Abstract:
A light measuring device for a single lens reflex camera wherein a reflection member having a slant surface is provided at the rear of an exit plane of a pentagonal roof prism and locating to be outside of the viewfinder light path, and the effect by undesired incident light is eliminated by a mirror formed by  the slant surface placed in front of the light receiving element or photosensitive device, which light once enters into the pentagonal prism from the ocular lens and is reflected at the bottom surface thus again comes out of the surface of the pentagonal prism facing the ocular lens to proceed toward the photosensitive device.

Description:
This is a continuation of application Ser. No. 288,444 filed Sept. 12, 1972, now abandoned. 
    
    
     The present invention relates to a light measuring device of a single lens reflex camera which employs a pentagonal prism to measure other light than the effective light flux at a viewfinder by placing a photo-sensitive device at the rear of such surface of a pentagonal prism of a single lens or eyepice reflex camera as facing an ocular lens. 
     In a conventional single lens reflex camera with a pentagonal prism the light, which is harmful to light measurement enters into the prism from an ocular lens or eyepiece side, and is internally reflected at a bottom surface thus coming out of the prism from its rear or exit surface again and impinges on a photo-sensitive device or cell such as CdS. Therefore, its measuring accuracy is poor and when the effect of such light is great, the measured amount of light is too large resulting in insufficient exposure for an image photographed by a single lens reflex camera. 
     As a means for improving the same, one measure taken is placing a mask at each reflective surface of the pentagonal prism and inserting a mask between the pentagonal prism and the ocular lens or eyepiece thereby eliminating a small portion of undesired incident light from the ocular lens side into a light measuring element, but this measure prevents a camera from being made smaller, or lowers the magnification of a finder, and thus it has not been completely satisfactory. There is also a method to provide an independent light measuring element other than the regular light measuring element of the finder which is placed to measure the counter light from the ocular lens or eyepiece for compensation of the amount of the regular light measured, but it has various disadvantages. For example, it is practically difficult to completely measure the counter light from the ocular lens and it requires an extra photocell and a compensation circuit. 
     In a conventional type of single lens reflex camera in which light flux is measured at rear of the ocular side of a pentagonal roof prism or at both sides of an ocular lens as mentioned above, a disadvantage has been that counter light from the ocular lens is received in the prism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are side views respectively of a light measuring device for a conventional single lens reflex camera showing reflection of undesired light within a pentagonal prism. 
     FIG. 3 is a side view of a light measuring device for a single lens reflex camera according to an embodiment of the invention. 
     FIG. 4 is a top view of the invention of FIG. 3. 
     FIG. 5 is a side view of a light measuring device according to another embodiment of the present invention 
     FIG. 6 is a top view of the invention of FIG. 5. 
     FIG. 7 is a side view of a light measuring device according to another embodiment of the present invention. 
     FIG. 8 is a side view of a light measuring device according to another embodiment of the present invention. 
     FIGS. 9 and 10 are perspective views respectively of wedge shape prisms used with the embodiments shown in FIGS. 7 and 8. 
     FIG. 11 is a side view of the applied embodiments of the present invention shown in FIGS. 7 and 8. 
     FIG. 12 is a perspective view of a wedge shape prism to be used in the embodiment shown in FIG. 11. 
     FIG. 13 is a side view of a light measuring device of still another embodiment of the present invention. 
     FIGS. 14 and 15 are perspective views of prisms, respectively, to be used in the embodiment shown in FIG. 13. 
    
    
     DETAILED DESCRIPTION 
     Explanation shall be made in reference to FIG. 1, in which 1 is an ocular lens frame, 2 is an ocular lens, 3 is a pentagonal roof prism, 3a is a roof or surface, 3b is a front plane or surface, 3c is a bottom plane, 3d is a plane or surface or eyepiece at ocular side and forms an exit surface, 4 is a light receiving element such as a photo-sensitive device such as a CdS cell, 6 is a condenser lens, and 7 is a focusing plate. 
     That is, the light flux of the counter upper light I (shown by broken line) from the ocular lens 2 is determined by the ocular lens frame 1 and goes through the ocular lens 2, then is internally reflected within the pentagonal roof prism 3 in the order of 3d → 3c → cb → 3a → 3b → 3c → 3d, thus enters into the photocell exit surface 4 from 3d. Or as shown in FIG. 2 the lower light flux (II) goes through the ocular lens 2 and is internally reflected within the pentagonal prism in the order of the plane 3b → 3a → 3b → 3c → 3d, then enters into the photocell 4 from the exit surface 3d. In both cases such light flux overlaps with the regular light from the focusing plate 7, causing measuring error by the photocell 4, and as a result it causes insufficient exposure in photographing. 
     Now the present invention shall be explained by the examples shown in the drawings. FIG. 3 is a side elevation of an example of using the prism of the present invention, and FIG. 4 is a top view of the same, wherein 5 is a prism used in the present invention, 5b is its totally reflecting plane or surface, 5a shows its incident plane or surface. In FIG. 3, upper counter light I enters into an ocular lens and enters into the pentagonal prism 3 through its surface 3d, then after a series of internal reflection it is reflected from the bottom surface 3c at an angle of θ o  degree against an imaging optical axis and goes out of the surface 3d. Then and there in a conventional device the counter light I directly enters into a photocell 4, but as a prism 5 is provided in the present invention the light transmits through the surface 5a of the prism 5 and is totally reflected at the surface 5b which forms an angle θ with the optical axis, therefore its direction is largely changed and will not reach the photocell 4, yet the ordinary regular light (shown by solid line) from a focusing plate transmits through and is refracted at the surface 5b and reaches the photocell 4. The angle θ formed by said surface must be within the scope defined by the following formula: 
     
         sin.sup..sup.-1 (1/N)- θ.sub.o ° ≦ θ ≦ sin.sup..sup.-1 (1/N) - ω° 
    
     in the above formula, N is the refractive index of the prism, and ω° is the photometric acceptance angle of the finder, while the left side of the formula indicates the condition for the undesired counter light to be totally reflected at the bottom surface 5b, and the right side indicates the condition for the regular light to be refracted and transmitted. ω° differs depending on the position of the photocell and on the finder. 
     As shown above in the present invention such prism, as having a surface which forms, with the optical axis on the plane of a pentagonal prism facing the ocular lens side or at rear of the same, an angle θ falling within the scope defined by: 
     
         sin .sup..sup.-1 (1/N) - 21° ≦ 1/4° ≦ sin.sup..sup.-1 (1N) - 10° 
    
     
         θ.sub.o ° &lt; 21° 
    
     
         ω° &lt; 10°, 
    
     or having an effect equivalent to the above, is placed between the pentagonal prism and the photocell thus the undesired light which is the counter light entering the ocular lens from upper direction and further entering into the pentagonal prism, and then tends to reach the photocell is totally reflected at the inner surface of the above mentioned prism in front of the photocell so that only ordinary imaging light transmits through the above-mentioned prism. Therefore, the measuring device of a single lens reflex camera with such arrangements just mentioned has the advantages that it can easily eliminate conventionally undesired effects of the counter light from the upper direction of an ocular lens or eyepiece by internal reflection within the pentagonal prism and it can largely reduce the error in measuring, and that in case of regular light, the position of the photocell can be moved upward from the conventional pentagonal bottom surface by the refractive effect of the prism so that leak in measurement is small and the arrangement (positioning) of component parts can be made compact. 
     Next another example of the present invention shall be explained by FIG. 5 and FIG. 6. FIG. 5 is a side elevation of an example of use of the prism of the present invention, and FIG. 6 is its top view, wherein 5 is a prism used in the present invention, 5b is a totally reflective surface, and 5a is an incident surface. 
     In FIG. 5 the lower counter light II from somewhat lower direction from the center of an ocular lens enters a pentagonal prism from its plane 3d and after a series of internal reflections is reflected at the plane 3c at an angle of θ o  ° against the optical axis and goes out of the place 3d. Now in the conventional method or device the counter light II directly enters into the photocell, but in the present invention by adopting the prism 5, the above mentioned counter light transmits through the surface 5a of the prism and is totally reflected at the surface (5b) which forms an angle θ with the optical axis, therefore its proceeding direction is greatly changed so that it will not reach the photocell 4, although the ordinary regular light (shown by solid line) from a focusing glass or plane is defracted at the surface 5b and transmits through the same thus reaching the photocell 4. The angle θ must be within the scope defined below: 
     
         sin.sup..sup.-1 (1/N ) - θ.sub.o ° ≦ θ ≦ sin.sup..sup.-1 (1/N) - ω°, 
    
     wherein N is refractive index of the prism. The left side of the formula shows the condition for the counter harmful light to be totally reflected at the surface 5b, and the right side of the formula shows the condition for the regular light to be refracted at and transmits through the surface 5b. As shown above, the present invention is so arranged that a prism, having a surface forming with the optical axis, on the surface of the pentagonal prism facing the ocular lens side or at the rear of the pentagonal prism an angle θ which falls within the scope shown below: 
     
         sin.sup..sup.-1 (1/N) - 29° ≦ θ° ≦ sin.sup..sup.-1 (1 /N) - 14° 
    
     
         θ.sub.o ° &lt; 29° 
    
     
         ω° &lt; 14°, 
    
     or having such effect as equivalent to the above, is placed between the pentagonal prism and the photocell so that the undesired counter light, which enters the ocular lens from a lower direction then enters into the pentagonal prism reaching the photocell, is totally reflected at the inner surface of the above mentioned prism before the photocell, thus only the ordinary imaging light transmits through the same. 
     FIG. 7 shows another example, wherein the present invention is applied so that the light flux proceeding to both sides (left and right) of the ocular lens, out of the light fluxes other than the effective flux used by the finder coming out of the surface of the pentagonal prism facing the ocular lens, is properly measured. 
     The undesired light I (shown by broken line) entering into the ocular lens from upper direction enters into the pentagonal prism 3 from the plane facing the ocular lens and is reflected at its inner bottom surface, thus the light is subjected to a series of internal reflections then is reflected again at the bottom surface and transmits again through the plane facing the ocular lens. While the light will be directly entering into the photocell 4 in the conventional device, the light is, by the wedge shape prism 5 for the examples to which the present invention is applied, totally reflected internally, thus the undesired light I is prevented from entering into the photocell 4. 
     On the other hand, the regular light (shown by solid line) from the focusing plate is after coming out of the pentagonal prism refracted at and transmits through the prism 5 thus reaching the photocell 4. 
     FIG. 8 shows an example wherein the present invention is applied in order to properly measure the light flux proceeding to the upper direction of the ocular lens out of the light fluxes other than the effective flux used at the finder coming out of the pentagonal prism at its surface facing the ocular lens. 
     The light II (shown by broken line) undesired for measuring entered into the ocular lens from somewhat lower portion thereof enters into the pentagonal prism 3 at its surface facing the ocular lens and after a series of reflections within the pentagonal prism 3 it is finally reflected at the bottom surface then comes to the surface of the pentagonal prism 3 facing the ocular lens, wherein while the light enters into the photocell 4 in a conventional device, the light is totally reflected by the wedge shape prism 5 in the example to which the present invention is applied. Thus the undesired light is prevented from proceeding to the photocell. 
     On the other hand, the regular light (shown by solid line) from a focusing plate is, after coming out of the pentagonal prism, refracted at and transmits through the prism 5 reaching the photocell 4. 
     In FIG. 7 and FIG. 8 a prism, having such surface that its angle θ falls within the scope of 
     
         sin.sup..sup.-1 (1 /N) - θ.sub.o ° ≦ θ ≦ sin.sup..sup.-1 (1/N) - ω° 
    
     (wherein N is the refractive index of the prism) or having such effect as equivalent to the above, is placed between the photocell and the pentagonal prism. 
     While the examples shown in FIG. 7 and FIG. 8 show a case wherein the light flux proceeding to both sides (left and right) of the ocular lens is measured and a case wherein the light flux proceeding to the upper direction of the ocular lens is measured, respectively, both kinds of light flux may be simultaneously measured. FIG. 11 shows such case as mentioned and FIG. 12 shows a modified wedge shape prism. 
     FIG. 13 shows, being different from the foregoing examples, an example wherein such a prism is provided as having the surfaces by which the counter light, which comes from the ocular lens and is totally reflected at the bottom surface of the pentagonal prism thus proceeding toward the ocular lens, is made to transmit the same and the regular light from a focusing glass and is totally reflected. In the drawing the undesired light I (shown by broken line) entering into the ocular lens from upper direction transmits through the reflecting surface 5b of the prism 5. And the undesired light II (shown by broken line) entering into the ocular lens from lower direction transmits through the reflecting surface 5c of the prism 5. On the other hand, the regular light (shown by solid line) is reflected at the reflecting surface and enters into the photocell provided at lower position in the drawing. The angle θ by the reflective surface must fall within the scope defined below: 
     
         sin .sup..sup.-1 (1/N) + ω° ≧ θ ≧ sin.sup..sup.-1 (1/N) - ω° 
    
     wherein N is refractive index of the prism and ω° is the measuring scope of the finder. 
     As have been explained above in the present invention a prism, having such surface as forming with the optical axis on the surface of the pentagonal prism facing the ocular lens side or at the rear of the prism an angle θ which falls within the scope defined by the above mentioned formula or having an effect equivalent to the above, is provided between a pentagonal prism and a photocell. The prism shown in FIG. 14 or FIG. 15 are modifications of the prism 5 of the foregoing examples. 
     As shown in the above mentioned examples the present invention provides a simple yet great improvement over the effect on the measuring error by the counter light from an ocular lens, which constituted disadvantages of such system as measuring the light other than the finder effective light flux coming out of the surface of the pentagonal prism facing the ocular lens.