Canon AF-On Button

Up until recently I had been a recomposing monkey when taking portraits or family photos. See, when using auto focus (AF), I like to use the center focus point, focus on what I’d like to be sharp, and then recompose the shot. The problem that crops up though, is that once I recompose and press the shutter button halfway down, the AF locks on something else (the new spot where the center focus point is), ruining my previous focus setting. To compensate for the problem I had been pressing the shutter button down halfway and holding it while I recomposed the shot, but that leads to a problem where I have to redo all those steps every time I press the shutter. What a pain in the neck, especially if my camera’s on a tripod. So, as a result, I had been searching for a solution to this problem and ran across the AF-On button, which allows me to separate the AF functionality from exposure and thus allows me to lock the AF so I am free to recompose the shot. I’ve been experimenting with using the button and am very pleased with the freedom it’s offering me.

Here’s what I now do:

  • Zoom to appropriate distance
  • Position center focus point on area of subject I want to be sharp
  • Press the AF-On button with my thumb to lock AF
  • Recompose shot to desired framing
  • Press shutter button for exposure and to trip shutter

As a result of my new process, I can now lock the AF and freely recompose shots without having to move my camera around every time I trip the shutter. I’ve also configured one of my camera’s custom settings as my own “portrait mode” with the appropriate AF-On custom function enabled (and configured another custom setting identical to this one but with AI Servo enabled). Canon has a great description of the settings in their Digital Learning Center. Here is a quote from the site regarding the back-button auto focus options:

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.

I use option #2 but I highly recommend visiting the Canon website for a more detailed description of the functionality.