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Rangefinder and Exposure Meter Coupling
Cameras in which the image is focused on a ground glass screen (technical and reflex models) need no special focusing provisions for the different focal lengths. Where the same focusing mount or focusing scale is used for different focal lengths, the distance figures are however not correct for all of them. Full interchangeable lenses on rangefinder cameras usually carry a differential coupling which moves the rangefinder mechanism through the same distance corresponding to a given focus change of the lens. Thus long focus lenses, which require an appreciable focusing travel for setting to nearer distances, still work correctly with a built-in camera rangefinder.
The same arrangement is not always possible with convertible lenses, since the focusing movement is also built into the camera and remains constant. A given focusing travel (extension of the front lens unit) thus corresponds to different distance settings according to the focal length of the lens. For that reason convertible lens systems have fallen largely out of use with rangefinder cameras; rangefinder focusing there involves focusing with the range-finder in the normal way, reading off a distance figure on the standard distance scale and resetting the focusing mount to a corresponding distance figure on another scale.
Exposure coupling, with cameras having built-in semi-automatic or automatic exposure meters, is however easier with a convertible lens system. There the shutter is firmly built into the camera and is easily linked with the iris diaphragm of the lens (sometimes even the iris is built into the camera). The same applies to interchangeable lenses that fit into the front of a shutter built into the camera. The coupling there usually takes place via some system of preselector diaphragm controlled by the exposure meter or by the meter setting control.
Complete exposure coupling with cameras taking fully interchangeable lenses in front of a focal plane shutter is rare, since such cameras are not normally intended for the user who requires exposure automation. Exposure coupling with focal plane shutter cameras is mostly confined to coupling the shutter speed setting control with the exposure meter, the aperture being pre-set and pre-selected separately on the lens. On one or two cameras however the aperture control on the lens can also be linked with the exposure meter.
Exposure coupling problems with technical cameras rarely arise, as these cameras are not normally fitted with built-in exposure meters. Viewfinders. Cameras other than single lens reflex and technical types need supplementary viewfinders or viewfinder adaptations to show the field of view taken in with the different focal lengths. Camera makers produce various types of multiple finder for this purpose; on many modern rangefinder cameras the viewfinder carries additional image frame lines to show the field of view with selected alternative focal lengths. For the very long focal lengths range-finder cameras often use a reflex housing for accurate focusing and viewing. Twin-lens reflex cameras need no special viewing provision, as long as the alternative lenses (or lens attachments) are available in matched pairs to take the place of, or fit over, the finder lens as well as the taking lens.