Last week I had an "optional" photo shoot at a civic event in our town, so I used the opportunity to experiment with back-button focus. I'd never used it, but a friend who's a sports photographer had told me how much he liked it (each of us has a 5D Mark III), so I set up the buttons and gave it a try.
Overall, BBF worked pretty much as I had expected; and while I didn't really notice any significant benefit for the kind of photography I do (landscapes, architecture, and events, mostly), I could see where it could be useful in certain circumstances. But one thing bothered me: When I edited the pictures in DPP 4, there were no red squares to indicate which AF points had achieved focus. (I was using the mode that divides the AF points into groups; and if any point in the selected group achieves focus, that's what's used.) I guess I can see the logic to it. With BBF it's assumed that that there's a high probability that you'll recompose, in which case a red square wouldn't necessarily mean that what's behind it is what was in focus. But I found it a bit annoying anyway. After all, you can recompose with shutter-button focus, and that doesn't make the red squares go dark. Is that really how it works, or am I missing something?
You need to be pressing the BBF when you activate the shutter. It is sort of lik maintaining a half press on the shutter to maintain a focus lock in One Shot Mode.
The focus point is still reported in EXIF, because Lightroom has an add-in that can distinguish between when you're holding the button, and when you're not, although the info isn't spelled out in that fashion. It reports multiple states of focus lock.
One state indicates a focus lock, and AF point was the one you selected. Another state says focus locked, but you did not have the point selected, which is the case when you have released the BBF button when you activate the shutter.
I am new to BBF and just figured out myself why the auto-focus point wasn't showing up in Digital Photo Processor. The question I have: if you release the BBF, does it lock on the original focus point. It seems to, but curious. Thanks.
Yes. If you release the BBF, the lens ring will remaiin focused in its' last position. I wouldn't call it a "lock" though, because with many lenses you can manually override the AF simply by turning the "unlocked" focus ring.
If you figured out that the AF point is not displaying in DPP because you releassed the BBF, then you figured corrrectly. The record of which AF point locked focus, if any, appears to always be written to EXIF It just seems like DPP does not display the AF point when, and if, the AF system is idle at moment the shutter is activated.
Thanks for confirming that the focus stays on the last focus point. I understand your point that it isn't really a lock. It's logical and also good to keep in mind.
Um, not quite the way I look at it. The focus does not stay on the last focus point. The lens remains focused at the last focused distance. The AF, focus points, are turned off and inactive, when you release BBF.
The subject that you had previously locked focused on could be in another part of the frame, or you could have turned arouind and be focusing elsewhere. The camera/lens combo will remain focused at the distance locked by the last active AF point. Distance, not focus point.
Be cognizant of the fact that all of this locked to the same distance can go out the window if your big zoom lens creeps on you.
Bob from Bean Town,
"I didn't really notice any significant benefit..."
Neither did or do I. Maybe it's because we are too old school as I don't use a lot of the newer functions available. I am currently content using with a camera that doesn't have all of them!
They don't come much more old school than me, 70yrs old and been playing with cameras since I was 18. One day I might just get that perfect shot I keep trying for.
I wouldn't be without back button focus, it's the perfect way to separate focus lock from exposure lock.
I must admit there are a lot of other new functions I don't care for though.
Instead of BBF, I reprogram the AF ON button to be AF OFF.
That gives me the ability to press the 'back button' when using AIServo and I need to temporarily suspend focus.
I find that the times I need to suspend focus are a lot less than the times I want continuous focus. So for me BBF would just mean having to press two buttons (back button and shutter button) most of the time. Where having the AF OFF button available just means I occasionally have to press two buttons (back button and shutter button).
To my knowledge all cameras that have an AF ON button allow you to reprogram it to AF OFF. I know the 40D/50D/6D/7D/7D Mk II for sure do.
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