Lenses over 100mm are considered to be Long. These lenses function much like a telescope in that they allow you to photograph distant objects. A Telephoto lens is a long lens whose actual physical length is shorter than its focal length. This is achieved through the use of a complex grouping of optical elements within the lens. Despite the shorter physical length, it will demonstrate the same optical properties as a long lens of the same focal length. These lenses are more convenient due to their compact size but can be more expensive and some quality may be sacrificed.