This is a question many users ask when back-button AF is first explained to them.  There are certainly many times where the standard method of operation — press the shutter button half-way down to focus, and then press fully to shoot — works perfectly well.  Everything is controlled by one finger, and if you like, when set for One-shot AF operation, you can lock the focus with a stationary subject by holding the shutter button half-way down.  Even dedicated supporters of back-button AF will sometimes change back to standard camera operation from time to time.

But back-button AF offers some significant advantages, especially for the experienced photographer.  Here are some frequently-mentioned ones:

Easier to lock focus
If you are shooting something like a series of portraits of a person, and you want them composed off-center, back-button AF makes it super-easy to take as many pictures as you want.  Focus on your subject by pressing the rear button (more on which button later in this article).  Once in-focus, take your thumb off the rear button.  Re-compose the shot to move your subject off-center. Shoot as many pictures as you like.  With focus activation removed from the shutter button, you now can fire any time you want, and remove your index finger from the shutter button after a shot is taken.  No matter what, the camera makes no effort to re-focus when you press the shutter button half-way down again.

Easier to avoid having Servo AF thrown-off by foreground distractions
For sports, wildlife and other photographers taking action pictures, back-button AF lets you halt continuous Servo AF whenever something might interfere with a moving subject you’re tracking — without requiring you to stop shooting. In sports, for instance, it’s common for a referee or another player to come between the camera and an athlete being photographed. With back-button AF, it’s easy to momentarily pull your thumb off the rear button, and you can still keep shooting by pressing the shutter button fully. The camera instantly stops focusing when your thumb comes off the back button. Once the obstruction is out of your way, you can immediately pick-up your primary subject by pressing your thumb on the back button again.

Easier macro and close-up focusing
Many times, you’ll find that it’s actually easier to get consistently sharp close-up pictures of small objects by pre-focusing, and then moving yourself and the camera forward or backward until you see the critical sharp focus appear in your viewfinder. Once again, with back-button AF active, you can use the AF for this pre-focusing to get within general range (press the rear button with your thumb, let the AF focus, then take your thumb off the button), and move a little bit to get things critically sharp. Most important, you can then shoot freely, without AF trying to re-focus each time you touch the shutter button. Finally, as just mentioned, touching-up focus with the full-time manual focus feature on certain Canon lenses is simple and quick, and the autofocus never fights you by trying to un-do what you just adjusted..

Back-button AF operation, or any of the methods to set it up that we’ll describe here, first require that the camera’s Mode Dial or exposure mode be set to what Canon calls a “Creative Zone” shooting mode — P (Program auto exposure), Av (Aperture-priority), Tv (Shutter-priority), Fv (Flexible priority auto exposure, on cameras so-equipped), or M (Manual exposure mode). 

Customization of controls isn’t possible in Basic Zone shooting modes, such as the full-auto Green Zone, Portrait mode, Landscape mode, or “SCN” (Special Scene modes), if your camera has those settings available.  Even if you previously set Back-button AF with the camera in a Creative Zone mode, if it’s then switched to a Basic Zone shooting mode, those customized adjustments are ignored and immediately returned to basic factory default settings.

Historically, once you’ve activated this feature, you press one of two buttons: either the rear AE Lock button (marked with an asterisk or star icon), or if your camera is equipped with it, the rear AF-ON button.  Most users will find either is relatively easy to reach with the right thumb on the back of the camera during shooting.

With the expansion of Button Customization in recent mid-range and higher-end EOS mirrorless cameras, it’s possible to select other buttons to separately activate AF when pressed, if you want.  The EOS R6 Mark II mirrorless camera, for example, allows you to assign Metering and AF Start to any of the following buttons:

• Shutter button (the factory default)
• AF-ON button (also active by factory default)
• AE Lock button
• AF Point Select button
• Depth-of-field Preview button
• Lens Function button (on select lenses, equipped with one or more of these buttons)

As we’ve said previously, traditional back-button AF means we remove AF activation from the shutter button and relocate it at a different button.  Usually, again, that was with the AF-ON button, or the AE Lock button.  With mirrorless Canon EOS R-series models, this is a 2-part process:

1. In the Customize Buttons Menu, select the button you’d like to have perform AF activation, press SET (to display the customized options for that button), scroll to the Metering + AF Start icon (displayed above), and press SET to lock-in that choice.  Note that the AF-ON button is set to activate AF, at factory-default settings, so you don’t need to re-activate that, unless that button has been previously changed.
   
   
2. In the same Customize Buttons Menu, remove AF activation from the shutter button.  Select the shutter button (it’s usually the first button displayed in the menu’s display of available buttons), press SET to display its options, and select Metering start (the metering icon that looks like a little bulls-eye, with NO “AF” included).  Press SET to register this choice.

Some photographers consider back-button AF activation to be a hallmark of being an advanced, experienced user.  We won’t go that far…  camera operations in modern cameras have many options, and the beauty is that users can individually set their cameras up to match not only their general preferences, but even the particular shooting scenario they’re working in.  A photographer taking candids at a wedding reception might prefer to activate focus and shooting via the shutter button;  that same user might prefer to switch to back-button AF activation if he or she was photographing wildlife or sports, with a long telephoto lens.

If you have not yet experienced back-button AF with your camera, we hope this article will inspire you to give it a try, and see if you like it.  Understand that it may take some getting used to, but there are plenty of users over the years who have done just that, and now enjoy that separation of AF starting and stopping from actual shutter activation.

We’ll have a 2^nd part of this article, where we’ll expand the discussion of back-button AF, and speak to an option some users of modern mirrorless EOS R-series cameras work with — dual back-button AF, where one button activates something like AF with a single AF point, and a second button starts AF but with technologies like subject detection, eye detection, and subject tracking active.  The possibilities are tremendous, but it all starts with understanding basic back-button AF operation, as we’ve discussed here.  Thanks for being with us!