What the heck is back button focus & why do I need to use it?

When most people make a photo with their camera, they aim, press the camera's shutter release button, wait for the camera to focus, then press the button the rest of the way. Whammo, the image is made.

While that's easy and direct, if you're using a DSLR (Digital Single Lens Reflex) or MILC (Mirrorless Interchangeable Lens Camera) camera, it may not be the best way to focus and shoot. Until 1989, it was the only way to activate autofocus and make a photo with an SLR (Single Lens Reflex) camera, the film based forbearer of today's DSLR and MILC cameras.

It wasn't until 1985 that autofocus was even available. That year, Minolta, having purchased Leica autofocus technology, released the first commercially successful autofocus SLR, the Maxxum 7000. Minolta configured the Maxxum autofocus system to work as most photographers know it today. Autofocus was activated by pressing the shutter release button.

Four years later, in 1989, Canon introduced back button focus. It removes the autofocus function from the shutter release button and assigns it to a button on the back of the camera. Hence the name, back button focus.

With back button focus, when you press the shutter release button, you only activate the shutter mechanism, while autofocus is activated, as desired, only via the button on the back of the camera. It's typically the AF-On button.

Today, virtually all DSLR and MILC cameras have back button focus capability, but most camera users don't utilize it, generally because they know nothing about it and its advantages.

I'm going to use five examples to explain why I think photographers should eschew the default autofocus activation setting and replace it with back button focus, where autofocus is decoupled from the shutter release button and activated by pressing the AF-On button or another button programmed to do so.

1. Resetting focus points is tedious and time consuming:
My DSLR has 153 focus points. Even when I used a DSLR with just eleven focus points for portrait sessions, for example, I found that taking time to move my focus point between shots, to be on my subject's eye when recomposing my images, took too much time, annoying my subject. By using back button focus with the center focus point to focus on an eye, recomposing is lightning fast. You point to the eye, focus, compose and shoot.

2. Ensuring you're using a cross-type focus point improves accuracy:
Cross-type focus points are the most accurate focus points. Back button focus allows photographers to preset their focus system to a cross-type focus point, typically the center focus point and lock it. (The center focus point is a cross-type focus point in all DSLR and MILC cameras.) That way you're assured you'll get the most accurate focus possible for the scene you're shooting.

3. Back button focus can eliminate refocusing:
When the shutter release button activates autofocus, every time you make a photo, then lift your finger from the button, your camera will automatically have to refocus the next time you make a photo, even if the basic composition of the image is unchanged. For example, when photographing an event such as a wedding we make group shots. Group shots require us to choose where in the scene we focus. While shooting a group, we often hold our camera away from our eye to give people a moment or two between shots or move people around for a more pleasing image. With back button focus once you achieve focus, it doesn't change by pressing the shutter release button. Unless you move or your subject as a whole moves, you're chosen focus remains.

4. Back button focus eliminates the need to swap between manual focus and autofocus:
In wildlife photography, when shooting perching birds, for example, there are may be many foreground objects very close to the birds, including branches, twigs and leaves. They often make accurate autofocus impossible. When using back button focus, you can activate autofocus to quickly get close to an accurate focus, release the button, then manually fine tune focus in a second or so. When you press the shutter release, focus will remain unchanged. Had you used the default focus method, in which the shutter release button activates autofocus, the focus would have changed unless you switched to manual focus. Taking the time to switch between manual and autofocus is tedious and could easily cause you to miss the shot.

5. Back button focus permits photographers to use continuous focus all the time:
Especially in both wildlife and sports photography, photographers often go back and forth between shooting subjects stationary or on the move.

To maintain focus on moving subjects, photographers utilize continuous focus, sometimes called AI-Servo. When using continuous focus, while autofocus is activated, the camera keeps adjusting focus continuously as the subject moves.

If you're shooting a stationary subject while in continuous focus, with shutter release button autofocus activation, any time your finger is pressing down the button the focus changes which makes it impossible to lock focus and recompose your shot. On the other hand, while in continuous focus with back button autofocus activation, before you recompose, you only have to stop pressing on the back button and the focus will lock. Your subject will remain in focus as you recompose.

Therefore, the combination of continuous focus and back button focus works well for shooting either stationary or moving subjects. With it, you're able to switch between subjects whenever necessary, without having to change focus settings giving you a better chance to not miss important photo opportunities.

Conclusion:

While it takes time to get used to back button focusing, if you've never used it before, I've yet to find any situations but one, where it isn't a more flexible, faster, better way to use autofocus on any camera that has back button focus ability.

What's the one situation? If you hand your camera to someone else to get a photo of you while you're traveling, it's likely they won't know how to focus your camera. You'll probably have to prefocus your camera for them.