[sassan]

Yes Foxy 10.5mm is still rectangular image and not circular field that comes with true fish-eye s.a. 8mm or smaller. It still covers a very wide field of view and gives you the distortion. So if circular view is your preferred one, go for a real 8mm. The older Non AF, Nikon 8mm, F2.8 is a gem. The even older f 5.6, so called Nikon's ugliest lens is a different story as it has a very protruding rear lens elements that need mirror lock up (Won't physically seat otherwise in its place despite the universal F mount).

But I think you are more into new AF lenses (Remember that in extreme wide angle focusing is really minimal especially with narrower F stop). So though I am not familiar with 10.5mm Niky, from the price, you can expect somewhat limitations.

Sigma 10-20mm is excellent. As for Canon (I know not your concern but could be important for others) the Canon's competitor lens, 10-22mm is only for small sensor bodies (x1.6 factor) and does not fit 5D or 1D full frames, but Sigma does! of course you can expect a bit vignette, thus the reason Canon specifically made it for EFs and not EF mount.

The problem I see with a true 8mm fisheye, is that picture is so small that you really hardly get to see what is going on and in fact I prefer to use 8mm on 1.6x bodies and a 10 to 16mm on a full frame body.

A lot of people are saying a lot of good things recently about Tokina's new ultra wide zoom; 11-16mm that comes with fixed F stop of 2.8 which gives a unique opportunity of indoor low light situations and also night or over caste sky situations that I think comes quite handy in Europe. The problem is lens is not readily available and who ever brings it in (At least in USA) the entire party solds out quick. I think that wonderful wide F stop plus excellent price or about $550, combined with the very sharp images it produce (Except a bit softness at full open stop) are the contributory factors to its popularity. You may want to have a closer look at this one too as one of the potential future candidates especially if budget matters.

No matter what your choice is, fisheye has a very unique charm and provides very special opportunities to photographer.