How does this compare to the Canon 20mm?
This is one of the sharpest lenses you can buy for Canon. It is very expensive in comparison to the 20 mm. It and the 100. Mm Zeiss are my sharpest lenses. It is a great lens for a full frame camera, but it would need to be wider for a cropped sensor (think Tamron 10 - 16). This at 2.8 can do what Canon leNses only dream of, and I truly believe that. It is only Manual focus, so beware if you can't do MF! I own the 50 f2, 100 f2, and 21 f 2.8, as well as 2 older zeiss contax lens.
The color quality of the Zeiss is truer to my eye than the Canon lenses. There is a distincy quality to these lenses--they are heavier. Look on the web for pics of these great lenses!