How to setup and use back button focus

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?

EOS 1200D back button focus – circled in red

EOS-1D X back button focus – circled in red

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.

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 to set back button focus on the EOS 650D

Custom controls to set back button focus on the EOS 70D

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.

Further reading…

• Canon USA’s Digital Learning Centre also explains back button focus
  
• Back button focus for action and sports photography