The Focal Length describes the distance from a lens to the point of focus, usually measured in millimeters (mm). The ratio of focal length (b) for the diagonal (d) corresponds to focal length and designation. The focal length of the lens in particular determines the angle of view and therefore the field of view. The standard focal length is defined as (35mm film corresponds to a 50mm objective). Outside this range are the telephoto and or wide angle lenses.