An ultra wide-angle lens is a lens whose focal length is shorter than that of an average wide-angle lens, providing an even wider view. The term denotes a different range of lenses, relative to the size of the sensor in the camera in question.
For 1" any 9mm or shorter is considered ultra wide angle.
For 4/3" any 10 mm or shorter lens is considered ultra wide angle.
For APS-C any lens shorter than 15 mm.
For 35mm film or full-frame sensor any lens shorter than 24 mm
For 6x4.5 cm any lens shorter than 41 mm
For 6x6 cm and 6x7 cm any lens shorter than 56 mm
