How Do Rangefinders Work?
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History
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Cameras that use a rangefinder as a focusing mechanism first appeared in the late 19th century. The first commercial rangefinder was introduced by Kodak around 1915. The term "rangefinder" refers to the focusing mechanism. This mechanism has been found in a variety of cameras using a variety of film formats. Contax and Leica are often what comes to mind when thinking of rangefinder cameras, but the mechanism has been used with cameras using many other film formats. Modern rangefinders are manufactured by Leica, Horseman, Zeiss and Voigtlander. Vintage rangefinders are also easily purchased but should be checked by a qualified camera technician to ensure there are no problems with any of the mechanisms.
Mechanism
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There are three lenses that gather light in a rangefinder camera. One is the viewfinder. The second is the main camera lens that gathers light to focus on the film plane. The third is a separate lens that gathers light and reflects it to overlay the image in the viewfinder by a prism and mirrors. The prism is connected to a cam shaft in the lens. As the lens moves to bring the image into focus on the film plane, the prism moves in the rangefinder. As the lens and prism move, the two images either coincide in the viewfinder or diverge. When the two images are perfectly overlaid, the lens is in the proper position and the image on the film plane is in focus.
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Pros and Cons
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Because the image seen in the viewfinder is offset from the lens, parallax can be a problem with rangefinders. Parallax isn't a problem with distant subjects, but as the subject is moved closer to the camera, a difference occurs in what the camera lens sees and what the photographer sees through the viewfinder. This difference is called parallax. Another problem with rangefinders is that the focal length of usable lenses is limited by a number of factors. Most 35mm rangefinders can't use lenses longer than 135mm and still focus accurately.
An advantage of a rangefinder over a single-lens reflex (SLR) camera is that there is no momentary blackout when a mirror moves up to allow film exposure. The image in the viewfinder remains visible, even when the shutter is opening to expose the film. Rangefinders are mechanically more simple than SLRs, and are much quieter. This makes them more suited for photography in which silence is required.
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