Some lens repair data. I have had both the Sigma and the Canon. I found the bokeh on both to be good for a wide angle but worse than most fast lens 50mm or above. The Canon has terrible CA and LoCA and not great aperture shape, the Sigma is nervous. The Canon focuses faster in good light and much faster in low light, in the dark I could get the Canon to focus sometimes, Sigma not as much. In decent light my Sigma focuses accurately(5D MarkII I mostly use centre point then crop, or slight recompose)The Sigma is sharper, noticeably. The Sigma "feels" well built and mine has been working just fine, but the Canon feels and works a little quicker and lighter in the field, plus it's weather sealed I think.