A Foca camera of 1947 at the Musée des Arts et Métiers in Paris.
A rangefinder camera is a camera fitted with a rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus. Most varieties of rangefinder show two images of the same subject, one of which moves when a calibrated wheel is turned; when the two images coincide and fuse into one, the distance can be read off the wheel. Older, non-coupled rangefinder cameras display the focusing distance and require the photographer to transfer the value to the lens focus ring; cameras without built-in rangefinders could have an external rangefinder fitted into the accessory shoe. Earlier cameras of this type had separate viewfinder and rangefinder windows; later the rangefinder was incorporated into the viewfinder. More modern designs have rangefinders coupled to the focusing mechanism, so that the lens is focused correctly when the rangefinder images fuse.
Almost all digital cameras, and most later film cameras, measure distance using electroacoustic or electronic means and focus automatically (autofocus); however, it is not customary to speak of this functionality as a rangefinder.
The rangefinder viewfinders usually have a greater field of view than the lens in use, with the photographer being able to see what is going on outside of the framelines and therefore better anticipate action. In addition, with viewfinders with magnifications larger than 0.8x (Leica M8) photographers can keep both eyes open and effectively see a floating viewfinder frame superimposed on their real world view. This is however possible, to a certain extent, with an SLR, using a 50 mm lens. Use of any other focal length on an SLR would result in a floating viewfinder with a different magnification than the human eye.
In contrast, the viewfinder pathway of an SLR transmits an image directly “through the lens”. This eliminates parallax errors at any subject distance, thus allowing for macro photography. It also removes the need to have separate viewfinders for different lens focal lengths. In particular, this allows for extreme telephoto lenses which would otherwise be very hard to focus and compose with a rangefinder. Furthermore, the through-the-lens view allows the viewfinder to directly display the depth of field for a given aperture, which is not possible with a rangefinder design. To compensate for this, rangefinder users often use zone focusing, which is especially applicable to the rapid-fire approach to street photography.
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