Ian, I had a Tokina 11-16mm f 2.8 for a couple of years. I purchased it thinking it was going to be a great wide field lens on my crop sensor Canon but it wasn't up to scratch
. I got reasonable results with it stopping down to f 8 but that defeated the point of having a fast wide angle lens. At open aperture, the stars suffered from severe coma and aspherical aberrations in the corners. I may have just purchased a bad one off the production line but in my experience with using it for astro work, I'd not recommend buying one.
You may be lucky in getting a good one which you can only establish by testing it on your camera before you purchase it. Good luck!
