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| p.1 #3 · Question regarding 85L |
Besides the manual, there are numerous online resources that mention this aspect of the EF 85/1.2L II. To recap:
This lens uses a mechanism commonly called "focus-by-wire." This means that the manual focusing ring is not mechanically linked to the focusing mechanisms in the lens. Instead, turning the ring simply sends an electronic signal to the ultrasonic motor in the lens, which then moves the focusing group.
There are a number of consequences to this design. Most importantly, because it depends on the USM operation for both manual and autofocus, the lens cannot be focused if no power is supplied to it. Therefore, it is strongly recommended that you remember to turn the focus to infinity before unmounting it, each time you are finished with it and are preparing it for storage. Second, this lens consumes more battery power than most other EF lenses. Third, the focusing ring is not geared to the sub-barrel, so it spins freely. And finally, even when the lens is mounted to the body, if the AF/MF switch is set to AF mode, turning the focusing ring does nothing unless you have already engaged AF through the body (e.g., with a half-press of the shutter). That is to say, the focusing ring will not move the focus in AF mode unless the camera has acquired focus via AF and is ready to shoot.