Real image type finder

Details Real image type finder Real image type finder

1998-09-23

2001-05-22

Gray, David M. (Department: 2851)

Photography

With viewfinder

Lens detail

C396S386000, C359S643000

Reexamination Certificate

active

06236813

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a real image finder of a camera.
2. Background Arts
A real image finder is usually composed of an objective lens, a field lens, an erect image optical system and an eyepiece. For example, Japanese Patent No. 2538525 discloses a zoom finder that is composed of an objective lens system consisting of a concave lens and a convex lens, a field lens, an erect image prism and an eyepiece, arranged in this order from the object side. Japanese Laid-open Patent application No. 7-159865 discloses a zoom finder that is composed of an objective lens system consisting of a concave lens and a convex lens, a prism integrally formed with a convex lens, an erect image prism and an eyepiece, arranged in this order from the object side. Japanese Laid-open Patent application No. 4-19705 discloses three embodiments of fixed magnification finders. The first embodiment is composed of an objective lens, a prism with a first reflection surface, and a prism with second to fourth reflection surfaces that functions as an eyepiece, which are arranged in this order from the object side. The second embodiment is composed of an objective lens, a prism with first and second reflection surfaces, and a prism with third and fourth reflection surfaces that functions as an eyepiece, which are arranged in this order from the object side. The third embodiment uses a reflection mirror in addition to an objective lens and two prisms.
As seen from the above examples, conventional real image type finders need at least four elements to be a zooming finder, or at least three elements for a fixed magnification. As the number of elements increases, the manufacturing cost of the finder increases. As the number of elements' surfaces and air gaps increases with the number of elements, the formed image gets dark because of the surface reflection.
OBJECT OF THE INVENTION
In view of the foregoing, a prime object of the present invention is to provide a real image finder that uses a smaller number of elements and thus forms a clearer and brighter image.
DETAILED DESCRIPTION OF THE INVENTION
To achieve the above object, the present invention provides a real image type finder that comprises an objective lens system including at least a convex lens and having a positive power as a whole, an erect image prism having four reflection surfaces and inverting an inverted image of an object formed through the objective lens system in horizontal and vertical directions, and an eyepiece having a positive power, the erect image prism and the eyepiece lens being integrated by forming an eyepiece surface on the image side of the erect image prism.
Since the eyepiece, which has conventionally been a separate element, is integrated into the erect image prism, the number of elements is reduced. Because the number of elements' surfaces is reduced, the loss in the light intensity is reduced, so that the finder of the present invention forms a brighter image.
It is general to dispose a field lens on the object side of the erect image prism, for effectively conducting light bundles from the objective lens to the eyepiece. Therefore, it is preferable to integrate a field lens with the erect image prism by forming a field lens surface on the object side of the erect image prism. Thereby, the light from the objective lens is effectively conducted to the eyepiece without increasing the number of necessary elements.
It is also possible to form most of the image side surface of the objective lens system as a field lens surface so that the objective lens system doubles as a field lens. Thereby the erect image prism may have a flat surface on the object side thereof, on which it is possible to print a parallax correction mark. In that case, the objective lens system with the field lens surface is preferably a double-convex lens whose object side surface, i.e. the field lens surface, has a larger radius of curvature than its image side surface.
The real image finder of the present invention preferably satisfies the following condition:
−0.01<{(1−Np)/Re}−(Np/Tp)<0  (1)
wherein Re is the paraxial radius of curvature of the eyepiece surface, Np is the refractive index of the erect image prism, and Tp is the axial thickness of the erect image prism.
Above the upper limit of the condition (1), the diopter would be shifted to the plus side where the image plane of the objective lens system is located on the object side of the erect image prism. Below the lower limit of the condition (1), on the other hand, the diopter would be shifted too much to the minus side. To prevent the diopter from increasing, the image plane of the objective lens system must be located away from the erect image prism. As a result, in order to obtain a sufficiently large magnification, the total length of the finder would be elongated.
Forming the eyepiece surface integrally with the erect image prism lens results in increasing astigmatism and coma aberration. However, astigmatism and coma aberration can be compensated for by forming the eyepiece surface to be aspherical. In that case, the real image finder of the present invention should preferably satisfy the following conditions:
0<&Dgr;Z−Re·[1−{square root over ( )}{1−(4/Re
2
)}]<0.01  (2)
wherein Re is the paraxial radius of curvature of the eyepiece surface, and &Dgr;Z is the sag value at the height of 2 mm from the optical axis.
Above the upper limit of the condition (2), spherical aberration would be over-compensated, though astigmatism and coma aberration would be compensated for very well. Below the lower limit of the condition (2), the compensating effect of the aspherical surface would not be expected.
When the present invention is applied to a fixed magnification finder, the objective lens system preferably consists of a single lens having a positive power and at least one aspherical surface. Thereby, aberrations are maintained in better balance even while the objective lens system consists of a single lens. Especially in the case where the objective lens system doubles as a field lens and the erect image prism has a flat object side surface, if the single objective lens does not have at least one aspherical surface, aberrations become so large that it is impossible to use the single lens as the objective lens system.
For a zoom finder, the objective lens system may be composed of a first or forward lens with a negative power and a second or rearward lens with a positive power arranged in this order from the object side, such that the second lens is movable along the optical axis for zooming. Thereby the zoom finder can be composed of a smallest possible number of elements. According to this configuration, the change in diopter during the zooming would be more reduced if the first lens is also moved by a small amount during the zooming. On the contrary, where the objective lens system is composed of a first lens with a positive power and a second lens with a negative power, the field lens must have such a strong positive power that it is difficult to form the field lens integrally with the erect image prism. Where the second lens has a negative power, it is impossible to make the objective lens system function as the field lens.


REFERENCES:
patent: 4751539 (1988-06-01), Yamada et al.
patent: 4842395 (1989-06-01), Sato et al.
patent: 4947198 (1990-08-01), Inanobe et al.
patent: 5034764 (1991-07-01), Inabata
patent: 5132838 (1992-07-01), Ohshita
patent: 5270758 (1993-12-01), Morooka
patent: 5563736 (1996-10-01), Lewis
patent: 5625486 (1997-04-01), Mori et al.
patent: 5689373 (1997-11-01), Takato
patent: 5796518 (1998-08-01), Kato
patent: 4-19705 (1992-01-01), None
patent: 7-159865 (1995-06-01), None
patent: 2538525 (1996-07-01), None

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