I have a Sigma 500mm f/4.5 lens. Just recently, my auto-focus does not seem to work. I know it is not the camera because I put it on my Canon 300mm f/4 IS lens and the auto focus works fine. Every once in awhile, the auto focus will start to work again. The battery is fully charged as well. While pushing down the shutter for focusing, the focus ring does not move at all. And yes, the lens is switched to AF.
Thanks for any suggestions.

