When you press the shutter release on your camera in its default set-up, it triggers the camera to focus the lens on the subject. Then when it’s pressed home, it trips the shutter to take a shot. When the back button focus is activated, the focusing is triggered by pressing a button on the back of the camera. You then use the shutter release to take the shot as normal.
So in essence, back button focusing splits the usual role of the shutter release across two buttons.
When to use the back button focus
Some photographers swear by back button focusing, while others never use it, but it can come in handy in some situations. For example, if you’re waiting for a moving subject to arrive at the perfect shooting location, it’s helpful to prefocus the lens and for the focus to not adjust as you take the shot.
If you’re using continuous AF to keep a moving subject sharp, you can keep your thumb on the back focus button so the AF is active but there’s no risk that you’ll fire the shutter until you want to. You can watch the subject and use the shutter release to take a shot when the composition is just right.
Back button focusing can also be useful when you’re photographing subjects that momentarily move away from the focus point. A flower swaying in the breeze is a good example. If the flower moves to one side when your camera is in the standard set-up, for instance, the focus will shift to the background as you go to take the shot. If you focus on the flower with the back button, however, the focus won’t move when you press the shutter release so it should still be sharp in your shot.
How to set up Back Button Focus
Back button focusing is a relatively advanced technique so it’s not possible with all cameras, but many do enable it.
Some cameras like the Nikon D850, Sony A7 III and Canon 5D Mark IV, for example, have a dedicated button on the back of the camera that’s marked ‘AF-on’. Other cameras like the Fujifilm X-T2, Olympus ON-D E-M1 Mark II and Panasonic G9 have a button marked AF-L (in some cases it’s AEL AFL). Pressing these buttons activates the AF system to focus the lens on the subject.
However, in the default settings, the shutter release button also still activates the focusing. This can be changed in the camera’s menu.
The exact option to make this change, and its location, varies by camera model. However, it’s usually in the custom menu or the button custom options. Here are a few typical examples:
Canon ESO 5D Mark IV
You can take AF activation away from the 5D Mark IV’s shutter button via the Custom Controls icon in the Quick Menu. Simply press the Q button then tap the Custom Controls icon on the screen. Now select the Shutter button icon in the top left of the on-screen grid. Then ensure that only Metering Start (the central option) is selected.
Quick Menu > Custom Controls > Shutter Button Icon > Metering Start
Head to the Set-up menu with the spanner icon and select Button/Dial Setting. Then tab right and scroll down to Shutter AF and tab right to reach the option to turn it off.
Set-up Menu> Button/Dial Setting > Shutter AF > Off
Navigate to the custom setting menu then find a8 AF activation and set it to AF-on only, it will show as ‘Off’ in the menu.
Custom Setting Menu > a Autofocus > a8 AF Activation > AF-on only
Olympus OM-D E-M1 II
Press the menu button and navigate to the Custom Menu (cog icon). Select AEL/AFL in menu screen A1 AF/MF. Tab right to select the autofocus mode (S-AF or C-AF) that you wish to use for back-button focusing. Then tab right and select mode 3.
Custom Menu > A1 > AEL/AFL > S-AF or C-AF > Mode 3
Press the menu button and locate the Custom Menu (spanner icon), then Focus / Shutter Release and Shutter AF, set it to Off.
Custom Menu > Focus / Shutter Release > Shutter AF > Off
Sony A7 III
Navigate to the 6th page of the first menu tab then scroll down to the bottom option, AF w/ shutter and set it to off.
Shooting Menu 1 > AF2 > AF w/shutter > Off