Posted 04 September 2013 - 12:47 PM
I would not be overly concerned with sharpness underwater--water quality (and your proximity to the subject) makes a larger difference than any difference between these three lenses.
I have the Sigma 15mm and find that it is the goldilocks lens on DX. Just wide enough to get super close, just narrow enough to keep extranious elements out of the frame, and the narrow FOV cuts down the fisheye effect. I personally like the focal length better than the 10.5mm. Still, for very big subjects and expansive reef scenics I still like the 10.5mm. But you really need to be much more careful with how you use it than at 15mm. Its easier to get a good shot with the Sigma, and now I use it on full frame too while my 10.5mm sits in the drawer waiting for that very special case. I do like the freedom of not having the zoom control to worry about and F2.8 is faster to focus and better with filters than a slower zoom.
However if I had the choice of the tokina when I bought these, I'd go with the zoom. I think this is a miricle lens underwater. While I think it has limited utility topside, I think it really is the only WA you need underwater. Also think about carrying less gear when you travel as dives are limited and so is luggage. This lens really does it all.
UWPhotoNewbie: Not such a newbie to diving and UW photography.
Nikon D70: 60 mm, 11-16mm, 105mm, 15mm, 10.5mm
Ikelite iTTL Housing, dual Ikelite DS125
Nikon D600 topside 14-24, 28-300, 70-200, 35,50,85