Why does it work? When it comes to pre-focus, I find that I'm often setting my focus on a certain point that is about the same distance the athlete will be away from me. The problem is that occasionally no focus points cover that location so I have to recompose the image after setting my focus. With the back AF button, I can simply lock in my focus and recompose the image. No more switching to manual focus or holding down the shutter half way to lock in the point. Instead, press the AF button on the location you want focus, release the button, recompose, and shoot away.
With this setting you have the ability to CHOOSE when you want to focus. The camera won't try to re-focus every single time you hit the shutter button. This makes things much simpler so that you will no longer have to switch to manual focus or press AF-Lock to maintain focus on the point. It even works better than the AF-lock button since you no longer have to hold the button to ensure it maintains focus.
Another benefit is the ability to override your AF focus with manual. Say your continuous AF is completely off all the sudden, you can simply twist the barrel and fine tune it manually. This way, when you go to press the shutter, the AF feature won't override your adjustments.
It will take me a little time to really get used to it. But After getting adjusted to it, Daniel had become so used to it that he forgot for a second how to focus the test cameras at the New York photo show with the shutter button. I would not recommend changing this before a major shoot. Change the settings when you have time to go shoot with some friends or just around the neighborhood for fun.
Once you've switched you probably won't go back. Below are instructions to change the settings on a Nikon, and further down is an explanation for Canon users.
Separate Your Focus & Shutter (Nikon Users):
Instructions provided by Simply Knot Photography
Step 1: Go into your custom settings menu
Step 2: Select A (autofocus)
Step 3: Select A5 (AF Activation)
Step 4: Choose AF-ON only
Instructions for Canon Users:
table provided by www.usa.canon.com
|EOS Model||Custom Function for Back-Button AF|
|EOS Rebel XS:||C.Fn 9|
|EOS Rebel XSi:||C.Fn 10|
|EOS Rebel XT:||C.Fn 04|
|EOS Rebel XTi:||C.Fn 04|
|EOS 20D:||C.Fn 04|
|EOS 30D:||C.Fn 04|
|EOS 40D:||C.Fn IV-1|
|EOS 50D:||C.Fn IV-1|
|EOS 5D:||C.Fn 04|
|EOS 5D Mark II:||C.Fn IV-1|
|EOS-1D Mark II (N):||C.Fn 04|
|EOS-1Ds Mark II:||C.Fn 04|
|EOS-1D Mark III:||C.Fn IV-1|
|EOS-1Ds Mark III:||C.Fn IV-1|