Distance from lens to focus.

Distance between X-ray source, film.

For an interchangeable lens camera, the flange focal distance (also known as the flange-to-film distance, flange focal depth, flange back distance, flange focal length, 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.