Does it autofocus? Does the manual focus "feel" loose? I don't know enough about lens construction to be of much help, but I can imagine that an internal friction stop might be missing or the helicoid lubrication inadequate. I have a used Nikon 50mm f/1.2 AIS MF lens that has a strange feeling focus ring but at the moment holds focus and has good optics, so I am not messing with it. It was Army Surplus, and may have been stored under conditions that degraded the helicoid lubrication. If yours is non-functional, you might be able to find some DIY fix-it info on the web. Lots of people in the manual lens world rebuild their lens housings, commonest is to de-click the aperture ring, also common is to change mount using adapter kit.