Ultra Wide Angle Photography

For a full-frame 35 mm camera with a 36 mm by 24 mm format, the diagonal measures 43.3 mm and by custom, the normal lens adopted by most manufacturers is 50 mm. Also by custom, a lens of focal length 35 mm or less is considered wide-angle.
By definition, wide-angle lenses differ from ultra wide angle lenses in that the latter have a focal length shorter than the short side of the film or sensor, which means that in 35mm, a wide-angle lens has a focal length between 35 and 24 mm, while an ultra wide-angle lens has a focal length shorter than 24 mm.
Common wide-angle and ultra wide-angle lenses for a full-frame 35 mm camera are 35, 28, 24, 21, 20, 18 and 14 mm. Many of the lenses in this range will produce a more or less rectilinear image at the film plane, though some degree of barrel distortion is not uncommon.
Ultra wide-angle lenses that do not produce a rectilinear image (i.e., exhibit barrel distortion) are called fisheye lenses. Common focal lengths for these in a 35 mm camera are 6 to 8 mm (which produce a circular image). Lenses with focal lengths of 8 to 16 mm may be either rectilinear or fisheye designs.