Many cameras these days give you the option to reprogram some of the buttons in the custom function menu. Apart from being able to lay out the settings in a way which is most intuitive to you it allows you to remove the focus function from the shutter. In other words, you can change the shutter button to only operate the shutter rather than the standard focus-to-shutter combination of the factory settings. You can then designate the focus button to be somewhere else (usually of the buttons easily reachable with your thumb). So why would you do this?
Easier to lock focus
There are many times when we find that we want our subject off-centre in the image frame. Most people know the standard routine to retain focus on the subject: focus on the subject, press the shutter half way, re-frame the image, and then fully depress the shutter. This is fine for a single shot but less efficient when you want to take a string of similar shots. Each time you fully release the shutter you need to go back to square one again. But with AF activation a separate button you need only follow this process once—after that you can just keep on shooting the re-framed scene until you get the image you want.
Renders the AF/MF lens switch obsolete
If I had a dollar for every time I’ve been shooting with the lens in manual mode (e.g. when using a filter), forgotten to switch it back to AF, changed scenes and then returned home to find most photos slightly out of focus. With back-button focus the AF/MF switch is redundant: if I don’t want the camera to auto-focus, I don’t press the AF button. It’s that simple. With the standard set-up I cannot avoid activating the AF when I press the shutter so was forced to turn the feature off on the lens.
Less AF focus errors
Let’s assume you’re photographing children playing. They’re going to be darting in and out of one another and if your subject is the child at the back then the constantly changing focal point is going to be a distraction. Back button focus lets you momentarily stop the AF system so you can carry on taking pictures until your main subject is back in clear view.
The only real disadvantage to back button focus is that you can forget handing your camera to a stranger to get a quick holiday snapshot (although it does make it easier to compose the scene so all they have to do is press the shutter!).