What is Back Button Focus all about?

Started Mar 25, 2014 | Discussions thread
Phhoenyxx Senior Member • Posts: 2,505
Re: What is Back Button Focus all about?

WilbaW wrote:

seeker moc wrote:

I think the misunderstanding comes from the ambiguity of the term 'lock'. I think that WilbaW is using the term as it the focus 'locks on' to a subject and continues to vary focus to tracked the 'locked' subject; whereas Phhoenyxx is thinking of 'lock' as in the focus 'locks' at the current focal distance, and doesn't change even if the subject and/or camera moves.

No, I mean "lock" as in hold the focus where it is now, like you get when One-shot confirms focus, or when you tell AI Servo to stop. Conventional BBF just means you trigger AF independently of pressing the shutter button. You don't press the back button to lock focus, you press it to start autofocus (though it can be configured that way but that's not the context of this discussion).

The whole purpose of the AF Lock button is to be able to lock focus where it is after you autofocused so that you can let go of the shutter button and not worry about it refocusing (re-autofocusing) the next time you hit the shutter button. What I was saying is that if you set the autofocus to the back-button, simply letting go essentially does that (it locks it there and you don't have to worry about it changing unless you press the back-button again). BBF acts as an autofocus button when you press it and an AF Lock button when you let it go, since pressing the shutter button will no longer try to refocus (re-autofocus). Therefore, the focus is essentially "locked" there until you press it again, but unlike the shutter button, you don't have to worry about it re-focusing (re-autofocusing) because you don't have to press it again to take a shot. You've decoupled its operation from the shutter button.

Is there more proper terminology to distinguish between the two?

One-shot locks, Servo tracks.

- One-Shot autofocuses when you hold the back-button. As soon as it achieves focus, holding it does absolutely nothing. It does not try to refocus even if you move the camera or if your subject moves.

- AI Servo CONTINUOUSLY autofocuses when you hold the back-button. If you move the camera or if your subject moves, it will keep adjusting the focus as long as you keep holding the back-button.

- Both LOCK (as in not change) focus when you let go of the back-button.

Note: When I say LOCK, I mean "not change." I do NOT mean track.

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