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How (and Why) to Use Back Button Auto-Focus

Most photographers auto-focus the traditional way: press the shutter down halfway, then compose your shot and pray your subject doesn’t move around. Or press down halfway and quickly take the photo in the same second. But Steve Perry of Backcountry Gallery is promoting a different method–one he says “makes auto-focus much easier to manage”:

The Three Auto-Focus Settings

Your camera should have three AF modes. (Note that Perry is working with a Nikon camera; Canons and Sonys use slightly different terminology, but the gists of each are the same.) The three modes are as follows:

  • AF Single Shot. Good for stationary shots, in this mode you press the shutter halfway down and it locks a certain depth of field in focus.
  • AF Continuous. The AF will follow a moving subject after you’ve targeted your subject with the AF icon in your viewfinder.
  • AF Auto. Ideally, this mode switches automatically from single to continuous, but it isn’t always accurate.

An example of why AF Single Shot doesn’t always work: if your subject moves, you’re screwed.

How Back-Button Auto-Focus Works

Perry recommends using back-button AF. Again, the term “back-button AF” only applies to certain Nikons, like the Nikon D800, which offers a literal button on the back of the camera that says “AF-On”. (Any camera can designate a back-button to AF, though; the Nikon D5100 has AE/AF Lock; Canons use their asterisk button.)

You have to activate the system first, though, by setting your camera on AF Continuous, then going into the “custom settings” menu, finding “AF Activation” and selecting “AF-On only”.

Now you’ll focus differently: when you want to focus a shot, hold the AF-On button rather than the shutter (which now only takes a photo), and release the button to lock your subject in focus. You can then freely compose your shot and your subject will stay in focus. If it’s moving, you can simply hold the AF-On button down and follow it, the same way AF Continuous works.

Some Nikon cameras have the AF-on button, making this process much easier.

Advantages of Back Button Focus

Perry lists off three reasons why this AF method is better than the conventional one.

  1. You have more accurate control. Your camera won’t re-focus between shots, the way the shutter necessitates; instead it will keep the same depth of field locked in so you can take multiple shots without worrying about finicky focus.
  2. It’s good for action shots. If your subject is moving, you can follow it by holding the button down to enable AF Continuous mode. Because you’re not switching back and forth between Single Shot and Continuous, you can spend less time dealing with mechanics and more time shooting.
  3. It also works for landscape shots. Because the shutter AF isn’t always accurate, focusing on either the foreground or center, but not necessarily the one you want, and then refocusing every time you release the shutter. This method allows you to lock in what’s in focus, then focus your own attention on composition.

Perry ends the video by warning that most people will find this system counter-intuitive, maybe even mess up a few good shots. He recommends sticking with it for a few weeks to realize its advantages, and testing it out on less important shots to get the hang of it. Anyone who’s converted can tell you: they’ll never go back.

[via PictureCorrect Photography Tips]

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