Definition of Back Focal Distance

Article ID: ART157647 | Date published: 06/09/2015 | Date last updated: 01/25/2016


Definition of Back Focal Distance


Back Focal Distance (Back focus)


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


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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