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I have a Canon 700d with 2 kit lenses, both having auto-focus feature. I read a book "Canon EOS Rebel T5i/700D For Dummies", which warns against switching auto-focus on/off while the camera is on. According to it, we should first turn off the camera before turning auto-focus on/off. That's quite cumbersome obviously.

A few days ago, a friend of mine who is professional photographer noticed me doing this, and told me that doesn't matter and I need not turn off my camera while switching auto-focus on/off. I am confused if this might harm my camera or lens in any way?

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    don't read books * for dummies, they are usually trash – Sarge Borsch Oct 25 '16 at 18:47
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    On a slightly different note, it is usually recommended to not attempt to manually focus a lens on a powered-on camera body if the lens is set to AF. – FreeMan Oct 25 '16 at 20:48
  • @FreeMan This is valid for lenses with tough gear between mototr and ring (for example set lens for EOS 500D). New and professional lenses are built to manual (de)focus while active AF. They have loose gear (for example EF 28-135 IS USM) or "fly-by-wire" ring (set lens for EOS 750D). – Crowley Oct 25 '16 at 21:04
  • I'm going to reword Crowley's answer: @FreeMan: You are right, on Canon micro-motor lenses you should not touch the focus ring when the switch is set to AF. Newer and professional lenses with USM (ultrasonic motor) can be refocused freely due to the full-time manual (FTM) feature. Focus-by-wire lenses (like the kit lens STM version) are always safe to manual focus, but the power need to be on, otherwise nothing happens. – Nayuki Oct 26 '16 at 0:36
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    @FreeMan The vast majority of Canon EOS EF lenses have full-time MF even when AF is turned on. The few that don't are the cheapest non-STM kit lenses and a few other isolated older models or a handful of specialty lenses (MP-E and TS-E) that have no AF capability whatsoever. – Michael C Oct 26 '16 at 1:22
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According to it, we should first turn off the camera before turning auto-focus on/off. That's quite cumbersome obviously.

That seems pretty ridiculous. Perhaps the authors are serious about their target audience being dummies.

I am confused if this might harm my camera or lens in any way?

You won't hurt anything. It's fine to turn AF on or off while the camera is turned on.

For evidence, see page 170 of the T5i manual (PDF), which covers manual focus operation. Note that it instructs you to switch the lens to MF but doesn't mention turning off the camera first.

As Rob points out in another answer, you should probably avoid changing the focus mode switch while the lens is actually autofocusing, as it would be if you keep the shutter release half-pressed or set the camera to Continuous AF mode for shooting video. So don't do that. Changing the focussing mode or other settings during use seems like a strange and unlikely thing to do, though, and it seems unlikely that you'd find yourself tempted to do it.

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    ` Perhaps the authors are serious about their target audience being dummies.` - Ha ha. thanks @Caleb – Mohit Bhardwaj Oct 25 '16 at 16:00
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    I feel this answer is incomplete, see my response for specifics. – rob j crowe Oct 25 '16 at 17:31
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    @robjcrowe Thanks for pointing it out. Updated the answer with a pointer to yours. – Caleb Oct 25 '16 at 18:29
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The warning you read applies to Live View shooting. If you look at the T5I manual it states on page 156 under the Continuous AF section , "...During Continuous AF, turn off the power before you set the len's focus mode switch to MF". I usually only use Continuous AF on my T4I so if I do switch to manual I don't usually remember to do this, and I have never had an issue. With that stated, following the camera manufacturer's advice is the safe way to go.

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    I suspect the reason behind this advice is that the mechanical switch may be in an undetermined state while the motor is actually moving with continuous AF, so turning it to MF while the trigger is (half) pressed might also be discouraged. But the chances of damage are very slim as you've found. – Chris H Oct 25 '16 at 18:04
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    Yes, this is a useful clarification: it's probably wise to avoid changing the focus mode switch which the camera is actually trying to autofocus. – Caleb Oct 25 '16 at 18:13
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    Wouldn’t that depend on the lens? What kind of motor tech, how it is clutched, etc. varies from lens to lens. – JDługosz Oct 26 '16 at 5:09
  • @JDługosz yes, of course. But if a significant proportion of entry-level lenses have a small but significant chance of having a problem, it would be worth mentioning it in the body manual. This is especially true if one of the lenses in question is a kit lens. Of course if it was a real issue, p156 of a manual most people don't read wouldn't cover it. – Chris H Oct 26 '16 at 10:30
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In some situations, it's convenient to autofocus then switch to manual focus so the focus doesn't change (timelapse, prefocussing for a remote-triggered shot etc.). This wouldn't make sense if you had to turn the camera off in between.

  • I think dummies usually don't need that feature. And I think it is more foolproof to switch the lens to MF and use AF points to detect the correct setup. In that case you cannot forget to turn AF off :) – Crowley Oct 25 '16 at 21:32
  • @Crowley I don't know if my current camera allows me to use AF points to check my MF (as configured it doesn't). My old one certainly doesn't and I still use it for both timelapse and group pictures with me in as it lives at work. – Chris H Oct 26 '16 at 5:47
  • My EOS 750D glows the AF point in focus and beeps when I reach it. My friend's Nikon (D5500 I think) couldn't do that. You can try it. If the AF switch turn whole AF system down, there is no other way. – Crowley Oct 26 '16 at 9:58
  • @Crowley I stand corrected. When I say old I mean old -- 350D. But that does highlight an AF point that's in focus, if that point is active (I usually have just the centre point enabled), and if the trigger is half-pressed. I never normally manual focus with the trigger half-pressed. (I also had to hunt for the beep option to turn it on, but it beeps as well). – Chris H Oct 26 '16 at 10:27
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The risky AF switching may be:

  • Interferrence between in-body motor and in-lens rings. it is simillar to changing gears in car without clutch pressed. All EF lenses have electronic interface only, so this is not your case.
  • Switching off the motor under load. When switching the switch, peak currents may be delivered to the motor or the control unit causing damage. This may be your case but it is risky if
  • you are switching the AF during autofocus routine;
  • you are switching the AF during live view and contionuous AF.

If the AF motor is not engaged it is 100% safe to switch the AF on/off on EOS lens. Same is valid for IS or any feature - do not switch the power off under load!

I don't think it will immediately kill the lens, but it is unnecessarily rude. In emergency - The Perfect Moment is approaching and AF still cannot focus - or -I shooting video and it focus on wrong point - do it. But take it as a lesson not as a routine.

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