I have a rebel xt and am looking to go wider and dont want to buy a lens that I cant take with me to my full frame later on. Is the 17mm wide enough on a crop senser for architecture? There are tons of really old catholic churches I am interested in shooting and the widest lens I own is my 28-90 film kit lens from my elan 7. I know I am coming into some money and am going to get this lens anyway, but I would like to know if I should be planning on something wider also? Any input is appreciated, thanks!