I experienced that particular problem when I bought a Sigma 15mm fisheye, but I think that was more a lens problem. It seems that the spring that controlled the aperture tab was not mounted properly, so the aperture "sticks" to wide open (f2.8). I've been sending that lens twice to Sigma USA. They did not do a good job in repairing the lens for the first time.
George Oei Madison, WI "Dude, you're getting a Nikkor !!!"