Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM Review
Image Sharpness
Both the Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM are pretty sharp from center to edge at all focal lengths and apertures. Both lenses use special low dispersion glass elements and each of them also uses three aspheric elements which are no doubt responsible for their good performance.
As you would expect with any lens, sharpness does increase a little as these lenses are stopped own by 1 or 2 stops, but both could be used wide open without major concerns over image sharpness.
17mm
Looking at the performance at 17mm with both lenses wide open I'd say that the Sigma may have a very slight sharpness/contrast advantage, but the differences are small and even a very small amount of focus error could easily mask any differences.
35mm
At 35mm both lenses were pretty sharp wide open in the center.At the edge the Canon was slightly sharper but both were still pretty good. There was no visible chromatic aberration. As expected, stopping down does improve sharpness a little, but if you are buying and f/2.8 lens, f/2.8 is where you want to do the comparison.
50mm
At 50mm wide open (f/2.8) the Canon EF-S 17-55/2.8 seems to have a slight sharpness edge over the Sigma 17-55/2.8, perhaps more noticeably at the edge of the frame than in the center, but as the lenses are stopped down the differences disappear. Again, at the long end of their zoom range these two lenses were very close in performance
Overall it's very difficult to say that one of these lenses is optically better than the other. Maybe the Sigma has a very slight advantage at 17mm, but then maybe the Canon has a very slight advantage at 50mm. Neither outperforms the other when averaged across the whole range of focal lengths and apertures.
Chromatic Aberration
Chromatic aberration is well controlled in both the Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM. Both use special low dispersion glass which offers better control over dispersion (chromatic aberration). Even looking at the images at 100% and at the edges of the frame, CA was low and unlikely to be a problem.
Below are 100% crops from the extreme edge of the frame shot at 17mm with each lens. In this case the Sigma 17-50/2.8 EX DC OS HSM shows slightly more intense colors than the Canon EF-S 17-55/2.8 IS USM, but both are pretty good.
At 35mm it was hard to see any chromatic aberration at all.
At 50mm CA was again hard to see. The images below are taken from the very corner of the frame (where CA would be most visible) and are 200% blowups which represent a crop from a very large print. Even under these extreme conditions, CA is only just visible. In this case he colors appear to be slightly more intense in the image taken using the Canon lens
