Great for outdoor and bright light, not so good inside
Kyle Biggs, Amazon
25 August 2012
Summary: I recently upgraded to a full frame camera and needed to replace my (That lens is designed for APS-C sensors, and was useless on the 5D). I figured that with a 1.6x crop factor, my old 10-20 should be equivalent to this 20mm. I am a real estate photographer and videographer.
Excerpt: I agree with other comments on the web about the lack of sharpness in this lens - certainly compared to the other Canon lenses I own. My one has a slightly "grabby" focus ring, but it is not really a problem in practise.
Conclusion: Vignettes badly at f2.8 and f4. Corners not sharp at all at f2.8 and f4. Good central sharpness at f4, f5.6 and f8. Corners ok at f5.6, slightly better at f8. Nice modeling effect and bokeh quality at f5.6 and f8. Looks worse at f11 and smaller stops - diffraction?
Summary: Reasonably sharp wide-angle lens. There is slight red-green fringing, similar to 15 mm fisheye. Purple fringing is absent. Strong vignetting at f2.8-f4, but this goes away at f8 and higher. Good centre sharpness, but the corners are a bit soft at f2.8, and this improves by f5.6.