Optical: systems and elements – Prism – With refracting surface
Reexamination Certificate
1998-11-30
2001-01-16
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With refracting surface
C359S431000, C396S296000, C396S377000
Reexamination Certificate
active
06175455
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an indication display device of a finder optical system used in a camera for photography or an electronic camera.
A finder for a camera needs indication display within the field of view to display an indication of a distance measurement range for autofocusing and an indication for parallax correction in photography at the closest focusing distance. Conventionally, a real-image finder is provided with an indication display member near an intermediate image formation plane. Known indication display methods include a method wherein incident light is blocked to display an indication, and a method wherein a wedge-shaped deflecting member is provided on the surface of an optical element of a Porro prism to deflect incident light in a direction in which it does not reach a viewer's pupil, as disclosed in Gazette Containing the Japanese Patent No. 2629690.
However, the method wherein incident light is blocked requires extra processing, e.g. vapor deposition onto a transparent member. This causes an increase in the cost. In the case of the method wherein incident light is deflected, if an indication display member is formed as a molded part, it is unnecessary to subject each individual parts to extra processing, and there is no increase in the cost. However, in the case of a prism in which an entrance surface and an exit surface are disposed close to each other as in a pentagonal prism, for example, deflected rays may be totally reflected by the exit surface to reach the viewer's pupil as light still having a strong intensity, thus causing ghost light. Rays deflected by a deflecting member for indication deviate from a visual field viewing light path but are still directed toward the pupil. In a Porro prism such as that shown in
FIG. 1
, an entrance surface
11
is surrounded with a first reflecting surface
12
and three diffusing surfaces
13
. Therefore, most of deflected rays impinge on the diffusing surfaces
13
to become diffused light of weak intensity. Accordingly, only a slight amount of light reaches the viewer's pupil and is not recognized as ghost light. In the case of a pentagonal prism such as that shown in
FIG. 2
, light that travels along a normal light path enters the prism through an entrance surface
21
and is successively reflected by reflecting surfaces
23
and
23
′ to exit from an exit surface
22
. However, the exit surface
22
has a mirror surface not included in the normal light path in the neighborhood of the entrance surface
21
. Therefore, as shown in part (a) of
FIG. 4
, deflected rays
40
may be totally reflected by an exit surface
41
(
22
) and reflected by a first reflecting surface
42
and a second reflecting surface
43
to reach a viewer's pupil without being diffused. In this case, a bright ghost image having a size equivalent to the indication is undesirably observed, giving rise to a problem.
SUMMARY OF THE INVENTION
In view of the above-described problems with the prior art, an object of the present invention is to provide an indication-within-finder made free from ghost light at a minimal cost even in the case of a prism having a mirror surface not included in the normal light path in the neighborhood of an entrance surface thereof as in a pentagonal prism.
To attain the above-described object, the present invention provides a real-image finder including an objective system having a positive power, and an image-inverting system including a prism. The real-image finder further includes an ocular system having a positive power. A deflecting member is placed near an intermediate image plane formed by the objective system to deflect incident light rays with respect to a visual field viewing light path. A prism is placed closer to the ocular system than the intermediate image plane. The prism has a mirror surface not included in a normal light path in the neighborhood of an entrance surface thereof. Light rays deflected by the deflecting member and totally reflected by the mirror surface are blocked or scattered before reaching the ocular system.
It is desirable that the deflected light rays should satisfy the following condition (1):
20°<&ohgr; (1)
where &ohgr; is an angle formed between the deflected light rays and the mirror surface.
It is more desirable that the deflected light rays should satisfy the following condition (1′):
20°<&ohgr;<50° (1′)
It is also desirable that the deflecting member should have a pair of refracting surfaces, and the refracting surfaces should satisfy the following condition (2):
&ohgr;′<40° (2)
where &ohgr;′ is an angle formed between each of the refracting surfaces and the mirror surface.
It is more desirable that the deflecting member should have a pair of refracting surfaces, and the refracting surfaces should satisfy the following condition (2′):
10°<&ohgr;′<40° (2′)
It is also desirable that the deflecting member should have a pair of refracting surfaces, and the refracting surfaces should satisfy the following condition (3):
40°<&thgr;<100° (3)
where &thgr; is a vertex angle formed between the pair of refracting surfaces.
It is more desirable that the deflecting member should have a pair of refracting surfaces, and the refracting surfaces should satisfy the following condition (3′):
50°<&thgr;<90° (3′)
In addition, the present invention provides a real-image finder including an objective system having a positive power, and an image-inverting system including a prism. The real-image finder further includes an ocular system having a positive power. A deflecting member is placed near an intermediate image plane formed by the objective system to deflect incident light rays with respect to a visual field viewing light path. A prism is placed closer to the ocular system than the intermediate image plane. The prism has a mirror surface not included in a normal light path in the neighborhood of an entrance surface thereof. The deflecting member has a pair of refracting surfaces. A ridge formed between the pair of refracting surfaces is not parallel to the mirror surface.
In addition, the present invention provides a real-image finder including an objective system having a positive power, and an image-inverting system including a prism. The real-image finder further includes an ocular system having a positive power. A deflecting member is placed near an intermediate image plane formed by the objective system to deflect incident light rays with respect to a visual field viewing light path. A prism is placed closer to the ocular system than the intermediate image plane. The prism has a mirror surface not included in a normal light path in the neighborhood of an entrance surface thereof. The deflecting member has a diffusing surface.
It is desirable that the deflecting member should be provided on the entrance surface of the prism.
It is desirable that the prism should be a pentagonal prism.
It is desirable that the prism should be formed by injection molding of a plastic material.
The reasons for adopting the above-described arrangements, together with the functions thereof, will be described below.
If the system is arranged to block or diffuse 80% or more of light rays reflected by the mirror surface, which is not included in the normal light path, after being deflected by the deflecting member, ghost light gives rise to no problem even at a position other than the pupil position assumed in design. The ocular system, which extends from the intermediate image plane to the finder exit part, can be arranged in a compact form by allowing the deflected light rays to impinge at a deep angle on the mirror surface, which is not included in the normal light path. Visual field rays near the intermediate image plane are approximately telecentric, i.e. approximately parallel to the optical axis. Therefore, in the case of a finder with a small-sized ocular system, as shown in
FIG. 5
Kato Shigeru
Suzuki Masahiro
Olympus Optical Co,. Ltd.
Pillsbury Madison & Sutro LLP
Spyrou Cassandra
Treas Jared
LandOfFree
Real-image finder does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Real-image finder, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Real-image finder will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2436018