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On my new Canon T6s/760d with 18-135 STM lens, I just noticed that in AF mode, when I hold the shutter button halfway, if I turn the focus ring on the lens, it does affect the focus!
I preferred to stop doing that, not knowing wether or not it could be dangerous for the lens (of course it didn't seemed as I was forcing any mechanism, since this ring can turn continuously on this model, and doesn't seem to be directly linked to the AF mechanism).

But I am just wondering why it is behaving like this ? Is this intentional ? If it's not can it be dangerous to do that?

  • It's called manual override. It's a feature. When AF fails, you can manually correct it without switching to MF. – null Aug 17 '15 at 10:26
  • For futur persons with the same question, it would probably be helpfull to habe a more detailed title. – Olivier Aug 17 '15 at 10:40
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    @Olivier - you can edit the question titles yourself if they aren't helpful. (Edits need to be reviewed until you get more reputation though) – Mark Fisher Aug 17 '15 at 11:31
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    @Oliver it is encouraged on every SE site to modify any content that you are able to edit to your liking. That explicitly includes other users' questions, answers, etc. With less than 2000 reputation your edit will be reviewed by users with 2000+ reputation, so you can't really breaks something at this point. Feel free to make an edit in addition to leaving a comment next time. – null Aug 17 '15 at 11:33
  • It's called focus-by-wire. Just one of the reasons why the "new and improved" STM lenses aren't always the best choice for every shooting situation. Great for video, good for stills of static scenes, not so great when manually focusing a dynamic scene. – Michael C Aug 17 '15 at 17:42
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That is intentional. As described here:

This new STM lens is an inch shorter and adds a zoom lock and instant manual-focus override to the older 18-135mm EF-S IS lens, however manual focus is electronic. The focus ring isn't connected to anything, and there is a tiny time delay between when you move the ring and the lens moving. The speed at which the lens moves isn't directly related to how far or how fast you turn the ring, so it's weird. It's not as good as having a real manual focus ring like the 15-85mm, 17-55mm or 17-85mm

Most lenses these days have manual over-ride. It does not damage the lens in any way.

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The Canon 18-135mm STM lens allows for Manual override when set to AF Mode. It will not harm the lens or the camera.

It is a feature that is commonly used by Videographers but also by some photographers,

A common example will be at a wedding. Here you focus on the bride, you hear the beep and see the red dot. Now with the shutter half pressed, you pan over to the Groom and manually refocus. It allows for the cinematic effect of the blur to continue from the Bride until you reach the groom and manually refocus again.

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