I bought a Sigma 17-70mm lens last year. It was fine to begin with, but whenever I have my Canon T2i in auto focus mode it focuses incorrectly, causing the image to be totally out of focus. As a result, I constantly have to keep the Canon in manual focus mode which isn't ideal as this was meant to be my quick multi-purpose lens. I do not have this issue with any of my prime lenses, or the kit lens that came with my original Rebel XSI. Is there a fix for this issue?