A quick guide on what back button focus is, why you might want to try it out, and how to set it up on your camera.
What is Back Button Focus?
As standard, your camera is set up to find focus when the shutter release is half-depressed, and once focus is achieved, you then push the shutter all the way down to take the picture.
However you can set up your camera in a different way, so that the shutter button doesn't handle the focus any more, but you assign another button on the back of your camera to do it instead. You still press the shutter to take the picture as before, it's simply that the button you push to set focus has changed, and it's at the back of the camera, and pushed by your thumb.
Why do I want to do this?
It can be easier to get crisp shots of moving subjects this way. When you have a subject who is moving about, you can keep your finger on the back button so that the camera is continually focusing, and simply use the shutter to snap and get the picture. As both actions are happening together, instead of having that small delay between half depressing the shutter and snapping the picture, in theory you are more likely to get a sharp, in focus picture.
For example, in the image below I locked focus using BBF, kept my finger on the back button, and snapped when I saw a movement or expression that I liked.
Secondly it's great for getting IN some of your pictures as it locks your focus. So, you can focus on something on the same plane as you intend to be using BBF, and providing the distance between yourself and the camera doesn't change, it won't attempt to auto focus when the shutter is pressed.
How do I change my camera to BBF?
Let's start with Canon's. Higher end cameras will probably have back button focus assigned to their AF-ON button on the back of the camera, but you will need to disable focusing when pressing the shutter button. To do this, go the Custom Controls, and change the shutter button function from "Metering and AF start" to "Metering Start Only"
If you don't have an AF-ON button, you can set the star button on the back to be your focus button. Again, go to Custom Controls, and find the option called "Shutter/AE Lock Button". The options here do vary, but generally you will want to choose "Metering Start / Meter + AF Start". If in doubt, check your camera manual, or head to this page on the Canon website which gives you a cheat sheet for the different camera models.
For Nikons, the process is similar - head to the Custom Menu Setting, and find the AutoFocus section. Find AF Activiation and select AF-On, then press OK. If you don't have an AF-ON button, then you will need to use another button. Under the Custom Settings find the Controls sections, choose AE-L/AF-L and scroll down to AF-On. Now your AE-L/AF-L button now acts like an AF-On button!
You will find details of you to do this for your particular model in your camera manual, so if you are unsure, have a look there.
Anything else I should know?
Use this with Al Servo (Canon) or AF-C (Nikon) modes on your camera for moving subjects. These modes continually track moving subjects, so this mode combined with BBF could be a winner. With still subjects, you can shift to One Shot (Canon) or (AF-S) and still use BBF to set the focus, and snap with your shutter. (You can read more tips for getting sharp focus on moving subjects here)
Back button focus is not for everyone, and some people don't like it. However, if you do take a lot of action shots (whether that be wildlife, sports or just kids running and jumping about) then it's worth checking out. It does take a few days to get used to, so give it a while before you decide. If you don't like it, just switch back to using your half shutter to lock focus and be happy that you tried it!