The lubricants used in these lenses do not last forever. In many of the old Nikkors they simply dry out. You can still use the lenses this way but you will eventually wear out the helicoids. Other lenses like old Canon FL or Minolta MC lenses can get very stiff. Those lenses also need a going over and relubrication.
"Those lenses also need a going over and relubrication."
These are beautiful lenses with no wear, still in the boxes, some with the instruction booklets. I'd like to use them.
Who would I send these old mechanical lenses to for an overhaul?
I send mine to my local Nikon Authorized Service center.
If I didn't use them, I'd go to Steve's Camera in Culver City, CA.
It's not brain surgery so any competent tech should be able to do the work correctly.
Essex Camera Service in Carlstadt, NJ does very nice work. If you are on the West Coast you might also consider Ken Ruth at Photography On Bald Mountain in Davenport, CA.
It is probably down to the age of the lenses. Not so much the use they have but the grease they lubricate the helical threads with Too much and they are stiff, too little and they get sloppy. All it needs is a good service to get them back up to standard.
If you're in the middle of the country, Havel's Camera in San Antonio, TX is reasonably priced, quick on the turnaround, and very experienced.
My old AI Nikkors always had very light focusing, from new. It is a little disconcerting, but they were precise. I got used to it quickly, coming to appreciate the ability to one-finger the focus ring if need be. Slop is clearly due to wear, but considering the lousy feel and wobble in so many auto focus lenses, I figure a little looseness is not that serious.
One of my Pentax SMC-M lenses, bought used, had some wear, causing a slight wobble. I asked Eric Hendrickson (one of the gods of Pentax repair), and he said it would not be a problem, and wasn't worth the cost to fix. Just for the heck of it, I took off the helicoid part (it is a zoom, and is front group focusing, single helicoid). I wrapped some teflon tape around the inner part, stretching it tight to thin it. I put the helicoid together, and worked it back and forth a few times. Then I lubed it and reassembled it. The tape filled the small gap, and the lens still feels great several years later.
Thicker grease will also fill the gap, but I wanted to keep my focus feel the way it was, plus I didn't have to buy thicker lube for just that one lens.
I have a 20mm f/4. Focus is, and always has been, somewhat loose---or as an optimist might say, "quick". Accurate though.