Flange focal distance
For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FFL), back focus or register, depending on the usage and source) of a lens mount system is the distance from the mounting flange (the interlocking metal rings on the camera and the rear of the lens) to the film or image sensor plane. The flange focal distance is a fixed mechanical specification of a given camera system, and it must be manufactured to a precision of hundredths of a millimetre—even small deviations can prevent lenses from achieving accurate focus across all focal lengths.