Back Button Focusing and IS

Nathan wrote in post #15758569
.. If no AF function is assigned to the shutter but is assigned to the back button, then only back button will activate IS.

This part is incorrect also.

Half press of shutter will activate AE and spin up IS.
The most common setting for using * or [AF ON] for "back button focus" results in IS spin up from pressing either button. AF is * and AE start is half press shutter.

Really? I'll have to test this one, just to make sure. What purpose does that serve except to waste my battery?

How often do you leave your finger pushing the shutter half way without using the camera that you would worry about battery life? I think the fact that it's always done this and you've never noticed would be a good indicator of how little impact it has on your shooting in general, and battery life in particular.

It does mean that when you decide you need to activate AF and shoot, you don't have to wait for IS to spin up.

Nathan wrote in post #15759146
Really? I'll have to test this one, just to make sure. What purpose does that serve except to waste my battery?

I make use of it to keep IS spun up even when I'm not actively focusing. Say, I've completed any AF (subject has decided to stop moving); so I've taken my finger off the rear/AF button. I can still maintain IS by keeping the shutter button partially engaged.

Snydremark wrote in post #15759181
I make use of it to keep IS spun up even when I'm not actively focusing. Say, I've completed any AF (subject has decided to stop moving); so I've taken my finger off the rear/AF button. I can still maintain IS by keeping the shutter button partially engaged.

Exactly.

To me, the whole point of BBF is to be able to set focus and then not have the camera keep trying to refocus every time you take another shot. If you want IS (and I presume that you do, if it is turned on) then you need it to work when you half press the shutter. Otherwise you have to keep refocusing, and hold the back button down, whenever you take a shot. If that was the case, you might as well have AF on the shutter button, it would completely defeat the purpose of BBF when you use IS.

You both explained it much better than I did. well done.

CyberDyneSystems wrote in post #15758806
It's also what internet forums are for

Yeah, I'm happy to answer when I can, but for my own questions, I'm a bit embarrassed to ask a question that I know is covered in an owner's manual that's easy and quick to search online.