Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-03-17
2001-10-30
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S682000
Reexamination Certificate
active
06310732
ABSTRACT:
RELATED APPLICATIONS
This application claims the priorities of Japanese Patent Application No. 11-078971 filed on Mar. 24, 1999 and Japanese Patent Application No. 2000-007665 filed on Jan. 17, 2000, which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power-variable finder and, in particular, to a simple power-variable finder suitable for low-cost cameras.
2. Description of the Prior Art
Recently, as low-cost cameras such as film-plus-camera boxes have come into widespread use, there has been a strong demand for finders suitable for such cameras. When a taking system is of a power-variable type, then such a finder is required to have a power-varying function for changing the field magnification while keeping a constant diopter, and it is also important for the finder to be small enough to be mounted in a narrow space and inexpensive.
An example of power-variable finders is disclosed in Japanese Unexamined Patent Publication No. 3-92807. This power-variable finder is an inverted Galilean finder having a simple configuration comprising one objective lens and two eyepieces, and is capable of varying its power by replacing the objective lens. However, it necessitates a shelter space for replaceable lens components, thus failing to save its space for reducing the dimensions of the finder. Though there have been known power-variable finders in which eyepiece systems are changed over instead of replacing objective systems, they are similarly problematic in terms of saving space.
On the other hand, Japanese Unexamined Patent Publication No. 61-167919 discloses an example of zoom type power-variable finders in which lenses of the objective system are not so large, thereby being suitable for making the finder smaller. This power-variable finder comprises, successively from the object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, and a third lens group having a negative refracting power; and reduces the fluctuation in diopter upon power variation, thereby attaining a favorable finder image. In this power-variable finder, however, a plurality of lens groups are movable groups, thereby complicating their moving mechanism and making it hard to cut down the manufacturing cost.
Japanese Unexamined Patent Publication No. 63-221313 discloses another example of zoom type power-variable finders comprising, successively from the object side, a fixed first lens group having a negative refracting power, a movable second lens group having a positive refracting power, and a fixed third lens group having a negative refracting power, in order to favorably correct chromatic aberration. In such a configuration, however, the positive second lens group comprises a plurality of lenses in order to attain a sufficient refracting power, and the lens moving mechanism is complicated since the lens spaces are changed within the second lens group so as to reduce variations in diopter while the second lens group as a whole is moved. In view of these points, it is hard to consider that this power-variable finder can be made inexpensively.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a small-size, low-cost, simple power-variable finder having a power-varying function, which is mountable in a narrow space.
The simple power-variable finder in accordance with the present invention comprises, successively from an object side, a first lens group having a negative refracting power, a second lens group, movable on an optical axis, having a positive refracting power, and a third lens group having a negative refracting power; and satisfies the following conditional expressions (1) to (3):
−1.3
<f
2
/f
1
<−1.0 (1)
1.6
<f
3
/f
1
<2.2 (2)
−2.4
<D
T
/f
1
<−1.7 (3)
where
f
1
is the focal length of the first lens group;
f
2
is the focal length of the second lens group;
f
3
is the focal length of the third lens group; and
D
T
is the distance on the optical axis between the surface closest to the object in the first lens group and the surface closest to an eye in the third lens group.
Preferably, the first lens group is constituted by a single negative lens having at least one aspheric surface and a concave lens directed onto the eye side, the second lens group is constituted by a single biconvex lens having at least one aspheric surface, and the third lens group is constituted by a single negative lens.
Preferably, a first field stop is disposed in front of or behind the first lens group.
At least two field stops may be provided, such that the first field stop is disposed in front of or behind the first lens group, whereas the second field stop is disposed in front of or behind the second lens group.
The first field stop, displayable as a finder field frame, may be disposed in front of the first lens group.
The first field stop, made of a transparent member, may be disposed in front of the first lens group, and the transparent member may be formed with a frame displayable as a finder field frame.
The surface closest to the object in the first lens group may be made concave onto the object side.
Preferably, the negative lens constituting the first lens group, the biconvex lens constituting the second lens group, and the negative lens constituting the third lens group are made of the same material.
REFERENCES:
patent: 4854680 (1989-08-01), Kikuchi
patent: 5796518 (1998-08-01), Kato
Japanese Patent Office, Patent Abstracts of Japan, Publication No.: 03092807A, Date of Publication: Apr. 18, 1991, Application No.: 01230158, Date of Filing: Sep. 5, 1989, pp. 41-46.
Japanese Patent Office, Patent Abstracts of Japan, Publication No.: 63221313 A, Date of Publication; Sep. 14, 1988, Application No. 62053993, Date of Filing Mar. 11, 1987, pp. 69-85.
Japanese Patent Office, Patent Abstracts of Japan, Publication No.: 61167919A, Date of Publication: Jul. 29, 1986, Application No. 60008838, Date of Filing: Jan. 21, 1985, pp. 103-109.
Fuji Photo Optical Co., Ltd.
Schwartz Jordan M.
Snider Ronald R.
Snider & Associates
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