The focal length describes the distance between the lens and the focus. A focal length is given in millimetres. The larger the focal length is, the larger the object will appear in the photo at the same distance. It is also an expression of the image magnification at a given distance. The focal length directly influences the angle of view of a lens. The larger the focal length is, the larger the object being photographed will be depicted, which reduces the viewing angle.
Many modern lenses allow the manual adjustment of the focal length (within certain parameters), which is known as zooming. A lens with a focal length of 50mm is labelled as a normal lens, since this kind of lens has a viewing angle almost the same as that of the human eye (46 degrees). Lenses with a focal length less than 50mm are called wide-angle lenses. These lenses enable the photographer to shoot photos with a very large angle. Lenses with a focal length less than 20mm are called ultra-wide-angle lenses. The most extreme kind of wide-angle lens is the fisheye lens, which has a picture angle of 180 degrees and mostly produces round-appearing photos. Due to their very high image angle, wide-angle lenses are especially ideal for landscape photography.
Lenses with a focal length larger than 50mm are called telephoto lenses. They enlarge the photographed object(s) exponentially. This enlargement, however, reduces the angle of view as well as the image detail when compared to photos taken with a wide-angle lens. A lens with a focal length of 300mm or more is known as a super telephoto lens.