Lenses of shorter focal length are called wide-angle lenses, while longer focal length lenses are called telephoto lenses.
In still photography, a normal lens is a lens whose focal length is roughly equivalent to the diagonal of the image projected within the camera. This roughly approximates the perceived field of view of the human eye.
Standard normal lenses for various film formats for photography are:
|Film format||Film diagonal (mm)||Normal lens (mm)|
|6 x 6||79||80|
|6 x 7||88||90|
|6 x 9||99||105|
|4" x 5"||163||150|
The 50 mm focal length was chosen by Oscar Barnack, the creator of the Leica camera, as a compromise between the theoretical value and good sharpness, as lens technology at the time was such that slightly longer focal lengths were able to achieve optimum sharpness.