Standard lens

A standard lens or normal lens for 35mm-format (called "full format" in digital age!) has a focal length of 40-58 mm.

A 35mm-format lens with focal length 43mm is considered to have a "natural angle of view", as well opposed to wide angle lenses as to telephoto lenses. The natural "field of view" of a human is 180 degrees. Maybe the "naturalness" of a 43mm lens is derived from the 35mm-format's 43mm diagonal. A lot very fast variants of 50mm, 55mm, and 58mm lenses exist. Thus the term standard lens may have been extended so far above 43mm since it was a practical focal length for superb fast lens constructions.

see also lens