For years, Canon EOS cameras have offered photographers an option to change the way autofocus is activated. Often referred to by pros as “back-button AF”, this feature lets the user customize the camera so that focusing is performed by pressing a rear button with the photographer’s right thumb. The shutter button still wakes up the camera with a half-press, and fires the shutter with a full press downward.

By separating AF activation from shutter release, it’s possible in some cases to be more effective with AF, and not have the focus thrown off if something momentarily enters the picture area while you’re shooting.

Canon was actually the world’s first camera maker to incorporate such a feature, launching it back in 1989 with the EOS 630 (35mm film SLR). All current EOS digital SLR models have this feature in the camera’s Custom Functions, including the EOS Rebel models, going back to the Rebel XT and XTi.

Why would anyone want to remove AF from the shutter button?
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, 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 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 like, 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 timing of shots 
Similar to point number one above, but yet another benefit of pulling focus away from the shutter button is that critical timing becomes simpler to manage. For example, if you were shooting a speaker at a podium, he or she might periodically look up or make a gesture that would be an ideal instant to capture. If you’ve focused with back-button AF, your index finger is free to shoot at the decisive moment. There are no worries about holding your finger half-way down and waiting, waiting, waiting in that position for your subject to do something interesting.

Even with a very animated subject that may be moving around, you can have your camera’s focus set to AI Servo AF (to track any movement), and just keep your right thumb on the back button to keep focus active, while your index finger can be ready to shoot with no worries about also preserving focus.

Less risk of focus errors with moving subjects 
For sports photographers and others taking action pictures, back-button AF lets you stop focus whenever something might interfere with the 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 over-riding of AF with full-time manual focus
More than half of Canon’s lenses have a neat feature called full-time manual focus*. Even if the lens’s AF/MF switch is in the AF position, these lenses allow the shooter to instantly adjust focus manually by simply turning the focus ring on the lens. There’s no need to first move the switch to MF.

With back-button AF, this becomes a nearly foolproof feature. Use the autofocus whenever you like by pressing the rear button with your right thumb. Shoot whenever you like by pressing the shutter button. And if you want to touch-up focus, or totally over-ride what the AF is doing, just pull your thumb off the rear button and turn the ring. No matter how many pictures you shoot, pressing the shutter button will not cause the AF to try to kick-in and re-set the focus you just adjusted manually.

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 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 to get within general range (press the rear button with your thumb, 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, 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.

Which button is used for back-button AF?
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. Either is relatively easy to reach with your right thumb on the back of the camera as you shoot.

Those cameras with the separate AF-ON button also have a Custom Function that lets you flip-flop the roles of the AF-ON button and the adjacent AE Lock button (with asterisk icon). This is called “AF-ON / AE lock button switch” in the Custom Function menu of most recent EOS models. If you find the AE Lock button easier to reach, you may want to engage this function as well.

Activating back-button AF
Back-button AF is engaged by setting the appropriate Custom Function in your EOS camera. Remember, to use any Custom Function, your camera must first be in one of the “creative zone” exposure modes – P (Program auto exposure), Av (aperture-priority mode), Tv (shutter-priority mode), or M (manual exposure mode). Custom Functions are totally locked-out if you’re in the full-auto “green zone”, or a picture-icon setting like the Portrait mode or Landscape mode.

The particular Custom Function number varies, depending on the EOS model in question. All digital EOS SLRs, with the exception of the original, 6-million pixel EOS Digital Rebel model, have a Custom Function for moving AF from the shutter button to a back-button. Be sure to check your camera manual for confirmation on the Custom Function number for Back-Button AF in your EOS model. Here are examples of the C.Fn menu selection for recent EOS models:

EOS Rebel T3: C.Fn 7 (option 1 or 3)
EOS Rebel T3i: C.Fn 9 (option 1 or 3)
EOS Rebel T4i: C.Fn 6 (option 1 or 3)
EOS 60D: C.Fn IV-1 (option 1, 2, 3, or 4)
EOS 7D: C.Fn IV-1 (Custom Controls — Shutter, AF-ON, AEL buttons)
EOS 6D: C.Fn III-5 (Custom Controls -- Shutter, AF-ON, AEL buttons)
EOS 5D Mark II: C.Fn IV-1 (option 2 or 3)
EOS 5D Mark III: C.Fn menu screen 2 (Custom Controls -- Shutter, AF-ON, AEL buttons)
EOS-1D X: C.Fn menu screen 5 (Custom Controls -- Shutter, AF-ON, AEL buttons)
 
Understanding the on-screen wording:
The terminology often used on the menu for this particular custom function may seem a little confusing, so an explanation is in order. In cameras without a separate C.Fn called "Custom Controls", the function is headed “Shutter/AE lock button”, or similar wording.

What this means is that anything before the slash mark refers to how the shutter button will behave. Anything after the slash tells you how the rear button will work if that option is selected. Using the popular EOS 50D as an example, here’s what you see on-screen, and here’s what it means:

0: Metering + AF start (note: there’s no slash here) 
Factory-default setting. You activate camera’s meter and AF by pressing shutter button half-way down. Rear AF-ON button also does same if it’s pressed, so you don’t get the benefits of removing AF activation from the shutter button when this option is set.

1: Metering + AF start / AF stop 
AF is still at shutter button. Pressing the rear button will actually LOCK the focus; potentially useful if you shoot a lot of moving subjects in AI Servo AF and prefer to activate AF with a conventional half-press of shutter button. Focus is unlocked by removing thumb from back-button.

2: Metering start / Meter + AF start 
Back-button AF activation. Shutter button no longer activates AF, but of course fires the shutter. Metering is continuously updated — if you shoot a sequence of pictures, the camera takes a fresh meter reading for each one. There’s no locking of exposure, unless you separately press the AE Lock button (this last item is not possible on some EOS models).
 

3: AE Lock / Metering + AF start 
Back-button AF activation. Difference between this setting and option 2 directly above is that when you press the shutter button half-way, your exposure is locked and won’t change until you pull your finger off the button entirely. Thus, if you shoot a sequence of pictures in any auto exposure mode, the exposure setting used for the first shot is used for each subsequent shot. Can be useful if you were using back-button AF to easily lock focus and shoot a series of portraits, where you wouldn’t expect lighting to change.

4: Metering + AF start / Disable 
Similar to setting “0” above, but now, the camera’s rear AF-ON button is disabled. AF activation is at the shutter button. Convenient if you’re worried about accidentally pressing the back-button and don’t want to use back-button AF.

Summary:
It can take a little practice to get the hang of back-button AF, but we suggest giving it a try if you haven’t done so already. Even if at first its operation seems unorthodox, in fact it can simplify certain types of shooting and allow you to work more quickly with fewer missed shots. Back-button AF was first suggested to Canon back in the late 1980s by sports photographers who saw the need for some way to be able to start and stop AF without interfering with shooting continuous pictures. The feature is now available on all current and many previous EOS models. It’s no longer just for pros — any photographer can experiment with it and benefit from it in certain conditions.

Finally, remember that like any Custom Function, you can always return the camera back to factory-default operation by returning that Custom Function to option “zero”, or returning the Custom Controls C.Fn for the shutter button, AF-ON, and/or AEL button to the first of each button's available options.