telephoto lens

telephoto lens is a specific type of long focus lens which the physical length of the lens is short than the focal length. the is achieved my incorporating a special lens group known as the telephoto group that extends the right path to create a long focus lens in a much shorter overall design.

telephoto lenses are sometimes broken into the further sub-types of medium telephoto: lenses covering between a 30° and 10° field of view 

A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length.