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 metal ring on the camera and the rear of the lens) to the film 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 in hundredths of millimeters and is known as the depth of focus (not to be confused with the similarly named depth of field).