Back-Button Autofocus Explained
Updated: February 28, 2019| Published: January 8, 2013
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, beginning with the Rebel XT camera of 2005.
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 halfway 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 halfway 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:
1. 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, lift your thumb off the rear button. Re-compose the shot to move your subject off-center. Shoot as many pictures as you like, fully pressing the shutter button as you always do. With focus activation removed from the shutter button, you now can press the shutter button any time you like, and remove your index finger from it after a shot is taken. No matter what, the camera makes no effort to re-focus when you press the shutter button halfway down again.
2. Easier timing of shots
Similar to point number one above, but yet another benefit of removing AF activation 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 fire the shutter at the decisive moment. There are no worries about holding your finger halfway down to keep the focus locked, 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 follow any movement), and just keep pressing the the back button with your right thumb to keep focus active, while your index finger can be ready to shoot with no worries about also preserving focus.
3. Less risk of focus error with moving subjects
For sports photographers and others who shoot 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.
4. 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’ 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 lens’ focus 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.
* Canon EF and EF-S lenses with a distance scale on the lens barrel normally do have full-time Manual focus available. Lenses with electronic manual focusing, such as Canon STM lenses, or the EF 85mm f/1.2L II, will need one or more Custom Functions active in-camera, to allow Manual focus with the lens’ AF/MF switch in AF mode.
5. 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 with back-button AF engaged, 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 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), Fv (Flexible-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 SL2 (EOS 200D): C.Fn 8 (option 0, 1, 2, or 3)
- EOS Rebel T6, T7 (EOS 1300D, 1500D): C.Fn 8 (option 0, 1, 2, or 3)
- EOS Rebel T7i (EOS 800D): C.Fn 12 (option 1 or 3)
- EOS 80D: C.Fn III-4 (Custom Controls — Shutter, AF-ON, AEL buttons)
- EOS 7D Mark II: Custom Controls — Shutter, AF-ON, AEL buttons
- EOS 6D Mark II: C.Fn III-4 (Custom Controls — Shutter, AF-ON, AEL buttons)
- EOS 5D Mark IV: Custom Controls — Shutter, AF-ON, AEL buttons
- EOS-1D X Mark II: Custom Controls — Shutter, AF-ON, AEL buttons
- EOS R: Customize Buttons — Shutter, AF-ON, AEL buttons
UNDERSTANDING THE ON-SCREEN WORDING
Especially for Canon EOS cameras that don’t have a “Custom Controls” area in their orange-colored Custom Functions menu area, 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,” look for a Custom Function 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 — when pressed halfway down. Anything after the slash tells you how the rear AE Lock button will work if that option is selected. Using the popular EOS Rebel T7i as an example, here’s what you see on-screen, and here’s what it means:
0: AF / AE Lock
Factory-default setting. You activate camera’s meter and AF by pressing shutter button halfway down. Rear AE Lock button (with asterisk icon) locks exposure if pressed.
1: AE Lock / AF
Back-button AF, when AE Lock button (with asterisk) is pressed. When shutter button is pressed halfway, there’s no AF. Metering is activated, and metering is locked at the initial exposure detected when shutter button is first pressed halfway (AE Lock moves to shutter button).
2: AF / AF lock, no AE Lock
AF when shutter button is pressed halfway down. Focus Lock activates when rear AE Lock button (asterisk icon) is pressed; there’s no AE Lock — in other words, metering stays continuously active whenever the camera is awake. This option is potentially useful for action shooters using AI Servo AF, who prefer to focus by pressing shutter button halfway. They can lock focus at any time with the rear AEL button.
3: AE / AF, no AE Lock
Back-button AF activation. Difference between this setting and option 1 directly above is that when you press the shutter button halfway, your exposure isn’t locked, and stays continuously active. For most users who want back-button autofocus activation, this would be the preferred setting — in any auto exposure mode (P, Av, etc.), your exposure is continually updated as light changes, and you focus via the back AE Lock (asterisk icon) button.
Keep in mind that other EOS models may display the menu options for shutter button half-press and rear AF-ON or AE Lock buttons a bit differently, but the basic concepts are the same as described in these examples.
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 continuous, AI Servo 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.