Quick question about back-button focusing

Hi all,

I've been using half-press AF on center focus point with no issues up to now but there's something I'm wondering right now that could make me change my position. If I leave the camera in AI Focus mode and move AF to the back button, does that mean that things will work as follows:
- track subject, press back button once: camera focuses at that moment and keeps focus at that distance.
- track subject, keep back button pressed: camera maintains focus on the subject.
Giving you, essentially, a manual "AI Servo" mode that allows you to switch seamlessly from Single Shot to AI Focus without your eye leaving the viewfinder, and without running the risk of the Servo mode getting confused and switching focus when it shouldn't (or the contrary).

Side question: in the above setup and on a fixed subject, would the accuracy and reliability of the AF be the same (or better) as if you shot in Single Shot mode?

Yes, as you describe it would work as single shot focus, this is the method that I use on my 7D, most of my shooting is birds, so it's permanent;y on servo, just a quick dab of the AF button when I want single shot focus. I've not noticed any particular problems with focus accuracy, however I've not tried in say macro situations with razor thin DOF, but then for macro I'm usually manual focus anyway.

Well macro isn't really a problem as I don't do macro work (don't have a macro lens in the first place, really). I'm just concerned that the AI Focus mode used as a single-press to achieve one-time focus would be less accurate than 1-shot.

armis wrote in post #13736363
Well macro isn't really a problem as I don't do macro work (don't have a macro lens in the first place, really). I'm just concerned that the AI Focus mode used as a single-press to achieve one-time focus would be less accurate than 1-shot.

No, I often use my 5d2 the way you describe.. you have to be careful though, if you often focus and then recompose like I do (using center AF point), make sure you stop pressing the back focus button before recomposing. I often recompose very quickly and I will frequently see focus drift from where I had recomposed a fraction of a second before I took my finger off the focus button.

I notice you say your using AI Focus? You'll find lots of posts here where people are saying AI Focus is not a good choice - that it fails to focus properly most times.

This is not something I have tested but have stayed away from it because a lot of photographers on this forum who have been shooting a lot longer than me have said it.

Any thoughts on that?

Also you don't mention what camera you have. The Back Button configurations work very differently in the 3 AF modes.

I agree with you Walt, I have read over and over again the "AI Focus" is not the best choice since the camera will determine wether the subject is moving or not; therefore it will switch back and forth between "One Shot" and "AI Servo". It's just better to manually tell the camera you're shooting still or action.

3Rotor wrote in post #13739160
I agree with you Walt, I have read over and over again the "AI Focus" is not the best choice since the camera will determine wether the subject is moving or not; therefore it will switch back and forth between "One Shot" and "AI Servo". It's just better to manually tell the camera you're shooting still or action.

I agree with the above two posts. AI Focus doesn't exist for me.

I use back button AF on my 30 D in that manner and always leave it on servo. Press the button and it focuses, let go of the button and it's locked. Keep pressing the button and it will track whatever the focus point is on. I feel it locks focus just as good and tracks fine. Seems to lock focus just as fast with a quick push of the button.

For questions as to how the Back Button works with AF or the different metering modes, I found this chart very helpful.

It is in one of the early "Back Button Focus - Whats it all about" threads but I can't seem to find it any more so I'm just posting the PDF that was in it.

I know its for the 20/30D but it explains a lot of stuff about using the back button properly.

Yes, "back button focus" plus AI SERVO will give you one shot and servo shooting without having to switch mode, via the steps you listed.

The only thing is, that if you're trying to use it for 'one shot' shooting, you've got to get a bit better about watching your actual focus through the viewfinder, since you don't get focus lock or the confirmation beep when shooting in SERVO.

+1 ^