Wide-angle Lenses
According to Wikipedia, a lens is considered wide-angle when it covers an angle of view between 64 and 84 degrees. Anything wider than that is considered ultra-wide-angle, with fisheye lenses being a special sector of the ultra-wide category. Popular wide-angle lenses for cameras with 36 x 24mm sensors have focal lengths of 35mm, 28mm and 24mm; ultra-wide-angle lenses include the 21mm, 20mm, 18mm, 15mm, and 14mm focal lengths.
For digital cameras, many of which have smaller sensors than the 35mm film on which these definitions are based, wide-angle lenses have focal lengths shorter than the short side of the sensor. This equates to about 15mm for an APS-C sensor or 13mm for Micro Four Thirds.
