All jokes aside, the late rubber focus ring non-AI and the AI lenses tend to have the stiffer focus rings. AIS lenses seem to have gone back to the smooth feel of the metal focus ring non-AI lenses, except for lenses like the 55f2.8, which are stiffer, but then again, so are the older 55f3.5's. Even my early compensating diaphragm 55f3.5's were pretty stiff in focus.

It's possible that the 50f1.4 that you're dealing with was dropped @ one time. That could cause a loose feeling focus ring. I once owned a 50f1.4 Nikkor-S that had the ring become disengaged from the helix. Had to pull the focus ring toward the camera body to focus the lens, otherwise it would just spin.

-J