From the very first time you picked up a camera, you were probably taught that pressing the shutter button halfway will cause the camera to auto-focus (AF), and that pressing down the button all the way will close the shutter to capture the shot. But what if there was a different method of focusing and capturing images?
Enter the back button AF!
Back button focusing removes the shutter button's ability to focus, freeing it up to act solely as a shutter release. Instead, a button on the back of the camera (hence the name) controls the focus. The shutter button will still wake the camera up if pressed half way, but won't activate the auto focus. So, in essence, there is one button dedicated to focusing, and one button dedicated to releasing the shutter.
When To Use Back Button Focus
Back button focusing may sound useless at first – why would you want to use two separate buttons to perform tasks that can be easily completed with just one finger? However, once you begin to use the back button technique, the benefits of separating the auto-focus and shutter become clear.
One of the major benefits of back button focusing is the ability to switch between a locked off focus point and continuous Auto Focus without having to touch your lens. Where you would normally have to flick the AF/M switch on your lens to jump between the two, back button focus makes this task much faster. By pressing the back button once, you can use the camera's AF to find focus, and if you don't press it again, the focus will stay fixed. To emulate a continuous auto focus, just keep pressing the back button and your camera's auto focus will track the subject.
There are some specific scenarios which are perfectly suited to using back button focus:
Shooting a sporting event, or a fast moving subject. Using back back button focus in the AF mode will allow you to track your subject, and will reduce the lag time (the second between when focus is locked and the shot is captured). Because both buttons have a separate function, you'll be less likely to miss the moment due to the camera lagging.
Photographing a still subject. If your subject isn't moving (a portrait, or still life for example), then using back button focus allows you to set your focus and continually re-frame and re-compose without having to re-focus. As long as your subject stays the same distance from your camera, this will save you time, as you will not have to focus, half-way hold the shutter, recompose, and take the shot for every single frame.
Shooting a scene in which the chance of something momentarily entering the frame could throw off your auto-focus. For example, if photographing a landmark, people, cars or animals are likely to cross the camera's line of sight, which could trick the autofocus. Using back button focus enables you to keep focusing on the subject you have chosen.
Capturing a scene where the auto-focus may get confused. Particularly scenes with busy foregrounds, or situations where you need to shoot through something (such as a fence, wire, etc). Using back button focus to lock the focus on to the background stops your camera's focus going back and forth every single time you want to take a shot, even if you recompose.
How To Enable Back Button Focus
Depending on what kind of camera you have, enabling the back button focus can be straightforward, or a little more complex. Most advanced camera models will already have the back button focus enabled by default, and many cameras will have a dedicated AF back button on the body, near the viewfinder. However, some cameras will need to be manually set up by assigning the function to a specified button. Some entry level cameras may not support back button focus, but check your camera manual if you aren't sure.
If you have a Nikon, you will need to turn on the continuous AF mode in your menu. It's often abbreviated to AF-C. If you don't do this, back button focusing won't work. If you own a Canon, you can use the AI Servo shooting mode (which is the equivalent), but this is not necessary for back button focusing to work.
Learning To Love Back Button Focusing
Using back button focusing will seem weird at first, especially if you are used to using the half-press method. However, with some patience, you can re-train your muscle memory and will be using the back button focus like a pro!
When you are first using back button focusing, make sure to test it out in a variety of shooting situations to really discover the advantages it brings, and get used to it, before using it in a situation where you need to shoot quickly.