Question: 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?

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

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> I am confused if this might harm my camera or lens in any way?
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**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)](https://static.bhphotovideo.com/lit_files/79893.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.
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.