Rangefinder optical system

Details Rangefinder optical system Rangefinder optical system

2000-03-02

2002-05-14

Evans, F. L. (Department: 2877)

Optics: measuring and testing

Range or remote distance finding

Base line instrument

C396S141000

Reexamination Certificate

active

06388738

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rangefinder optical system in which an in-focus state can be detected by superimposing two images formed by the first and second optical systems which are apart from each other by a predetermined base length.
2. Description of the Related Art
A rangefinder optical system, which is the combination of the first optical system (first window), a field frame optical system, and the second optical system (second window), is known in the art.
For example, a conventional rangefinder optical system, as shown in
FIG. 6
, includes the first optical system
20
through which a first image (finder image) is viewed, a field frame optical system
30
through which a field frame image is formed, and the second optical system
40
through which a second image is formed. The first optical system
20
, which is formed as an inversed Galilean type finder optical system, includes a negative objective lens element and a positive eyepiece lens element
15
. Through the eyepiece lens element
15
, an erected image of an object with the proper orientation thereof is viewed. Furthermore, the field frame optical system
30
includes a field frame member
32
which is an opaque member on which light-transmitting portions are formed, and a reflection member
33
, lens elements and the like. More concretely, one of the light-transmitting portions which is provided on the periphery of the field frame member
32
(hereinafter, the peripheral light-transmitting portion) is provided in the form of the field frame; and the other light-transmitting portion which is provided at the center thereof (hereinafter, the central light-transmitting portion) is for transmitting a bundle of rays carrying the second image. Further, in the first optical system
20
, an optical-path merging prism
24
is provided, and thereby the optical path of the first optical system
20
and that of field frame optical system
30
are merged. According to this arrangement, a bundle of rays which is incident, from the object, on the peripheral light-transmitting portion of the field frame member
32
is formed as a field frame image. Subsequently, the field frame image which is superimposed onto the above first image is viewed through the eyepiece lens element
15
.
The second optical system
40
includes a positive objective lens group
44
and a reflection member
41
having a plurality of reflection surfaces. The second image is formed in the vicinity of the field frame member
32
. Due to the peripheral and central light-transmitting portions of the field frame member
32
, the second image and the field frame image are combined thereat, and transmitted, along the optical path of the field frame optical system
30
, towards the first optical system
20
. The second image and the field frame image are superimposed onto the first image, and these superimposed images are viewed through the eyepiece lens element
15
. Since the field frame image is required to be viewed brightly and clearly, it is preferable that the incident optical axis of the field frame optical system
30
be made parallel with the optical axis of the first optical system
20
, and that the field frame member
32
be positioned to face the object. On the other hand, in order to increase the precision of the rangefinder, it is necessary to maintain the base length L between the incident optical axis of the first optical system
20
and the incident optical axis of the second optical system
40
as long as possible.
The base length L is substantially determined by the sum of the following distances:
(i) the distance from the incident surface of the second optical system
40
to the objective lens group
44
;
(ii) the focal length of the objective lens group
44
; and
(iii) the distance from the field frame member
32
to the first optical system
20
.
The focal length of the objective lens group
44
is determined so that the finder magnification of the first optical system
20
is the same as that of the second optical system
40
in order to make the size of the images (i.e., the first image and the second image) equal. The finder magnification is defined as the angular magnification. On the other hand, if an attempt is made to avoid increasing the size of the reflection member
41
, there is a need to make the distance from the incident surface of the second optical system
40
to the objective lens group
44
as short as possible. Therefore in order to maintain the longer base length L, it is necessary to increase the length P (in the direction of the base length L) between the first optical system
20
and the field frame member
32
.
However, in the above conventional arrangement in which the second image is formed in the vicinity of the field frame member
32
, the base length L cannot be made longer due to the following requirements to be satisfied at the same time:
(i) in order to view the field frame image clearly, the field frame member
32
has to be positioned to face towards the object so that much light from the object is incident thereon; and
(ii) for the purpose of obtaining substantially the same diopters, through the eyepiece lens element
15
, for the object image formed through the first optical system
20
, for the field frame member
32
, and for the second image formed in the vicinity of the field frame member
32
, the field frame member
32
and the first optical system
20
have to be positioned closer, since there is a need to make the position of a virtual image formed by the objective lens element of the first optical system
20
, and the position of the field frame member
32
optically equivalent with respect to the eyepiece lens element
15
.
In another conventional rangefinder optical system, as shown in
FIG. 7
, in order to make the distance between the field frame member
32
and the first optical system
20
longer, the optical axis from the optical-path merging prism
24
to the second optical system
40
is inclined. However, the field frame member
32
cannot be positioned to face towards the object, and therefore the field frame image becomes unclear.
Furthermore, in the case where the above-described finder optical system is provided in a camera, it is preferable that the dimensions of both the field frame optical system
30
and the second optical system
40
measured in a direction parallel with the optical axis of the first optical system
20
(hereinafter, the thickness of a camera) be as small as possible in order to achieve miniaturization of the camera, and to make space for providing other components. However, as explained, in front (the object side) of the field frame member
32
of the field frame optical system
30
, the second optical system
40
is positioned; and in the second optical system
40
, there is a need to provide an optical system to make the incident optical axis to be incident on the field frame member
32
substantially parallel with the incident optical axis of the first optical system
20
. As a result, it is difficult to reduce the thickness of the field frame optical system
30
and the second optical system
40
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rangefinder optical system through which a bright and clear field frame image can be viewed, and in which the base length can be made longer.
Further, it is another object of the present invention to provide a rangefinder optical system which can reduce the thickness of the field frame optical system and that of the second optical system.
The present invention is based on a conception that the base length between the first optical system and the second optical system is made longer by merging the optical path of the field frame optical system and that of the second optical system on the side of first optical system with respect to the filed frame member, and thereby the thickness of the field frame optical system and that of the second optical system are reduced.
In order to achieve the above-mentioned objects, there is pr

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