I understand that a fisheye gives you a 180 degree view, but with vignetting ie your field of view is round instead of rectangular. An ultrawide angle lens, on the other hand, gives you a rectangular field of view.

What puzzles me is why there is a 15mm fisheye lens, and a 14mm ultrawide angle lens. Which would give a wider field of view? If the 15mm fisheye gives you a 180 degree field of view, does that mean the 14mm gives you a field of view greater than 180 degrees?

Would much appreciate some clarification on this, or links to a site that may help.