Making the case for back-button focus
For serious nature photographers, the ideal focus point usually has little to do with the location of the camera’s focus point (unless you first take the time to position the focus point). So one of the first things I do when I get a new camera is set back-button focus to remove the act of focusing from the act clicking the shutter.
While at least 80 percent of my images are manually focused, when my entire scene is on the same plane and I’m in a hurry (for example, when light or clouds are changing fast, I’ll sometimes autofocus. With focus set to the shutter button, autofocusing means remembering to click the lens’ autofocus switch On before shooting, then remembering to click the switch back to OFF when I’m done. With the camera’s autofocus set to the AF-ON button on the back of my camera (and turned off the shutter-release button), I can leave my lenses’ autofocus switch ON all the time.
This is arrangement is particularly beneficial when I’m photographing moonlight—the landscape I’m photographing is illuminated by moonlight—bright enough for a long exposure to reveal detail, but not bright enough to focus—but the moon is at my back. To focus, I simply pop my camera off the tripod, turn around and point it at the moon, autofocus (back-button), return the camera to the tripod and click (it takes longer to describe these steps than it takes to execute them). Or let’s say that I’m on the rim of the Grand Canyon and I determine that to maximize depth of field (using the hyperfocal point), I need to focus eight feet away. But six feet into my frame there’s nothing but open air (and a 5,000 foot drop); in this case, I just pick a rock or shrub six feet in another direction, pop my camera from the tripod and focus there, return the camera to my tripod, and shoot.
Setting back-button focus for a Canon 1d and 5d camera
Here are the (not too intuitive) steps for setting back-button focus on a Canon camera (assuming you know your way around the Canon menu system):
I’ve also set back-button focus on a 50D and even a Rebel SL1 (100D), so while you may need to refer to the manual, it can indeed be done. Nikon shooters? You’re on your own (though I know it can be done on many/most Nikon bodies too).