Indicates the magnification and angle-of-view of a lens. The human eye sees things roughly the same as a 43mm focal length of a lens for a 35mm camera. Anything shorter is classed as a wide-angle, while longer focal lengths are telephoto. Because of the comparatively small size of the CCD in a compact digital camera it has a standard focal length of between 6mm and 8mm while a medium-format camera is around 80mm.
A distance between the rear nodal point of the lens and the focal plane, when the focus is at infinity.