[bain]

i think this question is for the canon 40d owners really : i have read the canon book on back focus button and how it works but i cant for the life of me figure how this actully works i real life, any you guys use this and can explain in lamen terms how this would benifit me in normal use of the camera. thanks.

[VikingPhotography]

It's quite simple and incredibly useful but, like most things, it'll take a little getting used to.

Back button focus is used for situations where you don't want to trigger the AF system every time you press (or half press) the shutter button. It's a staple of sports photographer and many others use it too to allow for better focus-recompose technique.

Think of it this way. The most sensitive AF point on your 40D is the central one... but the subject that you want in focus isn't always in the center of your frame. So you half press the shutter to acquire focus and then move the camera so that you have the composition you want... all the time keeping the shutter release half-pressed to maintain focus. What happens if your finger slips? You have to start again.

Using the back-button to focus, you thumb it whilst your focus point is over your subject and recompose your shot. At that point you can just trip the shutter release without the AF system re-triggering. Instantly useful!

What you're really doing is wrestling back control of where you want the focus to be and eliminating (or at least reducing) the chance of focusing somewhere you didn't intend to by accidentally re-triggering the AF system whenever you trip the shutter.

[silkstone]

Another use of it is for autofocusing in Live View.

[JMitchell]

I changed from using half press to back button focus after I heard 'some pro' talking about it, in my naïver (word?) days. I still use it now, because whether or not this guy really was all that, I think he was right, and used all the reasons Martin (VP) explained.
Useful stuff

[bain]

Thanks for the explanation. i will have to give it a go now , but even more interesting is what silky said about the auto focus in Live veiw, i understand that you can only manual focus on the 40d in live veiw ,proberly wrong though anyway, thanks again guys

[VinnyP]

I spent ages learning to use the seperate AF button then multi AF points came in and all of a sudden I had to go back to half press. Now I chop and change but my heart is with using the seperate button whilst my brain knows you will get better results using the half press and moving AF points with the thumb.

[Markulous]

Must admit that I came to the button late but now incorporate it, if suitable - the trick now is for me to remember whether I'm set for MF (which I still prefer as it always works!), half press release with AF points (and which point!) or button focus! Like all things, practice helps!

[VikingPhotography]

Originally Posted by VinnyP

I spent ages learning to use the seperate AF button then multi AF points came in and all of a sudden I had to go back to half press. Now I chop and change but my heart is with using the seperate button whilst my brain knows you will get better results using the half press and moving AF points with the thumb.

Vinny,

Why not plan your shot in advance a little and put the AF point where you want it to be in relation to the way you're going to frame your subject? Here's an example of what I mean



For this shot, I knew I needed the focal point to be Kerr's face so, as I moved into position, I thumbed the pad so that an AF point would be where I needed it. Once I got the distance I needed, I thumbed the AF-ON button and shifted the lens down a bit so that the framing was right, then tripped the shutter a few times.

Had I been using the half shutter press AF acquire mode, I'd have been re-tripping the AF system each time and possibly snaring a series of out-of-focus frames instead.

So: whilst I agree that there are obvious uses for each focusing method, using back button focusing can be made to work for almost all situations.

[VinnyP]

My way I can hold the half press and put the AF point where I want it to be and if the subject moves across the frame track them with the focus point or move the camera. If it's something very fast moving I want to be able to keep the focus point on the face or eyes as I track and reframe. I find I can do that quicker if I am not moving my thumb back and forwards but it is taking a while to learn.

[john crossley]

For moving subjects it is simplicity its self
Select your focus point
Set the camera to AI servo
Keep your thumb on the star button

[VinnyP]

Originally Posted by john crossley

For moving subjects it is simplicity its self
Select your focus point
Set the camera to AI servo
Keep your thumb on the star button

What if you want to keep the background framed and keep the subject in focus as they move? The only time there is an advantge to to using af/on is if the subject is outside of the area of coverage of the af points but then you can just flip the camera or lens to manual once you have focus.

Incidentally I am happier about this now as Andy Rouse today said he uses the same technique to follow focus with moving objects. (When he wasn't slating Canon)