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When looking at custom functions in Canon camera manuals, I've come across mention of setting functionality to have happen when the "AF Stop" button is pressed - a button that, apparently, exists on certain of Canon's telephoto lenses.
What is the purpose of this button, assuming I don't change its meaning in one of the custom functions?
On some Canon models you can reassign the AF Stop feature to other buttons, such as the AF button or the exposure lock button, so you can take advantage of the capability even if you're using a lens that, like most lenses, doesn't have a dedicated AF Stop button. The Canon 6D manual describes the feature like this:
AF-OFF: AF Stop
The AF will stop while you hold down the button assigned to this function. Convenient when you want to lock the focus during AI Servo AF.
According to Canon's Lens Performance page, this button is intended as a way to temporarily disable the autofocus, for example if you see that an object is about to pass in front of the lens that's not on the same plane as your primary subject. Thus, focus is retained (or close to retained, with a moving subject) while the obstruction passes, and autofocus can be resumed from a good starting point once it's gone.
Edit: Note, also, that the AF Stop functionality can also be achieved through other buttons on some cameras, via Custom Function settings, by setting a button (e.g. the * button) to "AF-OFF". (Thank you @caleb, for that information.)
