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 From Wikipedia
