Hey, I have an XSi and seem to be having a problem. Sometimes when I focus on a random point in the distance that has no real object to focus on, the camera seems to have a fit. Understandably, when it is on AF it wont get a lock and keeps focusing in and out. But if I switch over to MF sometimes it still refuses to take the picture..all it does is buzz like its trying to focus or something, and it will refuse to release the shutter. What gives?