I have had a rebel xt for quite a while. I recently upgraded to a 40D but I kept my xt to use as a backup/ to put a wide angle on at sporting events. My general purpouse/walk around lens is a sigma 18-50 2.8. It focuses correctly on the xt but when i put it on the 40D it front focuses quite badly. I realize that sending it in to sigma can correct this problem. My question is this: when i get it back will it back focus on the xt? This would be a problem because it is the wide angle I use for sports. Is there any way that I will be able to get this lens to serve as both a walkaround on the 40D and still focus correctly on the xt? also... probably more to sadisfy my own curiosity than to actualy solve my problem... why is there such a difference between the two cameras... I mean i realize that they use two different AF systems but really?.... the difference is pretty huge... and why is this the only lens affected? all my other lenses do fine on both bodies. Thank you in advance for any help you may be able to give me.