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I have an AI 50mm f1.2 sn 196548 which I have used on my D200. As I focus close to infinity (ie the lens comes closer to the camera) there is a noticible 'bump' of resistance in the focus ring. At first I assumed it was dirt in the helicoid, but when I take the lens off the camera, it is not there. My conclusion is that as the lens moves back toward the camera there is a physical interference, where something in the rear element is bumping something in the camera.

Any ideas what is going on, and if this can be eliminated? It works and all, it's just a bit disconcerting that there is an interference that should not be there.

Thanks,

Art