Pentagonal prism and related optical structures for single lens reflex cameras

A pentagonal prism holder for holding a pentagonal prism in horizontal and vertical directions is made of a transparent plastic material and at least a portion of the prism holder is formed as an element or elements of an auxiliary or supplemental optical system or a finder optical system. The auxiliary or supplemental optical system may be one that indicates photographic information in the finder view field, an optical system for photometry, an optical system for preventing a ghost from being observed in a viewfinder, or an optical system for preventing undesirable or stray light passing through an ocular lens from entering into the pentagonal prism. The element of the finder optical system may be a condenser lens or an ocular lens.

BACKGROUND OF THE INVENTION 
This invention relates to pentagonal prisms and related optical systems for 
single lens reflex cameras. 
Recently, single lens reflex cameras are being equipped with a variety of 
auxiliary optical systems operable in association with a finder optical 
system including a pentagonal prism. Typical auxiliary optical systems 
are, for example, an indicating optical system for indicating within a 
finder view field picture-taking information marked on a lens barrel as 
well as picture-taking information provided within the camera body; a 
light measuring optical system for directing light transmitted through an 
objective lens to a light receiving element; and a stray light eliminating 
optical system for shutting out ghosts or light which is inversely 
incident through an ocular lens. 
It has been customary to use a holding member for attaching such an 
auxiliary optical system to a camera body. Such structure has the 
following disadvantages: 
(I) The use of a holding member for supporting an individual optical system 
results in increased manufacturing costs and requires a large space. 
(II) Foreign matter, such as dust, is admitted into the auxiliary optical 
system or between the pentagonal prism and the auxiliary optical system. 
(III) It is difficult to make adjustment of the relative position between 
the pentagonal prism and the auxiliary optical system. 
Meanwhile, it has been proposed that an individual element in an auxiliary 
optical system, for example, a prism, be molded of plastic. This method 
presents another difficulty in that, because the individual element to be 
molded is small, a smooth molten plastic flow cannot be achieved upon 
molding, resulting in failure to produce an element of high precision and 
uniform in quality. 
In the vicinity of the pentagonal prism, on the other hand, are provided 
optical elements, such as a condenser lens and an ocular lens, 
constituting a finder optical system, besides the auxiliary optical 
system. The same difficulties as in the case of the auxiliary optical 
system are also experienced in supporting these optical elements and in 
the positional adjustment of these elements relative to the pentagonal 
prism. 
Japanese Utility Model Application No. 46-76297, which was laid open to 
public inspection on Apr. 21, 1973 discloses a single lens reflex camera 
having a device for indicating photographic information in a viewfinder, 
which device may avoid the above described disadvantages (I) and (II). 
More particularly, in the device, the frame on which a pentagonal prism is 
mounted is made of a transparent plastic resin and includes a portion 
which serves as an optical element to introduce light bearing photographic 
information or light from an indicating lamp to the pentagonal prism 
through the lower light incident surface thereof. This construction 
requires no means for holding the optical element and prevents foreign 
matter, such as dust, from entering between the optical element and the 
pentagonal prism. However, since the frame is so adapted that the 
pentagonal prism is only mounted thereon, i.e., the frame has no means for 
holding the pentagonal prism in the horizontal direction, adjustment is 
required to determine the relative position of the optical element with 
respect to the pentagonal prism in the horizontal direction, and thus the 
device will still have disadvantage (III). 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to provide a 
pentagonal prism and related optical structure for a single lens reflex 
camera, which camera will not have the above described disadvantages (I), 
(II) and (III). 
Another object of the present invention is to provide a pentagonal prism 
and related optical structure for a single lens reflex camera, in which 
camera, to avoid all of the above described disadvantages (I), (II) and 
(III), a pentagonal prism holder for holding the pentagonal prism in the 
horizontal and vertical directions has as a part thereof an element or 
elements of an auxiliary or supplemental optical system or an element of a 
finder optical system other than the pentagonal prism. 
To accomplish these objects, the pentagonal prism and related optical 
structure of the present invention is provided with a pentagonal prism 
holder made of a transparent plastic material. The pentagonal prism holder 
has as a part thereof means for holding a pentagonal prism in a horizontal 
and vertical direction and at least one optical element of an auxiliary or 
supplemental optical system or an element of a finder optical system other 
than the pentagonal prism. The auxiliary or supplemental optical system 
may be an optical system for indicating photographic information in the 
finder view field, i.e., at the outside or inside of the finder image, an 
optical system for photometry, an optical system for preventing a ghost 
from being observed in a viewfinder, or an optical system for preventing 
undesirable or stray light passed through an ocular lens from entering 
into the pentagonal prism. The element of the finder optical system may be 
a condenser lens or an ocular lens. In preferred embodiments of the 
present invention, the element of the auxiliary or supplemental optical 
system or the finder optical system is formed on either of the holding 
means or both of the holding means. 
The above described construction of the present invention requires no 
particular means for holding the optical element and prevents foreign 
matter, such as dust, from entering between the optical element and the 
pentagonal prism. Furthermore the construction of the present invention 
requires no adjustment of the relative position of the optical element 
with respect to the pentagonal prism in the horizontal and vertical 
directions because not only the optical element but also both of the 
holding means are formed on the pentagonal prism holder. 
These and other objects and features of the present invention will become 
more apparent from the following description taken in connection with the 
accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 through 5 show embodiments in which part of an indicating optical 
system is integrally formed with a holding frame for a pentagonal prism, 
the aforesaid indicating optical system being adapted to indicate within a 
finder view field picture-taking information marked on a lens barrel. 
Referring first to FIG. 1, pentagonal prism 1 is retained by holding frame 
2, which frame is formed integrally with arm 4 for supporting mirror 3 and 
prism 5. Holding frame 2 is made of a transparent plastic material such as 
acryl resin, methyl methacryl resin or styrol resin. Prism 5 is positioned 
in the upper portion of front wall 2a of the holding frame, which wall 
contacts the front reflecting surface of pentagonal prism 1. Prism 5 
constitutes, in combination with mirror 3 and concave lens 6 provided for 
diopter compensation and magnification setting, an indicating optical 
system for indicating within the finder view field the predetermined 
aperture value on diaphragm setting ring 7 fitted on a lens barrel. With 
this arrangement, the diaphragm value is indicated in the upper portion of 
the image field within the finder view field. Plate 8 adjusts the 
inclination of mirror 3. When screws 9 are tightened, mirror support arm 4 
is pressed by adjusting plate 8 to be deflected, so that the inclination 
of mirror 3 is varied. 
Holding frame 2 is formed on its peripheral side walls with a plurality of 
attaching lugs 2b, 2c and 2d having screw holes, so that frame 2 may be 
attached to a camera body by threading screws (not shown) into the 
respective screw holes. Holding frame 2 has bottom wall 2f having opening 
2e, through which the main light flux from the finder is incident on the 
lower light incident surface of pentagonal prism 1. 
With this arrangement, by simply inserting pentagonal prism 1 in holding 
frame 2 from above, the pentagonal prism may be set for its positions in 
the vertical, and horizontal directions, and hence for a relative position 
of prism 5 or mirror 3 with respect to pentagonal prism 1, thus dispensing 
with any positional adjustment of prism 5 and mirror 3. Holding frame 2 
also serves as a support for prism 5, thus reducing the number of parts, 
as compared with the prior art where a support member for prism 5 is 
provided separately. This affords a reduction in the manufacturing cost as 
well as compactness in the size of the camera. Furthermore, the thickness 
of a peripheral portion of prism 5 which requires high surface precision 
is increased by the thickness of holding frame 2. This allows smooth 
molten plastic flow, upon molding, with the improvements in surface 
precision or uniformity in quality of prism 5 as compared with prism 5 
molded separately. 
FIGS. 2 and 3 show modifications of the embodiment of FIG. 1, wherein front 
wall 2a of holding frame 2, which contacts the front reflecting surface of 
pentagonal prism 1 and prism 5, are formed integrally with each other, and 
part of the front reflecting surface is made transparent, so that light 
from the diaphragm setting ring 7 may be incident through the transparent 
portion onto pentagonal prism 1. These embodiments advantageously prevent 
admission of dust between front wall 2a of holding frame 2 and prism 5. In 
the embodiment of FIG. 3, mirror 3 and concave lens 6 are replaced by a 
single convex mirror 10. 
FIG. 4 shows a further modification, wherein front wall 2a of holding frame 
2 is divided into two portions, i.e., a portion contacting the front upper 
surface of pentagonal prism 1, and a portion contacting the front 
reflecting surface and lower light incident surface of pentagonal prism 1. 
The former portion is formed by convex lens 11, and the latter portion is 
formed by prism 12. In this embodiment, those portions of the front upper 
surface and front reflecting surface of pentagonal prism 1, which contact 
holding frame 2, are transparent, so that light coming from diaphragm 
setting ring 7 through concave lens 6 is reflected onto mirror 3, then 
past convex lens 11 and then through pentagonal prism 1, and eventually 
incident on prism 12. Light entering prism 12 is subjected to total 
reflection three times as seen in FIG. 4 and again incident on pentagonal 
prism 1. Convex lens 11 is provided for maintaining a negative power as a 
whole as well as for diopter compensation, and convex lens 11 and prism 12 
are integral with a not shown portion of front wall 2a of holding frame 2. 
FIG. 5 shows a still further modification, wherein all of the optical 
elements constituting the indicating optical system are formed as one 
block 13 in the upper portion of front wall 2a of holding frame 2. Light 
from diaphragm setting ring 7 is incident on a concave surface of block 
13, then reflected twice on the reflecting surfaces of the block, to 
eventually enter through the transparent portion of the front upper 
surface of the pentagonal prism into pentagonal prism 1. In this 
embodiment, all optical elements are formed integrally with holding frame 
2. Thus, this embodiment is superior to the preceding embodiments from the 
viewpoint of positional adjustment of the optical elements. 
In the embodiments shown in FIGS. 4 and 5, the optical elements contacting 
the front upper surface of pentagonal prism 1 are formed integrally with 
holding frame 2, while the rear wall of holding frame 2 is omitted, so as 
to afford ease of insertion of pentagonal prism 1 into holding frame 2. 
FIGS. 6 through 8 show embodiments in which optical elements in the 
indicating optical system disposed within a camera body for indicating 
picture-taking information in the finder view field are formed integrally 
with the holding frame for the pentagonal prism. 
In the embodiment of FIG. 6, front wall 2a of holding frame 2 functions as 
a light guide for guiding light from an information source (not shown) 
disposed thereabove. The light transmitted through the light guide portion 
is reflected on the reflecting surfaces provided on the front and bottom 
walls of the holding frame and then incident on the lower light incident 
surface of pentagonal prism 1. 
FIG. 7 shows an embodiment, in which the instant invention is applied to an 
indicating optical system for indicating picture-taking information in the 
side portion of the image field within the finder view field. In this 
embodiment, one side wall of holding frame 2 functions as a light guide, 
so that light from lamp 14 is reflected three times on the reflecting 
surfaces formed on the aforesaid side wall and the bottom wall of frame 2, 
then passes through transparent indicating plate 15 sandwiched between 
pentagonal prism 1 and a bottom wall of the holding frame, and is incident 
on the surface of pentagonal prism 1. The picture-taking information 
represented by characters or the like on indicating plate 15, in general, 
are seen illuminated by lamp 14. Since indicating plate 15 in this 
embodiment is sandwiched between pentagonal prism and holding frame 2, 
dust or other foreign matter does not adhere to the indicating plate. 
In the embodiment shown in FIG. 8, the rear wall of holding frame 2 
functions as a light guide, so that light from the information source (not 
shown) disposed above the rear portion of pentagonal prism 1 may be 
reflected on the reflecting surfaces provided on the rear and bottom walls 
of holding frame 2 and then may be incident on the lower light incident 
surface of pentagonal prism 1. 
FIGS. 9 and 10 show embodiments, in which optical elements of the light 
measuring optical system for guiding light from an object being 
photographed to a light receiving element are formed integrally with 
holding frame 2. 
In the embodiment of FIG. 9, block 16 is formed integrally with the upper 
portion of the rear wall of holding frame 2, on which block is disposed 
light receiving element 17. Thus, light flux A from the front portion of a 
focussing plate (not shown) is reflected on the roof surfaces and front 
reflecting surface of pentagonal prism 1, then incident on block 16 to be 
reflected onto the reflecting surface formed on the rear portion of the 
block, and enters light receiving element 17. Light flux B from the rear 
portion of the focussing plate will advance linearly through pentagonal 
prism 1 and the block 16 in the manner shown and enter the light receiving 
element 17. The block 16 in this embodiment functions as a support member 
for the light receiving element 17 as well. 
In the embodiment of FIG. 10, the front wall of holding frame 2 extends 
along the front upper surface of pentagonal prism 1, and this extended 
portion functions as a light guide for orienting light fluxes A and B from 
the front and rear portions of a focussing plate towards light receiving 
element 17 disposed on top of the extended portion. 
FIGS. 11 and 12 show embodiments wherein optical elements of the so-called 
undesirable or stray light eliminating optical system are formed 
integrally with holding frame 2 for supporting the pentagonal prism. 
In the embodiment of FIG. 11, the rear wall of holding frame 2 is given a 
slant surface on one side, so that prism 18 may be provided. Wedge-shaped 
prism 19 supported by a support member (not shown) is disposed in opposing 
relation to the rear surface of prism 18, with a small gap left 
therebetween. The light coming obliquely through an ocular lens (not 
shown) from above is incident on the prism 19 from the reverse direction 
and subjected to the total reflection on the slant surface of prism 10, 
thereby being oriented downwards, without being incident on pentagonal 
prism 1. To the extent that admission of inversely incident light alone is 
prevented, a single prism 19 can achieve the object, but lack of prism 18 
causes a deviation and hence coloring of the main light flux from 
pentagonal prism 1. Prism 18 is one of the auxiliary optical elements 
which is essential to the inversely incident light eliminating optical 
system. 
In the inversely incident light eliminating optical system of the prior 
art, the holding frame for a pentagonal prism is interposed between prism 
18 and the pentagonal prism. In this embodiment, however, prism 18 is 
formed as part of the holding frame. This contributes to reducing the 
length of the camera (in the front and rear directions) by the thickness 
of the holding frame. 
The embodiment of FIG. 12 is the same in construction as that of FIG. 11, 
with the exception that the inclination of the slanted opposing surfaces 
of prisms 18 and 19 are less than those in FIG. 11. Prisms 18 and 19 in 
FIG. 12 constitute a ghost-light eliminating optical system. The 
ghost-light is subjected to the total reflection on the slant surface of 
prism 18 as seen in FIG. 12, to be oriented upwards, but without entering 
the ocular lens (not shown). 
FIGS. 1 through 12 show embodiments, in which the instant invention is 
applied to an auxiliary optical system associated with a finder optical 
system. FIGS. 13 through 15 show embodiments, in which part of the optical 
elements other than the pentagonal prism constituting the finder optical 
system itself are formed integrally with a holding frame for a pentagonal 
prism. 
In FIG. 13, condenser lens 20 is formed in the bottom wall of holding frame 
2. In this embodiment, there is omitted the bottom wall of the holding 
frame which is interposed between the pentagonal prism and the condenser 
lens in the prior art. This yields another advantage, in addition to the 
advantages described, in that space is saved in the amount of the 
thickness of the holding frame, thus providing a low profile of the 
camera. 
FIG. 14 shows an embodiment in which ocular lens 21 is formed integrally 
with the rear wall of holding frame 2. FIG. 15 shows an embodiment in 
which concave lens 22 for use in aberration compensation is formed 
integrally with the rear wall of holding frame 2, and ocular lens 21 is 
disposed behind the aberration compensating concave lens. In these 
embodiments, the length of the camera (in the front and rear direction) is 
reduced by the thickness of the holding frame for the pentagonal prism, as 
in the embodiments of FIGS. 11 and 12.