On other lenses, this is a dangerous move. Can I move the focus ring, with AF ON with an L lens?
I heard that Canon L lenses (and others with an ultrasonic motor) can be in AF ON mode and I can move the focus ring, but I don't have a strong conclusion.
To the best of my knowledge all current Canon "L" lenses have ring type USM focus motors, other than the TS-E lenses that are designated "L". They have no focus motor of any kind as they are manual focus only lenses.
If the lens has ring USM then you can move the focus ring at any time without fear of damage. If the lens has Micro-USM and the focus ring does not move during autofocus it is also safe. Lenses with full time manual focus will also allow you to continue to turn the ring even when the focus is all the way to Minimum Focus Distance (MFD) or infinity.
If the focus ring moves during autofocus or if the ring hits a hard stop at MFD and infinity, then you should only move the focus ring when the lens is set to Manual Focus.
Yes it is safe to adjust the focus manually in AF mode of L lenses. The general rule is that for any lens if in AF mode you have to use force (The motor is not allowing to change focus, using force may cause damage) it is not safe. In L lens of any other USM motor this is not the case the focus ring will move freely and you may feel that this ring is not directly connected to the motor.
Yes, it's safe on just about any Canon L lens that has autofocus. Keep the shutter button half-depressed while you're re-focusing with the focusing ring, and then press the shutter button all the way down to take the photo. If you re-focus, release the shutter button, and then take a photo, the lenses will focus automatically and you'll lose the re-focus setting,
