Light metering device for camera

A light metering device for single lens reflex camera having photoelectric transducer elements arranged in the optical path of a finder thereof. The photoelectric transducer elements are arranged ahead of a focusing screen as viewed from the direction of finder light, and an area corresponding to a clear portion of the focusing screen is made either to have a photoelectric transducer structure of narrower width than in the environmental area, or to be light-metered by another or separate light metering optical system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to light metering devices, and, more particularly, 
to light metering devices for single lens reflex cameras. 
2. Description of the Prior Art 
In the past, the light metering devices using a light beam splitting 
member, for example, a micro beam splitter formed in, for example, the 
condenser lens so that light is directed to the photosensitive element, or 
a Fresnel lens positioned on the exit face of the pentagonal roof type 
prism to concentrate light, have been proposed. These two types had 
drawbacks that the light distribution over the field of view of the finder 
was degraded, or when altering the light metering mode, it was necessary 
to interchange the light beam splitting member. 
FIGS. 1 and 2 show an example of each of the conventional types of light 
metering devices. In FIG. 1, light coming from an object to be 
photographed and entering through a photographic lens 1 of the single lens 
reflex camera is reflected upward by a reflex mirror 2 and passes through 
a focusing screen 3, a light beam splitting member 4, a photosensitive 
element 5, a condenser lens 6, and a pentagonal prism 7 to an eyepiece 
lens 8. In this type, when the object is observed through the eyepiece 
lens 8, that part of the light which travels through the light beam 
splitting member 4 is shut out. Thus, there is produced a bad effect of 
casting a shadow on the central portion of the viewfield of the finder. 
Also the light metering modes of the camera are the center-weighted light 
metering with a heavy emphasis on the central portion of the area of the 
image format, and the average light metering mode with uniform emphasis on 
the overall area of the field. Because the region of the light rays to be 
taken in the photosensitive element 5 is determined by the shape of the 
light beam splitting member 4, when it is desired to change the light 
metering area, the condenser lens 6, having the light beam splitting 
member 4 formed therein, must be replaced. 
In the conventional example of FIG. 2, the object light emerging from the 
photographic lens is reflected by the reflex mirror 2 to a focusing screen 
9 and further directed through the pentagonal prism 7 to the eyepiece lens 
8. Meanwhile, by means of a Fresnel lens 10 positioned on the exit face of 
the pentagonal prism 7, light is concentrated on the photosensitive 
element 5 confronting the exit face. In this case, though the central 
portion of the viewfield of the finder is not darkened as in the case of 
FIG. 1, it is very difficult to selectively operate many light metering 
modes such as those mentioned above. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a light metering device 
for a single lens reflex camera in which photoelectric transducer elements 
for light metering are provided in the optical path of a finder optical 
system, whereby no bad influence of the photoelectric transducer elements 
is given to the field of view of the finder. 
Other objects of the invention will become apparent from the following 
detailed description of embodiments thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A prototype of the light metering device for a single lens reflex camera of 
the invention is disclosed in U.S. patent application Ser. No. 770,162 
filed Aug. 27, 1985 assigned to the assignee of the present invention, 
where photoelectric transducer elements formed to a fine width are 
provided in the optical path of the finder at a position adjacent the 
focusing screen. This proposal is that the photoelectric transducer 
elements, whose width is a few microns and which are made of, for example, 
amorphous silicon, are formed in comb teeth-like shapes not only in the 
central area of the view field but also the environmental area surrounding 
that central area. And, because this photoelectric transducer element is 
only a few microns in width, no bad influence is given to the finder 
image. 
The present invention is next described in detail in connection with 
embodiments thereof by reference to FIG. 3 and those that follow. 
FIG. 3 is a sectional view of the optical system of single lens reflex 
camera to which the present invention is applied as a first embodiment; 
FIG. 4 is a plan view of an example of arrangement of photoelectric 
transducer elements forming photoelectric transducer means; and FIG. 5 is 
a perspective view of the body of the photoelectric transducer elements. 
In FIG. 3, light coming from an object to be photographed and entering 
through a photographic lens (not shown) is reflected by a known reflex 
mirror 21 and passes through photoelectric transducer elements 22 provided 
at a position somewhat displaced from a focusing screen toward the lens 
side (reflex mirror 21 side), a condenser lens 23 arranged adjacent 
thereto, and a pentagonal prism 24 to an eyepiece lens 25. Photoelectric 
transducer elements 22 are constructed, as shown in FIG. 4, as an array of 
comb tooth-shaped photoelectric transducer element bodies 26, 26A and 26B. 
In the case of the photoelectric transducer element assembly 22 in this 
embodiment, it is in a central portion 22A of the photoelectric transducer 
element assembly 22, corresponding to a clear portion or split image 
portion which exists in a central portion A of a focusing screen shown in 
FIG. 11 positioned in a focal plane, that no photoelectric element body is 
provided in order to give no bad influence on the finder viewfield image. 
The structure of this photoelectric transducer element body 26, 26A, 26B is 
shown in FIG. 5 (where the element bodies 26A and 26B have the same 
structure as that of the element body 26, and here their explanation is 
omitted). On a glass plate 27 is arranged a lower electrode 28. On the 
lower electrode 28, there are then arranged a photoelectric transducer 
element 29 and an upper electrode 30, in laminated relation, having 
similar comb-tooth shapes. The width, d, of the corresponding constituent 
substance to the comb-tooth portions and back portions are formed by fine 
lines of about several tens of microns and are easy to fabricate. It 
should be noted that despite the photoelectric transducer element assembly 
22 having such a wide width, d, almost no shadow of the photoelectric 
transducer element assembly 22 is formed on the finder viewfield image due 
to the diffusing action of light formed on the matted surface B of the 
focusing screen (shown in FIG. 11) arranged on the rear (pentagonal prism 
24) side of said element assembly 22. Connected to the end portions of 
electrodes 28 and 30 are lead wires 31 (31A, 31B) and 32 (32A, 32B). The 
lower electrode 28, photoelectric transducer element 29, upper electrode 
30 and lead wires 31 (31A, 31B) and 32 (32A, 32B) are unified. The comb 
tooth-shaped photoelectric transducer elements 22 are arranged in a number 
of divided blocks so that they can correspond to various light metering 
modes. By changing over circuits connecting the lead wires 31 (31B) and 32 
(32B) connected to the respective electrodes 28 and 30, the photoelectric 
transducer element bodies 26 and 26B can be used either independently or 
in combination. It sould be noted that, the changeover circuit for the 
light metering modes has already been proposed and here its explanation is 
omitted. 
If CdS or amorphous Si is used as the photoelectric transducer element 29, 
due to their low transmittances to visible light, the viewfield of the 
finder is obscured. But, though the width, d, of the tooth portion of the 
photo-electric transducer body 26 is about several tens of microns, 
because the diffusing or matted surface B of the focusing screen (shown in 
FIG. 11) lies above it or at the focal plane, as has been described above, 
the naked eye cannot perceive the photoelectric transducer element 
assembly 22, and no bad influence is given to the viewfield of the finder. 
Concerning the light metering of the central portion 22A in the 
photoelectric transducer element assembly 22, which corresponds to the 
clear portion A or split image portion of the focusing screen (shown in 
FIG. 11), as shown in FIG. 3, light is conducted to the photoelectric 
transducer element body 26A shown in FIG. 4 by using a mirror 40 provided 
in the rear of the reflex mirror 21. At this time, even though the width 
of the photoelectric transducer element body 26A is several tens of 
microns, for the same reason as described above, because the presence of 
the matted surface B of the focusing screen which diffuses light in rear 
(on the pentagonal prism 24 side) of the element body 26A, it is 
impossible to see the details of the shadow of the photoelectric 
transducer element body 26A by the naked eye. Therefore, there is no 
possibility of giving any bad influence to the viewfield of the finder. 
Further, since CdS or amorphous Si is amenable to vacuum evaporation on a 
mother material, such as the electrodes 28, 30, the photoelectric 
transducer element body 26 can be formed with ease, and the pattern of 
each of the comb tooth-shaped regions can be constructed freely, so that 
the selection of the partial light metering, center weighted light 
metering, average light metering and other light metering modes can be 
realized with ease by combination. It should be noted that the 
photoelectric transducer element 29 may be made up by using another 
material instead of CdS or amorphous Si. 
In the arrangement of the photoelectric transducer elements shown in FIG. 
4, the split image portion of the focusing screen plate (shown in FIG. 
11), that is, the clear portion A, is not provided with a photoelectric 
transducer element body, while the corresponding portion to the matted 
surface B, other than the clear portion A, has the photoelectric 
transducer element bodies of several tens of microns wide. 
The reason why it is arranged as such is that if a photoelectric transducer 
element body of a few microns wide is provided in a corresponding position 
to the clear portion in order to prevent the shadow of the photoelectric 
transducer element body from being projected to the finder as in the 
previously proposed light metering device, as the fabrication of the 
photoelectric transducer element body becomes difficult, a problem of 
increasing the cost is produced. 
FIGS. 6 and 7 illustrate a second embodiment of the invention. A 
photoelectric transducer element assembly 22B, of FIG. 6 has photoelectric 
transducer element bodies 26C arranged in concentric relation. In the case 
of this embodiment, too, a central portion of the photoelectric transducer 
element assembly 22B corresponding to the clear portion of the focusing 
screen plate, is not provided with a photoelectric transducer element body 
in order to prevent a bad influence on the finder. 
An optical system for the photoelectric transducer element assembly of FIG. 
6 comprises, as shown in FIG. 7, a reflex mirror 21, a focusing screen 
plate 33, a condenser lens 23, a pentagonal prism 24 and an eyepiece lens 
25 arranged in this order in a direction in which light from an object to 
be photographed passes. In the case of this embodiment, the light metering 
of the corresponding portion to the clear portion of the central portion 
of the focusing screen plate 33 is performed by another photoelectric 
transducer element 41 (for example, SPD) arranged to receive the central 
part of the object light split by a half-mirrored central portion of the 
reflex mirror 21 of known construction and reflected by a sub-mirror 40 
positioned in rear of the reflex mirror 21. 
In the optical system of FIG. 8 as a third embodiment of the invention, the 
reflex mirror 21, photoelectric transducer element assembly 22, focusing 
screen plate 33, condenser lens 23, pentagonal prism 24 and eyepiece lens 
25 are arranged in this order from front to rear. Similar to the 
embodiment shown in FIG. 7, the light metering of the corresponding 
central portion of the photoelectric transducer element assembly 22 to the 
central clear portion (split image portion) of the focusing screen plate 
33 is performed by conducting a light which will enter the central portion 
to another photoelectric transducer element 41 (for example, SPD) by a 
sub-mirror 40. 
Next, referring to FIGS. 9 and 10, a fourth embodiment of the invention is 
described. This fourth embodiment is different from the above-described 
first to third embodiments in the method. A central portion of the 
photoelectric transducer element assembly 22 positioned below (reflex 
mirror 21 side) the focusing screen plate 33, that is, a region 
corresponding to the central clear portion A (split image portion) shown 
in FIG. 11 of the focusing screen plate 33, is provided with a 
photoelectric transducer element body 26B-1 whose width, d, is made 
several microns or less. That is, the region of the photo-electric 
transducer element assembly 22 corresponding to the diffusing matted 
surface B (shown in FIG. 11) of the focusing screen plate 33 has 
photoelectric transducer element bodies 26, 26B of several tens of microns 
width which are easy to form. Meanwhile, it is in a region corresponding 
to the central clear portion A (shown in FIG. 11) of the focusing screen 
plate 33 that the photoelectric transducer element body 26B-1 of a width 
of less than several microns is formed. Therefore, even in the region of 
the clear portion A of the focusing screen plate 33, it is made possible 
to avoid the bad influence of this photoelectric transducer element body 
26B-1 on the finder viewfield image. It should be noted that the 
above-described photoelectric transducer element bodies 26 and 26B of 
several tens of microns wide correspond to the diffusing matted portion B 
of the focusing screen plate 33. Therefore, similarly to the 
above-described first to third embodiments, no bad influence is given to 
the finder viewfield image. 
The fabricating and assembling of the above-described photo-electric 
transducer element body 26B-1 of several microns wide is not easy. But, 
this portion accounts for a very small proportion of the entire area of 
the photoelectric transducer element assembly. Therefore, the problem does 
not become very serious in the point of fabricating and assembling. 
As has been described above, the first to fourth embodiments have common 
advantages described below. Firstly, because most of the photoelectric 
transducer element bodies of fine width of the photoelectric transducer 
element assembly can be formed to several tens of microns in the width 
which is easy to fabricate, as compared with the method of fabricating the 
previously proposed photoelectric transducer element bodies which are all 
fabricated to a width of less than several microns, the price becomes 
cheaper, and the fabricating steps become simpler. 
Also, secondly, despite the photoelectric transducer elements for metering 
light being provided in the optical path of the finder optical system, no 
bad influence on the viewfield of the finder is produced by the 
photoelectric transducer elements. 
Also, thirdly, because the photoelectric transducer element bodies are 
divided into a plurality of blocks, their combinations allow for easy 
selection of many light metering modes. 
Also, fourthly, because the photoelectric transducer element bodies are 
formed by vacuum evaporation, they can be manufactured economically. 
Also, fifthly, because the light metering is taken near the focal plane, 
the proportionality of the light metering to F-number is good, and 
correction of the full open number becomes unnecessary. 
Also, sixthly the light metering optical system of the camera becomes 
simple, so that a simplification of the lens structure can be achieved.