This is without doubt the best 12mm rectilinear lens which is available for full-frame cameras. In fact, it's the only one!

Sigma are really pushing the envelope on this lens design. A focal length of just 12mm on a full-frame camera such as a 5D or 1Ds is just insanely wide, but Sigma have managed to make a lens that delivers pretty decent optical performance. It's sharp when stopped down, there's no distortion to speak of (now that is impressive!), chromatic aberration is controlled very well, and the colour rendition is very good. On a crop sensor camera - either the 1.6x crop of the 20/30/40/50D and the 350/400/450D, or the 1.3x crop of the 1D-series - the vignetting and contrast fall-off towards the corners of the picture are minimal. On a full-frame camera these defects are more noticeable, but not a major issue, and they disappear when the lens is stopped down.

But let's ignore the techy stuff for a moment. A lens like this is supposed to be about having fun. It's immensely challenging to use on a full-frame camera - the angle of view is so wide, it might be difficult to avoid the images looking a bit gimmicky - but very rewarding when you can get the hang of it.