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S Dilworth wrote:
There's a clutch, but there must also be gearing somewhere, because the focus ring turns faster than the distance scale.
The focus ring of the 24-70 mm f/2.8 is loose – too loose, in my opinion – but it's smooth and free from rotational play. The ring seems to be directly connected to the focusing elements and distance scale. Makes focusing a bit easier when using zoomed-in live view, although the throw is too short at longer focal lengths.
It's not so much that there is a difference of clutch, gearing, etc...but...as far as I can tell, a completely different motor type. But yeah you are right, in what you said, the 24-70mm is directly connected and the 24-120mm is not...has to do with the motor type as far as I can tell.
The 24-70mm, 70-200mm, 14-24mm (I think), 16-35mm, and the super tele lenses have real ultrasonic ring motors in them, like most of the canon USM lenses.
The 24-120mm, the 24 1.4, 85mm 1.4, 28-300mm, and some others... have micro-USM motors, similar to Canon's 50mm 1.4 lens. Why Nikon chooses to go with that type of focusing even in some of the expensive primes, I am not sure. Perhaps it saves a lot of space, who knows. But I believe that is why. Look up how the 50mm 1.4 Canon lens gives full time manual. There is so much going on that there will be some play. The tolerances and gaps in things will build up going one way, so when you reverse directions there will be some slack.
Just my opinion and best guess.