An ultra-wide-angle lens aims to provide an image free of distortions, whereas a fisheye lens does not concern itself with such considerations.
Fisheye

A fisheye lens produces distortions. Straight lines curve, as in this photo. This effect can be desired for artistic reasons.
Photo taken with an Arsat 30mm F3.5
ultra-wide-angle

An ultra-wide-angle lens will try to limit these distortions as much as possible while maintaining the very wide field of view. Thus straight lines remain visually straight.
Photo taken with Vivitar 19mm F3.8



















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