This is the fast way to use back button AF to switch between one optimised AF setup and the next, so you are ready for different kinds of subjects and motion.
Back button AF
I love and use back button AF on my cameras, but sometimes you want to do more than just choose when to focus track or not. Several advanced EOS models offer a deeper configuration of the AF-ON and AE-Lock / * button.
Access the deeper functions, through the usual custom controls screen. This is from the EOS 5D Mark IV, but similar on the other models.
Select either the AF-ON button, or the * button to customise. You’ll see that when you select metering and AF start the INFO. Detail set button appears. Just press the INFO button on your camera to get to the deeper functionality.
With this configuration, your AF-ON or * button not only activates focus, but can switch any or all of four AF functions too.
The four selectable functions are;
- AF start position
- AI Servo AF characteristics
- AF operation
- AF area selection mode
AF start position
AF start position can be chosen to retain the current AF point location, or switch to a previously registered AF position. This is useful if you need to change the AF point to one of your choice, possibly even the centre point.
To register an AF point, check the manual for your camera, or look at the end of this post. Please remember that if you are using orientation linked AF points, there are different registered AF point locations available for each orientation of the camera.
AI Servo AF characteristics
This option lets you choose to retain the currently set AF case, or switch to one of the other AF cases in your camera. Perhaps you want to switch between birds flying across the sky, where you are likely to be using AF Case 3, to AF Case 2 if the bird starts flying amongst some trees.
This option will retain the current AF mode, or switch to either ONE SHOT or AI SERVO AF. Potentially you may want to use this to switch to ONE SHOT, but I don’t think this is needed with back button AF, since you can do this by simply lifting off the back button. Perhaps more helpful is to ensure that AI Servo is selected whenever the back button is pressed.
AF area selection mode
This setting retains the currently selected AF area, or switch to one that you choose. Maybe you are tracking a moving subject with Zone AF, and now the subject is static you want to switch to a single AF point. Remember that you can’t use a registered AF point and one of the Zone AF area selections.
Are you mentally adept enough to use this?
I know of some photographers who have the mental and motor skills to use the AF-ON button for their regular back button AF, but switch their thumb to use the * button when the subject changes. One in particular tracks birds in flight using the AF-ON button, and then uses the * button to switch the AF to a single point when the bird lands in a tree.
You might also look at this capability as a way to simply ensure that your desired settings are selected automatically whenever you use the back button. Perhaps that’s AI Servo, AF Case 3 and Zone AF.
It might be time to think about a brain memory expansion… custom shooting modes C1/C2/C3 each have their own settings for custom controls. So 2 buttons, four exposure modes would give 8 different combinations of settings. Even my brain would struggle with that.
Which cameras have this capability?
The cameras that support this extended level of configuration for back button AF are;
- EOS 7D Mark II
- EOS 5Ds / EOS 5Ds R
- EOS 5D Mark IV
- EOS-1D X Mark II
Selecting and reseting a registered AF point
EOS 5D / 7D models : Determine the registered AF point, by selecting the location of the point you want to use in the normal way. Then press and hold the AF selection button, next to the * button, and press the button that illuminates the top LCD on the camera. In the viewfinder you’ll see that the text in the bottom changes from SEL AF to SEL HP. Once it’s selected you will also see that the registered point blinks so you know where it is. You can clear a registered AF point by pressing the AF selection button and tapping the ISO button.
EOS-1D X Mark II : Determine the registered AF point, by selecting the location of the point you want to use in the normal way. Then press and hold the AF selection button, next to the * button, and press the ISO button to register an AF point. In the viewfinder you’ll see that the text in the bottom changes from SEL AF to SEL HP. Once it’s selected you will also see that the registered point blinks so you know where it is. You can clear a registered AF point by pressing the AF selection button and tapping the exposure compensation button.
Also worth a read – REGISTERED AF POINTS
Back button AF refresher from Canon USA