Essential Photography: Back Button Focus
By default, almost every camera arrives from the factory with two important functions assigned to one big conspicuous button: the shutter release. It’s conveniently located where the right index finger would rest when holding the camera comfortably in your hands. This button will engage autofocus when pressed halfway and fire the camera when fully depressed.
By configuring your camera for back button focus, you separate these two functions by 1) disabling autofocus on the shutter release button – perhaps the most important part of adopting back button focus – and 2) reassigning autofocus to a button on the back of the camera where the thumb comfortably rests. So now the shutter release button ONLY fires the camera when depressed by your index finger and no longer starts autofocus. The autofocus is all in your thumb.
Every camera is different, but they all should allow you to disengage autofocus from the shutter release button and reassign it to another button on the rear of the camera, preferably near your thumb. Consult your camera’s manual to find out now.
At first glance, it might seem like a more complicated way of doing things, but it’s not. After a minor adjustment period, it will soon feel as natural as the default set up, maybe more so. It’s just so easy; thumb focus, finger capture. There are many things we do in everyday life which involves using the thumb and index finger simultaneously. And some of these things are actually good.
I find back button focus most useful for rapidly-changing scenes or quick-moving subjects commonly found in street and wildlife photography. You now have the ability to autofocus and recompose the scene without having the camera refocus again when you take an image. Just focus on your subject, make a slight change to the composition by moving the camera while your subject stays in focus, and then shoot. Pressing the shutter release only fires the camera. It doesn’t activate autofocus again.
The frog photograph above is a superb example of when back button focus will serve you well. Place one of the autofocus points over the frog’s red eye (you always want an animal or person’s eye to be tack sharp, even if other parts of the subject are not) and focus using your thumb on the designated autofocus button on the back of the camera. Then take your thumb off the autofocus button (essentially locking in focus) and quickly recompose. In this case, using the green branch from the bottom left corner to lead the eye into the scene. Take the shot with your index finger on the shutter release. Even though the focus indicator is no longer on the frog’s eye after moving the camera, the focus won’t change because that function has been removed from the shutter release.
As I said, I find this way of focusing most valuable when I’m doing wildlife photography but I use it for any scene where I’m using autofocus. Focusing with my thumb is now a grooved habit that has taken quite a bit of time of set in. The middle of an expensive Africa safari is not a good time to experiment with back button focus if you’ve never used it before. Under stressful situations (a lion stalking your safari vehicle with a setting sun over it’s left shoulder, as an example) you will defer to the familiar without even thinking and this shot of a lifetime will be out-of-focus because you assumed it was focusing with the shutter release button. Practice, practice, practice. Practice so much that it becomes second nature and you don’t even have to think about it.