I think it's inconsistent quality control when it comes to manufacturing. I've had several AI Nikkors since bought new from 1979 to 1984.
One of them has had a loose element since I bought it. Two of them (50mm and 28mm) focus very smoothly. A third (105mm) focuses slightly stiffly. A fourth (20mm) is very stiff.
Optically, they're all fine, but it's obvious that final checks on each lens were not performed.
In the case of your lens, it's hard to say. The age of the lens might or might not play a role. I'm just speaking from my experience. |