I recently acquired a used 100mm f2.8 USM macro lens. When trying it out at home, I started getting an err 01 message, but not with every shot. I then systematically started shooting with different aperture settings, and realized that at f2.8, there was no problem, but at any other F the error message came up. I just want to make sure that I'm not missing anything here, as I am quite new to DSLR photography. I have already tried cleaning the contacts, on the body and lens, with a soft cloth, with no difference. Any suggestions?