Hello, my xti is working perfectly until I put my 70-300mm lens on then it won't focus. It tries but the lens just keeps going in and out and it won't take a picture. It will if I manually focus it. The lens is a quantaray and I have used it many times in the past. Hasn't been dropped or damaged or anything. It still works fine with my 35-135 lens. Is there a certain setting I should check and adjust or does this seem to be a lens failure?
After to clean the lens and camera's mounts by soft and dry cloth and try again. If this don't work you need contact the dealer to repaire but if it (the lens) is under warranty, otherwise you just use it in manual focus.
As I know, the quantaray lens has this common error. This brand lens are made by Sigma in Ritz standard (no in Sigma standard quality).