Focal Length

The focal length is the distance, measured in millimeters, between the optical center of the lens and the focal point (film or sensor), when the lens is focused on infinity.

Various lenses have various focal lengths. Wide angle lenses have a focal length of less than 35mm in film cameras and full-frame DSLRs; lenses are considered standard if they have a 50mm focal length; telephoto lenses generally have focal lengths of 70 or more.