Nearly all modern DSLR and mirrorless cameras have numerous buttons. And normally, a few - and often many - of those buttons are programmable. Most of the time, I'm happy with the original setting. But there is one setting that I've changed and it makes a world of difference in how I approach photography. That change is to set back button focus.

What Is Back-Button Focus?
With most DSLR and mirrorless cameras, automatic focus is applied by pressing the shutter button halfway down. This causes the lens to automatically focus on whatever area is selected. The shutter button is then pushed the rest of the way to take the picture.
As the name implies, switching to back button focus (BBF) means moving the focusing option from the shutter button to another button, usually on the back of the camera. Separating focus from triggering the shot results in several advantages.
While back button focus won’t guarantee sharper shots, it can help you avoid focusing errors. It also helps you shoot more efficiently.

It Allows You to Easily Lock Focus
For me, this is the greatest advantage. Using traditional shutter button focus, you must keep the shutter button pressed to the halfway point to maintain focus. So let’s say you’re using your center focus point (the most accurate) to focus on an object. Then you want to reframe the shot to take advantage of the ‘rule of thirds.’
To do this, you have to keep pressure on the shutter button (halfway, but not too far so as to take a photo) in order to maintain focus.
With BBF, you press the focus button. When you release it, the focus plane stays locked in so you don’t have to concentrate on maintaining pressure on a button.

It Helps Prevent Focus Errors
With traditional focus, if something moves in the foreground, autofocus might lock on that object rather than your original focus point. With back button focus, the focus adjustment remains locked until you press the back button again.

It Allows You To Easily Swap Between Continuous And Single Autofocus
With back-button autofocusing, holding the button down turns on the continuous autofocus. And when you don’t need continuous focus, tapping the button once locks the focus in on still subjects. That means sports photographers can shoot the game and the sidelines without taking their eye off the viewfinder to adjust the focus mode.

It Makes It Easy To Switch To Manual Focus
With many lenses, you don’t have to flip a switch to change to manual focus if you’re using back button focus. BBF locks the autofocus when you release the button, but you can then ‘tweak’ the focus with the manual focus ring.
I use this quite often in macro photography.

How Do I Set Back Button Focus?
Naturally, setting up BBF varies from camera to camera. While the camera manual (if you actually got one with your camera) will explain the process, many photographers find it advantageous to look on YouTube for instructional videos on the process.
Additionally, a company called CreativeLive has created a series of short video courses, specific to each camera model. They call them Fast Starts, and whatever camera model you have, there is a good chance that CreativeLive has created a course for it.
For those who would like a general description, CreativeLive’s Justin Katz offered explanations for Nikon and Canon cameras in a recent article. My thanks to Justin Katz and CreativeLive for allowing me to reprint their instructions here:

Setting Up a Nikon
Since the back button focusing method isn’t the traditional way of shooting, there are a few steps to telling your camera that you don’t want to focus on the shutter release. On a Nikon, first you need to set the focus mode to continuous, or AF-C. That allows the back button to focus continually when held down, and just once when pressed just once. (Most Nikon DSLRs have a button on the auto-to-manual focus switch towards the front left — hold this button and use the back control wheel to select AF-C).
Next, you need to tell the camera which button you want to use for focusing. On high-end Nikon DSLRs and the new Z series mirrorless cameras, there is a dedicated AF-ON button. (The photo for this article shows the AF-ON button location on my Nikon Z6 - circled in yellow)
If that is not available, the AE AF Lock can also be used for BBF. To set the control, go to the menu, then Custom Settings then Controls. Set the control for the AF-ON or AE AF Lock to AF-ON.
Then, go back to the custom settings menu and select the autofocus submenu. Under AF C Priority Selection, select ‘Choose Release.’ This lets you take photos even if your autofocus point isn’t on the subject, such as when you lock the focus then recompose.
One last step — still inside the autofocus menu under custom settings, go to AF Activation and select AF-On only.

Setting Up a Canon
Canon was actually the first to introduce the back button option in 1989, and that feature is available on all but the original digital EOS Rebel. To set up BBF on a Canon DSLR, locate the Custom Controls or C.Fn option in the menu (the wording varies depending on what model you are using), then select the shutter button/AF-On option. Under this menu, two options will set up BBF.
The Metering Start/ Meter + AF Start option turns on BBF and will continuously adjust the metering on programmed auto, shutter priority, and aperture priority modes. The AE Lock / Metering + AF Lock will lock the exposure in the first frame, while still using the back button focusing method.

Conclusion
It takes a little practice to get used to the process of using your thumb to focus and forefinger to shoot. But once you develop the muscle memory, you’ll probably never want to go back to traditional ‘half press’ focusing again. 
I know I’m convinced.

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