Canon DSLR cameras like the EOS 50D 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 50D and 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.
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).