Canon’s Back Button Focus Explained

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Canon DSLR cameras like the EOS 50D, 60D, 7D and 5D Mark II have an option to change the way auto focus is activated. This setting lets you customize the camera so that auto focusing is initiated by pressing the rear “AF-ON”  button with your right thumb instead of by half pressing the shutter button. “Back-Button Focus” as its called, offers several advantages such as making it easier to lock focus, making it easier to override auto focus with lenses that provide full-time manual focus and making it possible to switch between focus lock and focus tracking when in AI Servo mode.

Activating “Back-Button Focus” is done on the EOS 5D Mark II by changing C.Fn IV-1 to one of the settings shown below. I’ll attempt to explain what each of these mean since they definitely seem a little confusing at first.

Back-Button Focus

0: Metering + AF start
This is the default setting where you activate the camera’s meter and AF by pressing the shutter button half-way down OR by pressing the rear “AF-ON” button.

1: Metering + AF start / AF stop:
Again, you activate the camera’s meter and AF by pressing the shutter button half-way down but now, pressing the rear “AF-ON” button locks the focus. Focus is unlocked by removing your thumb from the “AF-ON” button. This is very useful when shooting in “AI Servo” mode when you need to switch from “AI Servo” to “One Shot” mode back and forth. (Hint: This is the setting I use most often for birds in flight)

2: Metering start / Meter + AF start:
The shutter button no longer activates auto focus, but still fires the shutter. Auto focus is activated solely by pressing the “AF-ON” button. There’s no locking of exposure, unless you separately press the “AE Lock” button.

3: AE Lock / Metering + AF start:
Auto focus is activated solely by pressing the “AF-ON” button. The difference between this setting and option 2 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. If you shoot a burst of pictures in any auto exposure mode, the exposure setting used for the first shot is used for each subsequent shot.

4: Metering + AF start / Disable
This is the same as the first option except the rear “AF-ON” button is completely disabled. I suppose this protects you from accidentally pressing the “AF-ON” button (Hint: Dumb setting since this rarely happens).

10 thoughts on “Canon’s Back Button Focus Explained”

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  3. Hello. I have a question about AI Servo and back button AF. If a subject is moving, do I keep the back button pressed about 1-2 sec then may I release and the Servo function will still follow the subject? Or do I keep the back button focus pressed the whole time? I ask because the second way limits me in that I have to try to keep the subject mostly in the center of the frame (since I use center point AF most of the time). I’d like to be able to do it the first way, so that I can release the btton and shoot the still in focus (hopefully) subject more to the side of the frame.

  4. Very helpful, thank you! Purchased a macro lens that tries to auto-focus when in MF mode – non-Canon lens which is the issue for sure – but can’t beat the price or optics! Switching to #2.

  6. Thank you so much for this article! I own a Canon 5D and rented the 5D Mark 2 for a wedding. I’ve been wanting to upgrade but wanted to play around with it first. The camera was set in Metering Mode with the back button and I couldn’t figure out where in the menu to switch to the first option. I SO appreciate this article!

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