Camera viewfinder with leveling indicator
FIELD OF THE INVENTION 
     The invention relates generally to the field of photography, and in particular to a leveling indicator visible in the viewfinder of a camera for indicating whether the camera is oriented level relative to a subject to be photographed or is tilted relative to the subject. More specifically, the invention relates to the use of an optical prism as the leveling indicator. 
     BACKGROUND OF THE INVENTION 
     It is generally known for a leveling indicator to be visible in the viewfinder of a camera for indicating whether the camera is oriented level relative to the subject to be photographed or is tilted relative to the subject. 
     In some cameras a leveling indicator is visible in the viewfinder in the form of two pointers. When these are in line, the camera is level. In other cameras, the leveling indicator is a pendulum or a spirit level. 
     SUMMARY OF THE INVENTION 
     According to the invention a camera comprising a see-through viewfinder for viewing a subject to be photographed, and a leveling indicator visible in the viewfinder for indicating whether or not the camera is oriented level relative to the subject, is characterized in that: 
     the leveling indicator device has integral means for viewing through the viewfinder two images of the subject which appear to be at the same elevation when the camera is oriented level relative to the subject and appear to be at different elevations when the camera is not oriented level relative to the subject. 
     More specifically, the leveling indicator is an optical prism with two similarly-angled front faces which crest at a common boundary to deviate individual light rays reflected from the subject onto the respective front faces towards each other as they emerge at a rear face of the optical prism, whereby two images of the subject will be seen through the optical prism which appear to be at the same elevation when the camera is oriented level relative to the subject and appear to be at different elevations when the camera is tilted level relative to the subject. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevation view of a camera with a level-indicating optical prism located in the viewfinder; 
     FIG. 2 is a rear elevation view of the camera; 
     FIG. 3 is a schematic perspective view of the viewfinder including the optical prism; 
     FIG. 4 is a top view of the optical prism showing the ray paths of respective beams of light passing through the prism; 
     FIG. 5 is a rear elevation view of the optical prism showing two identical images of a subject seen through the viewfinder when the camera is oriented level relative to the subject; 
     FIG. 6 is a rear elevation view of the optical prism showing two identical images of a subject seen through the viewfinder when the camera is tilted relative to the subject; and 
     FIGS. 7 and 8 are front elevation views of the viewfinder with alternative prisms. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is disclosed as being embodied preferably in a photographic camera. Because the features of a photographic camera are generally known, the description which follows is directed in particular only to those elements forming part of or cooperating directly with the disclosed embodiment. It is to be understood, however, that other elements may take various forms known to a person of ordinary skill in the art. 
     Referring now to the drawings, FIGS. 1 and 2 show a camera 10 having a commonplace direct see-through viewfinder 12 for viewing a subject to be photographed. The see-through viewfinder 12 is bounded by a front viewfinder opening 14 and a rear or eye viewfinder opening 16. The front viewfinder opening 12 is larger than the rear viewfinder opening 14, and both of them have the same aspect ratio, i.e. the length L--to --height H ratio. As can be seen in FIG. 3, the height H is substantially shorter than the length L. 
     A leveling indicator visible in the see-through viewfinder 12 for indicating whether or not the camera 10 is oriented level relative to the subject, is an optical prism 18 located within the rear viewfinder opening 16. See FIGS. 1-3. As shown in FIG. 4, the optical prism 18 is a transparent triangular-shaped body having two similarly-angled equal-length front faces 20 and 22 which crest or meet end-to-end at a common boundary or mid-line 24 and a rear base face 26 which extends between opposite ends of the angled front faces. The two angled front faces 20 and 22 form identical acute angles A with the rear face 26, preferably within the range of 2-5 degrees. Individual light rays 28 reflected from the subject onto the respective front faces 20 and 22 are transmitted through the optical prism 18 and emerge at the rear face 26 deviated towards each other. Each of the light rays 28 consists of an incident ray 30 impinged onto one of the front faces 20 and 22, a refracted ray 32 passing through the optical prism 18, and an emerging ray 34 exiting at the rear face 26. Consequently, two identical images 36 of the subject will be seen through the optical prism 18 which appear to be at the same elevation when the camera 10 is oriented level relative to the subject, as shown in FIG. 5, and appear to be at different elevations when the camera is tilted relative to the subject, as shown in FIG. 6. 
     FIG. 7 shows an alternative optical prism 38 located within the rear viewfinder opening 16 only along a top-most section of the rear opening, as compared to the optical prism 18 which occupies the entire opening. FIG. 8 shows the alternative optical prism 38 located within the rear viewfinder opening 16 only along a bottom-most section of the rear opening. The alternative prism 38 is similar to the optical prism 18 except for its height. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
     10. camera 
     12. viewfinder 
     14. front viewfinder opening 
     16. rear viewfinder opening 
     18. optical prism 
     20 &amp; 22. angled front faces 
     24. mid-line 
     26. rear face 
     28. light rays 
     30. incident ray 
     32. refracted ray 
     34. emerging ray 
     36. identical images 
     38. alternative prism 
     L. length 
     H. height 
     A. acute angles