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.
For a 35 mm camera with a diagonal of 43 mm, the most commonly used normal lens is 50 mm, but focal lengths between about 40 and 58 mm are also considered normal.
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.