GB2320584A

GB2320584A - Prism with part recessed rear face for viewfinder

Info

Publication number: GB2320584A
Application number: GB9726885A
Authority: GB
Inventors: Isamu Hirai; Etsuro Nishio; Yutaka Kamijo
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1996-12-20
Filing date: 1997-12-18
Publication date: 1998-06-24
Anticipated expiration: 2017-12-18
Also published as: DE19756567B4; DE19756567A1; JPH10186454A; GB9726885D0; GB2320584B

Abstract

A prism 20 maybe provided with a plane parallel plate 25 which is applied to a central portion of a rear surface of the prism. A surface of the plane-parallel plate is a first emergent plane 22, which faces an eyepiece 12 of a viewfinder. A portion of the prism positioned above the plane-parallel plate is a second emergent plane 23, which faces a condenser lens 15 of a photometry optical system. Thus, the first emergent plane is projected with respect to the second emergent plane. The plane parallel plate may be integral with the prism (fig.7). Photoreceptor 16 is shown.

Description

2320584 - 1 PRISM to a pentagonal roof camera.

pentagonal roof prism of a single-lens reflex camera, which serves as a part of a photometry optical system. Namely, a photoreceptor element for photometry is disposed behind a rear surface of the pentagonal roof prism, close to an eyepiece lens of the viewfinder optical system, and a condenser lens is interposed between the rear surface and the photoreceptor. Thus, a light beam passes through a focussing glass of the viewfinder, is reflected by an inner plane of the pentagonal roof prism, and emerges from the rear surface to become incident on the photoreceptor so that a photometry value is sensed.

At least two portions of the rear surface of the pentagonal roof prism, a portion facing the eyepiece and a portion facing the condenser lens, should be transparent. Usually, the transparent surface is manufactured by grinding and polishing the surface. However, since the operation, by which a stepped surface is polished, is very cumbersome, the portions facing the eyepiece and the condenser lens, i.e. the major part of the rear surface of the pentagonal roof prism, are formed as a continuous flat surface.

The size of the rear surface is limited so that the The present invention relates prism mounted in a single-lens reflex Conventionally, there is known a pentagonal roof prism is formed as small as possible, and a major portion of the rear surface must face the eyepiece of the viewfinder. Therefore, the condenser lens of the photometry optical system is disposed at the edge of the rear surface. Thus, in a conventional device, the area of the portion of the rear surface facing the condenser lens for the photometry is not sufficiently large, thereby restricting the amount of light received by the photoreceptor and causing difficulty in the enhancement of the sensing accuracy of the photometry value.

Therefore, an object of the invention is to provide a prism which enables a higher proportion of the incident light to be subjected to photometry, while retaining the limited size of the prism.

According to one aspect of the present invention there is provided a prism mounted in an optical device, the optical device being provided with a viewfinder, having an eyepiece, and a photometry optical system, having a condenser lens. The prism comprises a first emergent plane facing the eyepiece, and a second emergent plane facing the condenser lens. The second emergent plane is recessed relative to the first emergent plane.

According to another aspect of the present invention there is provided an optical system disposed in front of both a viewfinder, having an eyepiece, and a photometry optical - 3 system, having a condenser lens. The optical system comprises a prism having a rear surface, a sectional area of the prism, being parallel to the rear surface, which decreases as the rear surface approaches. The prism has a first emergent plane and a second emergent plane. The first emergent plane faces the eyepiece and the second emergent plane faces the condenser lens. One of the first or second emergent planes is coplanar with the rear surface. The remaining emergent plane is parallel to the rear surface. The second emergent plane is 10 recessed relative to the first emergent plane.

Examples of the present invention will be better understood from the description of the preferred embodiments of the invention set forth below, together with the accompanying drawings, in which:

Fig. 1 is a partial sectional view showing a pentagonal roof prism and a construction associated with the pentagonal roof prism, to which a first embodiment of the present invention is applied; Fig. 2 is a side view showing the pentagonal prism of 20 the first embodiment; Fig. 3 is a perspective view showing the pentagonal prism of the first embodiment; Fig. 4 is a perspective view showing the pentagonal prism of the first embodiment, viewed from a rear side 25 thereof; Fig. 5 is a partial sectional view showing a conventional pentagonal roof prism and a construction associated with the pentagonal roof prism; Fig. 6 is a view showing a path of selected rays in a 5 pentagonal prism; Fig. 7 is a partial sectional view showing a pentagonal roof prism and a construction associated with the pentagonal roof prism, to which a second embodiment of the present invention is applied; Fig. 8 is a side view showing the pentagonal prism of the second embodiment; Fig. 9 is a perspective view showing the pentagonal prism of the second embodiment, in a disassembled state; and Fig. 10 is a perspective view showing the pentagonal prism of the second embodiment, viewed from a rear side thereof.

The present invention will be described below with reference to embodiments shown in the drawings.

Fig. 1 shows a viewfinder optical system mounted in a single-lens reflex camera to which a first embodiment of the present invention is applied. This prism 20 is made of glass.

A focussing glass 11 faces an incident surface 21 located on an undersurface of the pentagonal roof prism 20. A lens group 12a of an eyepiece unit 12 faces a f irst emergent plane 22 provided on a rear surface of the pentagonal roof - prism 20. The eyepiece unit 12 has a housing 13, which is provided with an opening 14 at an upper portion thereof. The opening 14 houses a condenser lens 15. A photoreceptor 16 is disposed behind the opening 14 so that a photometry value can 5 be sensed. The condenser lens 15 faces a second emergent plane 23 formed above the first emergent plane 22. Namely, the pentagonal prism 20 is utilized as a part of a photometry optical system as well as a viewfinder optical system.

As shown in Figs. 2 through 4, the top ridge 27 of the pentagonal roof prism 20 is inclined with respect to the rear surface so that a sectional area of the pentagonal roof prism 20, being parallel to the rear surface, decreases as the rear surface approaches. The rear surface of the pentagonal roof prism 20 is approximately triangular.

A trapezoidal plane-parallel plate 25 is applied to the central portion of the rear surface with adhesive having photo-transmissibility. A surface of the plane-parallel plate 25 is a first emergent plane 22. The two inclined sides of the trapezoid of the plane-parallel plate 25 are coincident with the two sides of the rear surface of the pentagonal prism 20 allowing the second emergent plane 23, which has a shape similar to that of the rear surface, to be formed at a portion above the plane-parallel plate 25 on the rear surface, i.e. at a portion to which the plane- parallel plate 25 is not applied. A recessed portion 24, which is a six sided strip shape extending in a horizontal direction, is formed beneath the plane- parallel plate 25.

Thus, the first emergent plane 22 facing the lens group 12a of the eyepiece unit 12 is projected relative to the second emergent plane 23 facing the condenser lens 15 of the photometry optical system. Although the first and second emergent planes 22 and 23 and the surface of the recessed portion 24 are ground and polished to be transparent, the surface of the recessed portion 24 may remain frosted or nonpolished. Surfaces, which are formed adjacent above and beneath the first emergent plane 22 and which are in parallel with the optical axis of the eyepiece unit 12, are frosted.

As shown in Fig. 5, a portion 42a facing the condenser lens 15 of the photometry optical system, and a portion 42b facing the lens group 12a of the eyepiece unit 12, for an emergent plane 42 of a conventional device, are coplanar, and a recessed portion 24 is formed beneath the emergent plane 42. Comparing this pentagonal roof prism 40 shown in Fig. 5 with the pentagonal roof prism 20 shown in Fig. 1, the distances from the tip portions S to the emergent planes 42 and 22 of the pentagonal prisms 40 and 20, respectively, are equal to each other. Further, in the conventional device, shown in Fig. 5, the condenser lens 15 is disposed at a position subsequent to the first emergent plane 42 in comparison with the embodiment device, shown in Fig. 1. Therefore, in the conventional device, an area of the emergent plane facing the condenser lens 15 is relatively small, and thus, the amount of light received by the photoreceptor 16 is not suitably sufficient.

Conversely, in the first embodiment of the present invention, the second emergent plane 23 is recessed relative to both the first emergent plane 22, and the emergent plane 42 of the conventional pentagonal roof prism 40. In fact, some further portions of the rear surface of the pentagonal roof prism 20 are able to be recessed because, as shown in Fig. 6, the significant portion of the light from the object to be photographed travels along the path bounded by the doublechained lines K and L, and becomes incident on the rear surface without penetrating the hatched areas M. Therefore, by removing the hatched areas M, the object image, realized in the viewfinder, is not perceivably deteriorated. The reason behind the first emergent plane 22 being preferably projected relative to the other portions of the rear surface is to prevent stray light, which is indicated by a chained line i in Fig. 5, from being internally reflected, as occurs in the prism 40.

Thus, with the present embodiment, the condenser lens 15 and the photoreceptor 16 can be disposed closer to the tip portion S of the pentagonal roof prism 20. As a result, an area of the emergent plane facing the condenser lens 15 becomes larger in comparison with the conventional device so that an increased proportion of the light entering the pentagonal prism 20 can be directed, via the condenser lens 15, to the photoreceptor 16, and thus, photometry can be performed with a greater accuracy and also at a lower luminance.

Fig. 7 shows a pentagonal roof prism 20 mounted in a single-lens reflex camera to which a second embodiment of the present invention is applied. Figs. 8 through 10 show the pentagonal roof prism of the second embodiment. Components common to the first embodiment bear common reference numerals.

In the second embodiment, a first recessed portion 26, which is triangular, is formed close to the peak of the rear surface of the pentagonal roof prism 20, and a second recessed portion 24, which is a six sided strip shape extending in a horizontal direction, is formed at the bottom portion of the rear surface. In other words, a trapezoidal projection is formed between these recessed portions 24 and 26 with the surface of the projection being a first emergent plane 22.

Note that the recessed portions 24 and 26 are formed by grinding in order to create a frosted surface, i.e. a nonpolished surface.

A reverse face 32 of a transparent plate 31, which is a plane-parallel plate which is made of glass, is applied to the triangular surface of the first recessed portion 26. The transparent plate 31 is triangular and slightly smaller than that of the triangular surface. The transparent plate 31 has a thickness which is less than the depth of the first recessed portion 26, the depth extending in parallel to the optical axis of the eyepiece unit 12. The transparent plate 31 has an obverse face 33, which is a transparent (i.e. polished) plane, and a reverse face 32, which may be either a transparent plane or a frosted (i.e. non-polished) plane. The reverse face 32 is applied to the first recessed portion 26 with adhesive which exhibits photo-transmissibility. It is preferable that the adhesive does not substantially shrink when hardening.

Thus, by applying the transparent plate 31, which has the transparent obverse face 33, to the surface of the first recessed portion 26, with the adhesive having phototransmissibility, a transparent plane is produced on the first recessed portion 26. Further, the condenser lens 15 of the photometry optical system is disposed close to and facing the transparent plate 31. Accordingly, light having less turbulence enters the photoreceptor 16 for photometry.

The depth of the first recessed portion 26 depends upon the size of the focussing glass 11 or the second recessed portion 24, the depth extending in parallel to the optical axis of the eyepiece unit 12, and the depths of the first and second recessed portion 26 and 24 being calculated so as not to disturb a light beam which enters the incident surface 21 and emerges into the eyepiece unit 12.

As described above, in the embodiment, since the transparent plate 31, which has a thickness less than the depth of the recessed portion 26 and has the transparent obverse face 33, is applied to the surface of the first recessed portion 26 of the pentagonal roof prism 20, the condenser lens 15 can be disposed substantially in the first recessed portion 26. Therefore, light, the amount of which is enough to sense the photometry value of the object to be photographed, is led to the photoreceptor 16 so that the photometry can be performed with a high accuracy. Further, since the second embodiment is constructed in such a manner that the transparent plane is produced by applying the thin transparent plate 31 to the first recessed portion 26, a polishing process carried out on the surface of the pentagonal roof prism 20 can be minimized as much as possible, and thus, the manufacturing cost for the optical system incorporating both the pentagonal roof prism 20 and the portion associated with the prism 20 can be reduced.

Note, that the present invention is not restricted to a prism having a specific shape, although the pentagonal roof prism 20 is utilized in the first and second embodiments.

Although the embodiments of the present invention have been described herein with reference to the accompanying - 11 drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention.

Claims

1. A prism for mounting in an optical device having a viewfinder and a photometry optical system, said viewfinder having an eyepiece, and said photometry optical system having a condenser lens, said prism comprising:first emergent plane to face said eyepiece; and second emergent plane to face said condenser lens, said second emergent plane being recessed relative to said first emergent plane.

2. A prism according to claim 1 wherein said first emergent plane is a surface of a planeparallel plate, said plane-parallel plate being applied to a first part of a rear surface of said prism, and said second emergent plane is defined by a second part of said rear surface to which said plane-parallel plate is not applied.

3. A prism according to claim 1 wherein said first emergent plane is formed by applying a transparent plate, which has at least one transparent surface, to a frosted surface of said prism with adhesive having phototransmissibility in such a manner that said transparent surface faces outward from said prism.

4. A prism according to claim 3 wherein said adhesive does not substantially shrink when hardening.

5. A prism according to any preceding claim wherein said - 13 prism is made of glass.

6. A prism according to any preceding claim wherein said prism is a pentagonal roof prism mountable in a single lens reflex camera.

7. A prism according to any preceding claim wherein a magnitude of the depth in which said second emergent plane is recessed is insufficient is selected whereby said second emergent plane does not disturb a light beam which enters said prism and becomes incident on said eyepiece.

8. An optical system disposed in front of a viewfinder and a photometry optical system, said viewfinder having an eyepiece, and said photometry optical system having a condenser lens, said optical system comprising:- a prism having a rear surface, wherein a sectional area 15 of said prism parallel to said rear surface decreases towards said rear surface; said prism having a first emergent plane and a second emergent plane, said first emergent plane facing said eyepiece and said second emergent plane facing said condenser lens, one of said first or second emergent planes being coplanar with said rear surface, the remaining emergent plane being parallel to said rear surface, said second emergent plane being recessed relative to said first emergent plane.

9. A single lens reflex camera including a prism according to any one of claims 1 to 7.

- 14

10. A prism substantially as herein reference to figures 1 to 5 and 6 to 10.

described with