I have a Nikon D300 with a Sigma 18-200MM AF lens, both new, but I wanted to get a little closer to my subjects, so I bought a Nikon 12MM extension tube. I got the desired results as far as magnification goes, but now the camera won't adjust the lens' aperture, and all my pictures are completely dark. I don't care about the autofocus, I don't use that much anyway, but any ideas on how to get the aperture working again would be very much appreciated.