Canon Ultra Wide.. which lenz to go with?

[Village Idiot]

what defines "worth it"?

Less barrel distortion at the wide end and a wider aperture. The Sigma can almost give a fish eye type look if you get your angles just right. It's not super sevre, but you can see it. Any wide angle will have barrel distorion at the wide end, but Canon's is controlled a little better.

 

[Village Idiot]

I'm going to buy an ultra wide for my Canon 30D any day now. It's going to be either a Canon 10-22 or a Sigma 10-20. Both seem very popular but I have a final question before I make my decision. Does the Sigma have a "floating" focusing ring? Ie not spinning during AF focusing yet still usable to fine tune focus in AF mode? The Canon lenses I own all have this feature and I really wouldn't want it any other way.

I believe so. I just used one of these two weeks ago, but I don't remember exactly.