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), 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. This value is different for different camera systems. The range of this distance, which will render an image clearly in focus within all focal lengths, is usually measured to a precision of hundredths of millimeters and is known as the depth of focus (not to be confused with the similarly named depth of field).
Lenses can be adapted from one mount (and respective FFD) to another. FFD determines whether infinity focus can be accomplished with a simple non-optical adapter. Optics to correct for distance introduces more cost and can lower image quality, so non-optical lens adapters are preferred. A simple non-optical adapter holds the higher FFD lens the appropriate additional distance away from the sensor or film on the lower FFD camera. A camera body with a lower FFD can accept a larger number of lenses (those with a higher FFD) by using a simple adapter. A lens with a higher FFD can be more readily adapted to a larger number of camera bodies (those with a lower FFD). If the difference is small, other factors such as the sizes and positions of the mounting flanges will influence whether a lens can be adapted without optics.