Back button focus is often used by advanced photographers as it provides a means to have separate control of the autofocus and shutter release.
By default Canon EOS cameras use “front button focus”; the shutter release button activates the autofocus and exposure metering. The shutter button has two distinct switch levels, so-called “half press” and full press.
When using the camera for static subjects photographers need to select ONE-SHOT focus mode, and for moving subjects that need to be tracked then AI SERVO is used. Switching between ONE-SHOT and AI SERVO as action happens in front of your camera will slow you down. Back button focus eliminates the need for switching focus modes.
Where is the back button?
The back button was historically the AE lock button, marked with a *. Since the EOD-1D Mark III some cameras have two buttons; AF-ON and the * button. Both buttons can be configured to provide back button focus depending on your thumb length.
For the EOS 7D, EOS 5D Mark III and EOS 70D I often configure both back buttons to work the same – it helps me when I’m switching between cameras with slightly different button placements.
ONE SHOT & AI SERVO autofocus
First it is necessary to understand the two main kinds of autofocus, one for tracking moving subjects and one for static subjects. If you already know about this then skip ahead.
read more about ONE SHOT & AI SERVO autofocus modes
ONE SHOT – Half press the shutter to activate the AF using ONE SHOT focus and the camera will acquire focus and then lock the AF on that subject while the shutter button is kept “half pressed”.
This allows a photographer to recompose their image but keep the focus locked on the subject, even if the AF point moves off the subject. This process is sometimes referred to as focus lock & recompose.
AI SERVO – Half press the shutter to activate the AF using AI SERVO focus and the camera will acquire focus and keep tracking the subject while the shutter button is kept “half pressed”.
If you try to change the composition of the picture and move the AF point off the main subject the camera will reacquire AF on the new subject under the AF point.
AI SERVO focus also determines if the subject is moving, the speed of movement and the direction to or away from the camera. With this is can predict where the focus needs to be when the actual picture is made.
There is a third AF mode; AI FOCUS. In my experience it tends to work like ONE SHOT focus most of the time and rarely and unpredictably decides to switch to AI SERVO mode. I prefer to be in charge of the focus on my camera.
Back button focus setup
The ability to change which button controls the AF is a Custom Function (C.Fn) and more recent cameras have also offered an additional way to configure it via the Custom Controls settings. These customisations are only available when a camera is used in the P, Tv, Av, M, B or Custom Shooting Modes.
Custom Function back button focus
In the case of the single custom function there are four options available. It’s simple to read this as the text before the / is the function on the shutter button at half press, and the text after is the function on the * button. So options 1 and 3 will move the AF to the back button, and have either AE lock or not on the shutter button.
If the subject is in a consistent light and you shoot in Av, Tv, or P modes then you can select option 1 and for a sequence of pictures each frame will have the same exposure. In a floodlit stadium the light is pretty consistent so this is a reasonable approach.
If you choose option 3 there is no AE lock, so as a subject moves from light to dark while you track it the metering will adapt for each frame. If your subject is a performer on stage and the lighting is changing then this is a better approach to back button exposure.
Custom controls back button focus
The options available for cameras with multiple back buttons and custom controls are extensive. Typically the shutter button can be assigned to AF and AE lock, exposure metering start or exposure metering lock.
The back button(s) have more functions depending on the camera model.
Using back button focus
Back button focus is most commonly combined with AI Servo AF. When the back button is pressed the AF is tracking the subject, but when you want to pause or recompose the scene you simply lift your finger off the back button and recompose the frame prior to taking the shot. Back button AF makes AI Servo AF usable for both moving subjects and stationary ones.
If you shoot and your subject passes behind a closer object as you track then release your thumb off the back button and press it again when the subject is visible again. This tops the AF system going off the intended subject but also requires some practice.
In my experience switching from front button to back button focus takes time to be instinctive
If you lose patience due to missed focus you won’t develop the muscle memory to use it correctly. If you have a battery grip on your camera take care as the buttons are sometimes slightly different in feel or placement.
Also if another person is asked to take a photo with your camera they may take out of focus pictures if they expect focus on the shutter button. Switching back to automatic “green square” or creative auto modes will deactivate back button focus until P, Tv, Av or M is selected again.
I often find the need to think about whether I need AE lock or not on the shutter button. But perhaps I just need to use manual exposure mode even more often.
- Canon USA’s Digital Learning Centre also explains back button focus
- Back button focus for action and sports photography