Back focal length

Also referred to as back focal distance, back focal length describes the distance between the rearmost surface of the lens’ rearmost element and the film/sensor plane. Back focal length should not be confused with focal length, which – in practical terms – simply describes the field-of-view produced by a lens.

Back focal distance is a hugely impactful concept in practical lens design, because lenses for all kinds of cameras that employ an articulating mirror between lens and film/sensor, will inescapably need to have a long back focal distance. This, in turn, makes it impossible to use some lens designs on systems that have a long back focal distance.

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