USM有兩種，Ring USM同Micro USM。
Ring USM扭力大和精確度高，適合快速切換焦點的操作，但未做到完全無聲 ...
harryytm 發表於 2015-3-27 01:53
My 300/2.8 IS has fixed AF drive speed. It would vibrate if u are tracking slow objects.
However, the movie focus ring of my 300/2.8 IS II have slow and fast mode.
I hv nvr tried it though so not sure what they mean!
The first I've seen it in any lens for still photography, the focus is compensated electronically as the lens is zoomed. If the camera's meter isn't on, the lens goes completely out of focus as you try to zoom. So long as the camera is on and awake it stays in focus as you zoom, but if it's off, it will only be in focus at one focal length setting. This isn't related to the autofocus system, it's part of the basic design of the lens. More at Usage.
Focus is continuously compensated as you zoom, and the camera has to be ON for this to happen.
If you zoom the lens with the camera OFF or asleep, focus won't hold as you zoom. If the camera is awake, focus holds as zoomed because the advanced electronic programs are compensating focus electronically as you turn the mechanical zoom ring.