So you want an ultra-wide lens AND you want a fast f/2.8 aperture? Well, the Tokina 11-16mm is the one for you. It doesn't have a very wide zoom range, but it has a lot of positives: it's fast, very well-built, and it's very very sharp: probably the sharpest ultra-wide lens available for cameras with APS-C sized sensors. And you've probably got a zoom starting at 17mm or 18mm anyway; this is just for the really wide shots.
COMPATIBILITY NOTE (1): This Tokina lens is for Canon cameras only. We also stock the same lens for Nikon cameras: click here.
COMPATIBILITY NOTE (2): This lens is designed for cameras with crop sensors (7D, 20D to 70D, 100D, 300D to 700D, 1000D to 1100D). It will physically fit on the 6D, 5D series and 1D series cameras, but will not deliver a full-frame image except at 16mm (and even then there is heavy shading in the corners).