I own and sometimes use a 400mm f/3.5 ED-IF Nikkor made in 1982 in AiS mount. This lens has internal focusing with a very,very nice,adjustable focus tension system and a single, pre-settable focus distance detent, which may be "over-run" or focused past if needed with the application of a bit of extra force. I seldom use the pre-set distance detent, and instead normally set the focus tension adjustment screw to "OFF" so that the focus races to and fro with a feather-light touch and I can go from infinity to minimum focus at any time. The silky smoothness and mechanical advantage the focusing ring has, and its precision over the focusing distances,and the rate at which the lens focuses throughout its distance range, is better I think than the manual focusing systems on the big-glass AF Nikkor lenses....the manual focus 400 3.5 has a superb manual focusing system,as good as any I have ever used,bar none.It weighs about what an AF Nikkor or AF-S Nikkor 300 2.8 weighs or about 7 pounds,and not 13 pounds like a 400 2.8 does. That slight 2/3 stop loss of speed from 2.8 to 3.5 is well,well worth it to me.The 400/3.5 is the BEST-handling and best-focusing manual focusing big telephoto that I've ever used,with a better MF system than in the 300/2.8 AFS-II or 200/2 VR.