Focal lengths are usually specified in millimetres (mm), but older lenses might be marked in centimetres (cm) or inches. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as a:
Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the image diagonal.
Wide-angle lens: angle of view wider than 60° and focal length shorter than normal.
Long-focus lens: any lens with a focal length longer than the diagonal measure of the film or sensor. Angle of view is narrower. The most common type of long-focus lens is the telephoto lens, a design that uses special optical configurations to make the lens shorter than its focal length.
