There is an excellent discussion in the forums on wide-angle image sharpness.
Paul Kay writes, “As you might have seen from other discussions, I contend that no wide-angle lens with a field of view exceeding 90 degrees will give really sharp frame corners unless substantially stopped down, and even then they may not be up to the frame centre. The key to optimising any wide-angle lens, is correct dome/lens placement, appropriate aperture, minimising important corner information being too close. But for ultra-wides this becomes difficult. Having looked through a lot of books with published underwater images I have realised that many exhibit softish corners unless a fisheye lens was used.”