I have a 150mm sonnar that recently jammed up a 500 body. I was able to remove the lens and the body works fine with other lenses. The 150mm shutter seems to be the culprit. I tried cocking and releasing it manually, and if i roll the shutter shaft screw it will fire, at all speeds, but when i put it on the camera it will not fire and get stuck half way. Any idea why this is happening? Or perhaps the shutter on that lens is kaput?