Originally Posted by
Tannin
Slow? Who cares? This is an ultra-wide we are talking about, not a portrait lens or a super-telephoto. What do you want speed in a 12-24 for?
1: Depth of field control. At these focal lengths, you don't get any. At 12mm you have huge DOF all the time, whether you want it or not. No.
2: Low-light work without pushing the ISO. At 12 or 16mm, you can hand-hold down to insanely slow shutter speeds even without IS. At 12mm f/5.6 you can hand-hold a shot that you would battle to manage at 50mm f/2.8. No.
3: Faster focus for action photography. On a UWA? Nope. Not required. In any case, you have a very good AF system, and ultra-wides have such huge DOF that exact focus really isn't an issue.
So what's left? Honestly, I can't think of any reason why I would want a fast ultra-wide.
By the way, not really the question you asked but both my Tokina lenses (see my sig) work brilliantly on the 1D III. The Fisheye vignettes until about 13mm or so, and the 35mm macro doesn't vignette at all. I don't use the 1D III for ultra-wide, mostly it goes with the 100-400 for landscapes and a little wildlife, and I use the 20D and the 10-22 or the Tokina fish.
12-24 sounds like the obvious way top go to me. Hey - if you are going to go wide, why not go wwwwwwwiiiiiiiiiiiiiiiiidddddddeeeeeee?