When you press the shutter button, your camera does multiple things before taking the picture. The camera first tries to focus on your subject, and depending on the camera settings, it may also measure the light to properly expose the photo. After doing all of that, the camera will finally release the shutter. There can be a slight delay between you pressing the shutter button, and releasing the shutter depending on how long the camera takes to find focus.
Back Button Focus (BBF) is when you separate the camera focus from the shutter button, and assign it to a second button on the back of your camera. When the focus and shutter are separated, and you press the shutter release, the camera will immediately shoot a picture. There will not be a delay while the camera searches for focus. When the focus is linked to the shutter button, the camera will attempt to focus before firing the shutter. When every millisecond counts, you can not afford to wait for the camera to think and miss the shot.

If you photograph fast moving subjects like sports or wildlife, than using back button focus is absolutely necessary. By decoupling the focus from the shutter button, you are able to shoot a lot faster and not miss the action. It can make all the difference in catching a photo of that Red Tailed fox as it slips into the bushes, and not getting it at all.

Back Button Focus and the camera focusing modes.

When you are shooting a motionless subject, it is better to have your camera set to One Shot or AF-S (Single) focusing mode. This mode ensures a tack sharp focus on your subject and is ideal for non-moving subjects. When focus is found, the camera will beep to confirm focus lock. As long as your finger is still half pressed on the shutter button, the focus will not change. Now you can recompose to put your subject on the side or in a corner creating a more compelling photo. But every time you press the shutter button, the camera will attempt to focus.

But once your subject starts to move, you will want to be in AI Servo or AF-C (Continuous) focusing mode to continuously refocus on your subject and follow it through the frame. The camera does not beep to confirm focus when it is in continuous mode. Instead the camera just keeps searching for the subject as it moves. This mode does not lend it self well for recomposing a shot. If you did try to recompose the shot after focusing on the subject, the camera will just attempt to focus on something else.

When focusing with the half pressed shutter button, you will have to manually change the focusing mode from Single to Continuous mode on the camera when the subject starts to move. This can be slow and cumbersome potentially making you miss the shot.

When using Back Button Focus, you can keep your camera in Continuous mode all of the time no matter if your shooting a mountain, or a fast moving football player. Using Back Button Focus gives you more creative control over your photos. You are limited when using the half-pressed shutter focus. This is because you can independently control the focus separate from the shutter button. This means you can lock the focus on your subject and then recompose to make a more interesting photo. But once the subject starts to move, you press your finger back onto the BBF to track the player as he sprints towards the end zone. You don’t miss one second of the action when using Back Button Focus.

I started using Back Button Focus when i was still a new photographer and I have never looked back. It took some getting used to, there were times when I forgot to press the BBF and ended up with a bunch of out of focus shots. But by now it has become a muscle memory. Back button focus has become second nature, I almost never have to think about pressing the BBF. You just need to spend the time trying it out and getting used to it. BBF can be frustrating if it means you missed a shot. So lots of practice will be needed.

How To Use It

Every camera body, and camera manufacture has a different method to enable Back Button Focus. I shoot on the Canon system, and with Canon, I have to go into the cameras settings —> Custom Functions —> Custom Function Operation/Others option.

In the Operation/Others Menu, select the Custom Controls tab.

The Custom Controls menu allows you to change what happens when you press different buttons on the camera. Select the shutter release button.

First you will need to separate the focusing from the shutter button. The default function is to meter the scene and to start autofocus Metering and AF Start.  Change the selection to either Metering Start or AE Lock. I have mine set to AE Lock.

Next we need to assign the autofocus to a different button. I use the AE Lock button as my back button focus. But you can assign it to several different buttons if you so choose.

Now that you know what Back Button Focus is, and how it can benefit your photography, Go out and give it a try. Back Button Focus will take some time to get used to, but once you have mastered it you will have precision control over your camera’s focus.

Tips

• Practice Practice Practice! Back Button Focus needs to become second nature. Only by doing something over and over and over again can something become second nature.
• Other people will not know how to focus your camera. You can always pop the camera into AUTO mode when letting other people use your camera and focus will be temporarily reconnected with the shutter button.