I've been reading about it on and off, and I'm wondering... 1. Would a 40D be close to a full frame sensor? 2. If I made an upgrade from my XT to a 40D, could I still use my Sigma 10-20mm? 3. On a XT, you multiply the crop sensor 1.6x by the lens focal length (I.E a 70-200mm on a XT becomes 112-320) Would using a 70-200 on a 40D ACTUALLY be 70-200? Thanks.