Back Focal Distance (Back focus)

DEFINITION:

Distance from the rear surface of the last element of a lens to the image focal point of the lens.

DESCRIPTION:

Back focal distance differs from flange focal distance or focal length. Back focal distance value is used when designing an optical system. Retrofocus systems are generally used for wide-angle lenses for single-lens reflex cameras so that the long lens back does not touch the mirror. When the back focal distance is short on an interchangeable lens of a single lens reflex camera and it touches the mirror, a mirror-up lock mechanism is required.

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