I bought a sigma 70-200 F/2.8 used from a friend of mine, and it is a superb lens that renders crisp, sharp images. However, there is a problem. The connection between the main lens and the mounting part (between the rear and and the zoom ring) is a tad loose and I can rotate it about 1mm in either direction in relation to the rest of the lens. I am wondering if anyone has seen or heard of this problem before and if anyone has any experience fixing anything fimilar. thanks.