formed by the objective optical system) and the eyepiece optical system, i.e., the focal length of the eyepiece optical system, is made shorter, and thereby the finder magnification and the apparent visual angle are made larger; and
(ii) the optical path length of the erecting optical system is made shorter, and the erecting optical system itself is made smaller, while a longer eye relief and a larger apparent visual angle are secured.
In order to attain the first conception (item (i)), among reflection surfaces required for the erecting optical system, at least three reflection surfaces are positioned on the side of the object with respect to the image formed by the objective optical system. According to this arrangement, the erecting optical system is constituted even if the focal length of the eyepiece optical system is made shorter.
In order to attain the second conception (item (ii)), the number of reflection surfaces between the image and the eyepiece optical system is reduced.
In order to achieve the above mentioned object, there is provided a real-image finder optical system including a positive objective optical system in which an image of an object, being upside down and reversed from left to right, is formed; an erecting optical system, having a plurality of reflection surfaces, in which the image formed by the objective optical system is erected to the same orientation as the object; and an eyepiece optical system through which the image erected by the erecting optical system is viewed. A shape of a finder field of view is a rectangle having major sides along the transverse direction in the finder field of view. Along the optical path on the side of the object with respect to the image formed by the objective optical system, at least three reflection surfaces of the erecting optical system are provided, and further, the most object-side reflection surface is positioned so that the optical axis is deflected in a direction along the major side of the rectangular finder field of view.
In the reflection surfaces of the erecting optical system, the second and third reflection surfaces, in this order from the object, are preferably two reflection surfaces of a prism. Moreover, between the most object-side reflection surface and the second reflection surface, a positive lens element is preferably provided. This positive lens element can integrally be formed on an incident surface of the prism.
Including a case where the positive lens element is integrally formed on the incident surface of the prism, when the positive lens element is provided between the most object-side reflection surface and the second reflection surface, the following condition is preferably satisfied:
0.1
<L/fc<
0.7 (1)
wherein
L designates the distance between the most image-side surface of the objective optical system and the positive lens element; and
fc designates the focal length of the positive lens element.
An emitting surface of the prism is preferably provided on an image forming position of the objective optical system. Further, on the emitting surface of the prism, finder-field information can be formed.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-11007 (filed on Jan. 19, 1999) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 5752107 (1998-05-01), Hasushita et al.
patent: 6018417 (2000-01-01), Yano
patent: 6058273 (2000-05-01), Abe
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Mathews Alan A.
LandOfFree
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