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 Sigma 10-20mm f/4-5.6 EX DC HSM Lens for Canon Digital SLR Cameras (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. When I'm doing outdoor pictures, this lens can get the job done.
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? Distortion is much less than any of the zoom alternatives. Color rendition and contrast are good at f5.6 and f8. Not a good reputation, so very inexpensive second-hand.
Summary: This is one of the first two lenses I bought when I switched to a Canon SLR about five years ago. I've had plenty of opportunity to use it since then, both with film and (full frame) digital. The idea behind this lens is a good one -- 20mm is wide enough to allow for landscape and architectural photos, and it focuses close enough for entertaining photos of people's faces, taken about a foot away from their noses (note that you'll need either a ring light or lucky ambient...
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. Gives a bit of a reddish/magenta color cast, e.g. skies look slightly purple, colors are warm. I shopped around quite a while before I bought this lens.