Focal length
From The Glossary of Digital Photography
A measurement from the point at which a lens focuses to the center of the lens. The measurement is most often made in millimeters (mm). With full-frame digital SLR cameras, lenses with focal lengths less than 50 are considered wide-angle; those larger are considered telephoto. Digital cameras with less than full-frame sensors have focal length multipliers and thus multiple 50 mm by their focal length multiplier to get the focal length of their 'normal' lens. See also 35mm equivalent; focal length multiplier
