Entocentric lens with a focal length equal to the sensor diagonal.
For SLR cameras (film 24x36mm, 42mm diagonal) is an f = 42mm a normal lens. Previously f = 50mm lenses are called normal lens (well, f = 50mm was close to f = 42mm)
Third inch lens (6mm sensor diagonal) of f = 6mm focal length are normal lenses.
Half-inch lenses of f = 8mm are normal lenses.
One-inch lenses of f = 16mm are normal lenses.
Normal lenses help to define “wide angle lens” and “telephoto lenses”:
Lenses with a focal length less than that of a normal lens is called wide angle lens.
Lenses with a focal length greater than that of a normal lens are called telephoto lens.
However, an f = 8mm lens on a 1/3 sensor is a “(slight”) “tele lens and on a 1” sensor, a wide angle lens.