Reflex camera light detecting arrangement

A light detecting system for a single lens reflex camera in which an image of the focusing screen is reflected to the normally unused surface of the pentaprism from a partially reflective surface in the viewfinder system. The reflective surface in one form may be glass-to-air interfaces on the viewfinder axis in the eyepiece, but also may be a thin partially reflecting coating.

This invention relates to cameras and more particularly relates to imaged 
light detecting and measuring systems. 
It is conventional practice to provide image light detecting systems in 
cameras to meter the light on a subject entering the lens of a camera. In 
cameras of the single lens reflex type, light sensitive devices have been 
positioned within the camera at a variety of locations to detect the light 
intensity of an imaged object. 
The measured light intensity is utilized to determine automatically lens 
aperture or shutter speed or the readings may be used to provide 
information to the photographer so that he can manually set shutter speed 
and/or aperture opening dependent on the film used and exposure desired. 
Alternatively, the camera may include a so-called match needle system or 
other type indicating device to indicate when a correct aperture has been 
set in relation to selected shutter speed. 
Various light metering systems may operate to provide center spot metering 
of an object, averaging of an entire scene, or weighted averaging of the 
scene. 
In single lens reflex cameras, the viewing system has an optical path which 
is partially coincident with and a continuation of the objective optical 
path. Thus photodetectors positioned within the camera to detect the light 
entering the lens are also subjected to some light entering the eyepiece 
of the viewing system. Various locations of the photosensitive elements 
have been proposed to eliminate detection of unwanted and reflected light 
in the viewing system in attempts to obtain more accurate light intensity 
readings. 
A further consideration is the desirability of obtaining true spot readings 
to enable good film exposure for back-lighted subjects. Since the spot 
detection is obtained on or from the optical axis of the viewing system or 
objective, prior positioning of the spot detecting device has also been 
subject to light entering the eyepiece. 
In view of these known difficulties in light detecting and metering systems 
of single lens reflex cameras, the present invention provides a light 
detecting and metering system which substantially eliminates or reduces to 
a very low level the effect of light from the eyepiece on the light 
detecting and metering system, and further enables simplified true spot 
metering. 
Briefly stated, the invention makes use of the normally unused surface of 
the pentaprism of a single lens reflex camera to detect the light 
intensity of an imaged object. A surface which is inclined to the optical 
axis of the viewfinder is defined behind the second (forward) reflecting 
surface of the pentaprism. This inclined surface includes a partially 
reflecting portion preferably on or near the optical axis of the 
viewfinder which reflects a reduced image of the viewing screen to the 
normally unused surface of the prism. The position of the image on the 
normally unused surface permits true spot metering, as well as other types 
of total area averaging. Additionally, baffling means may be provided on 
said unused surface of the prism to eliminate internally reflected light 
from light detectors. 
An object of this invention is to provide a new and improved light 
detecting system for a single lens reflex camera which decreases to a very 
low level the effect of light entering the eyepiece on the metering 
system. 
Another object of this invention is to provide a new and improved light 
detecting system for obtaining a true spot indication of the image. 
A further object of this invention is to provide a light detecting system 
for a single lens reflex camera in which the surface of the prism utilized 
for metering purposes in effectively baffled from internally reflected 
light in the prism.

FIG. 1 exemplifies the viewing system of a single lens reflex camera 10. 
The camera includes a body 11 having an objective lens 12 interchangeably 
mounted thereto, a pivotal mirror 13 before focal plane FP, and a 
viewfinder system 14. 
The viewfinder system comprises a viewing and focusing screen 15 having a 
fresnel surface and a central microprism or other focusing arrangement 
(not shown), a field lens 16, a pentaprism 17, and an eyepiece 18. 
An image of an object seen by the lens 12 is focused on screen 15, and 
thereafter viewed by the photographer through eyepiece 18. 
The pentaprism 17 has an entrance face or surface 19, first reflecting 
surfaces 20 and a second reflecting surface 21. The surfaces 20 are the 
usual roof type defined at angles of 90.degree. to each other. The 
surfaces 20 and 21 are joined by a normally unused or non-reflecting 
surface S. The optical axis of the system is indicated by the axis O, and 
it is apparent that the optical axis of the viewfinder is essentially a 
reflected continuation of the objective axis. Since the viewfinder system 
is a magnifier there may be a non-coincidence without detrimental effect. 
Instead of the normal exit window in the vertical plane the rear of the 
prism is formed with the surface 22 defined in a plane or surface 
substantially at a small angle to the perpendicular of the optical axis. 
Cemented to surface 22 is an element 23 having a surface 24 complementary 
to surface 22. Surfaces 22 and 24 may be planar but preferably are 
slightly curved. Surfaces 22 and 24 may be effectively coated with a 
non-reflective coating except for portions 22a and 24a. The portions 22a 
and 24a are illustrated as being on the optical axis but may be positioned 
anywhere on the surfaces to reflect to surface S. The center portions 22a 
and 24a define glass-to-air and air-to-glass interfaces, respectively, in 
one form. The spacing between surfaces 22 and 24 is only about fifty 
microns, the thickness of the bonding cement. 
The non-reflective coating may be a coating of a cement or film as, for 
example, zinc sulfide and magnesium fluoride. Alternatively, the surfaces 
22a and 24 may be a plurality of alternate films of zinc sulfide and 
magnesium fluoride to provide partially reflective surfaces. A plurality 
of such coatings will be partially reflecting. The glass-to-air or 
film-to-glass surfaces are angled so as to reflect a small image of the 
screen 15 to surface S of the prism. Photodetectors 30 in any 
configuration thereof may be positioned on or adjacent to surface S to 
measure the light intensity thereat in any desired pattern. One detector, 
as hereinafter shown, is positioned to read the exact center of the screen 
only and thereby give a true spot indication of the center. 
The photodetectors may be positioned in any desired pattern and selectively 
electrically connectable to give several different types of readings, for 
example, spot, average, center weighted average, etc. With this 
arrangement it will be seen that if one retraces the optical axis O from 
eyepiece 18 there is little possibility of light entering the prism 
through the eyepiece 18 reaching surface S. However, to further minimize 
this possibility one or a plurality of slits or trenches 36 are defined in 
surface S on the side adjacent reflecting surface 21 and filled with a 
light absorbing substance to baffle any stray light from surface 21 
reaching the photodetectors at surface S. Such baffling may be 
accomplished in other ways, as hereinafter described. 
Reference is now made to FIG. 2 which exemplifies another viewfinder system 
comprising a viewing and focusing screen 40, a field lens 41, pentaprism 
42 having reflecting surfaces 43 and 44 with surface S therebetween, exit 
surface 45 and an eyepiece 46. 
Eyepiece 46 comprises three elements 47, 48 and 49. Surfaces 47a and 48a of 
elements 47, 48 and optionally coated with a non-reflective coating 50 as 
previously described except for the small section 51. Alternatively, a 
small area between surfaces 47a and 48a has partially reflecting coatings 
thereon. Surfaces 47a and 48a are defined on radii of about 204 mm from a 
point approximately 10.degree. to 11.degree. above axis O, and in 
accordance with the geometry of the prism reflect an image 40' of screen 
40 to surface S, as shown in FIG. 3. 
Center section 51 is substantially on or close to the axis O, and is 
preferably in the range of five to seven millimeters in diameter. 
Alternatively, the section 51 may be defined on a rectangle. In FIG. 2, 
the reflecting surfaces are moved closer to the viewing position of the 
eye and therefore will be hardly noticeable in the viewed field. By way of 
example, in the system of FIG. 2, the axial dimensions of the elements 47, 
48 and 49 may be 
______________________________________ 
Element Axial Dimension 
______________________________________ 
47 4.600mm 
48 5.900 
49 1.500 
______________________________________ 
and the distance to the optimum eye viewing position is 12.6mm. The spacing 
between element 47 and prism exit surface 45 is about 0.800mm, a clearance 
dimension. 
In FIG. 2 a baffling member 53 comprising a plurality of parallel vane-like 
members is disposed between surface S and photodetector elements 54 and 55 
to block any light reflected from surface 44 from affecting the 
photodetectors. 
Where the reflecting surfaces of the eyepiece are glass-to-air and 
air-to-glass, slightly less than eight percent of the light incident 
thereon will be reflected to surface S. Each interface will reflect about 
four percent of the light incident thereon. This small reflection coupled 
with the closeness of the reflective spot to the pupil of the eye will 
render the spot 51 unnoticeable to the normal eye and darkening of the 
center of the focusing screen will not be noted by most eyes. The same 
holds true if a partially reflective coating is applied to the reflecting 
area. It is also to be understood that the entire surface may be made 
partially reflective since only about 4% of the light need be reflected. 
FIG. 4 is a view of the clear aperture at surface 47a, and exemplifies the 
small area 51 of the reflective portion of surfaces 47a and 48a. 
A viewfinder system as shown in FIG. 2 designed for a 24.times.36mm image 
frame single lens reflex camera has clear apertures of substantially 
24.times.36mm for screen 40 and field lens 41 and about 11.times.16mm for 
the eyepiece elements. The partially reflecting spot 51 of about 5.0mm 
size spaced only about twenty millimeters from the eye would not be 
noticeable to normal vision or eyeglass corrected normal vision. 
FIG. 2 exemplifies the positioning of a center spot photodetector 54 on the 
center of image 40' to obtain a true center spot reading, while 
photodetectors 55 are arranged for average readings or center weighted 
average in combination with photodetector 54. 
FIG. 5 exemplifies the invention in a viewfinder system 60 where the field 
lens is positionable in accordance with the position of the objective lens 
exit pupil and the eyepiece is positionable in accordance with the 
position of the field lens. Such viewfinder system is disclosed and 
claimed in copending applications Application Ser. Nos. 674,229 and 
648,779 filed Jan. 13, 1976, the disclosures of which are incorporated 
herein by reference. 
The system 60 comprises a viewing and focusing screen 61, field lens 62 
which is positionable between screen 61 and a negative power lens 63. A 
prism 64 having entrance surface 65 and exit surface 66, reflecting 
surfaces 67 and 68 with non-reflecting surface S therebetween and an 
eyepiece 69. Eyepiece 69 comprises elements 70 and 71, and 72 and 73 which 
are positionable with field lens 62 to avoid change in magnification of 
the image when the field lens is moved. Also elements 74, 75 move with 
field lens 62 to maintain focus. Elements 70 and 71 are cemented and have 
non-reflective coatings thereon at the mating surfaces except for the spot 
76. The surfaces 70a and 71a are defined on a radii of approximately 
seventy-one millimeters from a point at an angle of about 11.degree. above 
the axis O. The lens group defined by elements 70 - 73 is movable 
approximately 2.3mm away from a normal spacing of 0.500mm from exit window 
66. This small movement of the spot 76 produces very little movement of 
the reflected image of the screen on surface S. 
The surfaces 70a and 71a of FIG. 5 and also 47a and 48a of FIG. 2 have the 
slight curvatures specified to give the best available image of the 
focusing screen at the surface S. 
The distance of the spot 76 to the optimum viewing position of the eye is 
30.4mm. 
The reflectivity of the spots 51 and 76 may be increased by partially 
silvering or otherwise placing a partially reflective coating thereon if 
greater light intensity is desired on surface S. The only limitation to 
the degree of reflectivity of the spot is the degree of darkening that 
would appear in the middle of the focusing screen to the viewer. 
The invention may also be utilized in viewfinding systems where no separate 
field lens is provided and where the entrance surface of the prism may be 
made convex for positive power. 
The partially reflecting surface or surfaces may be round or rectangular. 
It is preferred that the major dimension of the reflecting surface or spot 
be no greater than one-half the major dimension of the clear aperture of 
the total surface on which it is defined; however, the entire surface may 
be partially reflective. Additionally, the reflective area may be defined 
on a convex or plano lens surface dependent on the lens power and the 
eyepiece element surface utilized. 
From the foregoing it may be seen that the invention provides a viewfinder 
system in which the image of the focusing screen is derived preferably 
from an area on or close to the optical axis of the viewing system and 
reflected to the normally unused front surface of the pentaprism in such 
manner that is effectively out of the path of any light entering the 
eyepiece. However, the reflective area may be positioned on a selected 
surface at any location from which it can reflect to the prism surface S. 
Moreover, the location of the image of the focusing screen is such that 
true and accurate center spot metering may be accomplished as well as 
averaging center weighted averaging or any other desired metering 
arrangement. 
It may thus be seen that the objects of the invention set forth as well as 
those made apparent from the foregoing description are efficiently 
attained. While preferred embodiments of the invention have been set forth 
for purposes of disclosure, modification to the disclosed embodiments of 
the invention as well as other embodiments thereof may occur to those 
skilled in the art. Accordingly, the appended claims are intended to cover 
all embodiments of the invention and modifications to the disclosed 
embodiments which do not depart from the spirit and scope of the invention 
.