I'm having autofocus issues with my T3i for the first time. Whether in low light, or plenty of light, it searches back and forth for focus, and most of the time never finds it. When I put it in manual focus and hold the button down half way while searching for focus, only sometimes do I get a beep and red light when I hit focus. I thought it may be a lens issue, but when I tried them on my 7D, there were no issues at all. What could the problem be? The contacts look clean, the lens is locked in completely, and the firmware is up to date.
