17-55 predictions

vs. Sigma 18-50mm f/2.8

This is the big question. Without image stabilization and ultrasonic focus the Sigma won't be able to compete on handholdability and autofocus speed. Obviously, the Canon can't compete on weight, size, or price. I expect build quality to also be in favor of the Sigma EX. As with all Sigma lenses I've seen, this one seems to have a brownish cast compared to the better-balanced Canons. I expect the lenses will have similar contrast - good, but not top-notch. Finally, sharpness: judging from the MTF charts and sample photos, it looks like these are very close, but that the Sigma has the edge at the wide end. Overall: provided the Sigma focuses accurately in low light, doesn't look good for the Canon.

vs. Canon EF 24-105mm f/4L IS

Image-quality-wise the EF lens will be better on a cropped-frame camera. Sharper, especially at the edges, and more contrasty, but then the focal length range is not really comparable to the EF-S's. The comparison is much closer with the 24-105 on a full-frame. The EF-S wins stopped down at the wide end and overall at the telephoto end. It is mostly a question of more focal length range and better build quality versus a one-stop-wider aperture and the ability to use on far cheaper bodies. In the balance, the EF wins for full-frame body owners, and the EF-S wins for cropped-frame body owners.

vs. Canon EF 16-35mm f/2.8L

The EF has better color, is a little smaller and lighter, has better build quality, and goes a tiny bit wider, but this doesn't begin to make up for a $220 higher price, lack of IS, and loss of the entire telephoto range. The EF will function as an ultra-wide on a full-frame camera, but by all credible reports its optical performance is mediocre in this application. I believe the EF-S 17-55mm f/2.8 IS will deeply hurt sales of both the EF 16-35mm f/2.8L and its 17-40mm f/4 little brother.