I was just going to start using my 135mm (Canon F/2.5) the other day, so I mounted it, looked through it, and moved the focus ring, and it just spun around without changing the focus. It had worked perfectly the day before when I was doing some perspective shots. It works (sorta) if I push it away from the camera, and even then, I can't tell the focal distance. It seems to lock a bit at infinity and a few other places, but not enough to make it usable. Is there any way to tighten it? I don't really want to go to a shop and pay half what it's workth just to get it tightened.