As some of you who have helped me on my "wide angle lens" thread, I'm trying to plan out what "pro"-level lenses I'm going to buy over the next few years. What I'm doing is effectively going through catalogs of lenses that work with Canon cameras and excluding lenses that are specific to the APS-C sensors (so that the lenses will work when I upgrade to a full-frame digital sensor in a few years). Before I post my final candidates for comments and suggestions, I have perhaps a basic question: What's up with the speed of Canon vs. Sigma lenses? Comparable Canon and Sigma "Wide Angle-Telephoto" lenses (like ~25-70 mm) seem to be around f/3.5-4 for Canon and f/2.8 for Sigma. Is there something I'm missing here? As far as I can tell, other than the name, Sigma seems to have faster lenses than Canon, and I'm really looking to get as fast a lens as possible without spending over $800 for a single lens. So yeah, what's up with that? Is there something obvious I'm missing?