How to use back button focus on your dSLR

Did you know you don't have to half-press the shutter button to focus? Here is an introduction to the brave new world of back button focus.

What is it?

Normally, photographers learn to half-press the shutter button on their dSLR to find focus, and then fully press the button to take the photo.

Back button focus frees up the shutter button to serve just one task: taking the photo.

The photographer then uses one button at the back of the camera to set focus, and the shutter button simply takes the photo.

Why should I use it?

Back button focus gives you the best of both focusing worlds: single and continuous AF. Keep pressing the back button to use continuous AF if the subject is moving back and forth in the frame, otherwise simply press the back button once to lock the focus. Then, you can take the photo using the shutter button and recompose to your hearts content without needing to keep finding focus.

There are many scenarios where back button focusing is superior to the traditional shutter half-press:

• Shooting a portrait with a subject that is only making minor adjustments to their pose between photos. You want to take a rapid succession of shots and don't want the AF system to choose a different focus point each time.
• Fast-moving action, kids or sports photos. Using a back button to focus will help reduce the lag time between half-pressing to focus and fully pressing the shutter button to take the shot.
• Situations where the AF system can get confused with what to focus on. For example, shooting through a busy foreground like a net or a gate, it's easy for your dSLR to want to focus on that rather than what's behind it. With back button focus you can choose to keep the focus locked on the background, even if you recompose the photo.
• When you need tack-sharp images. Half-pressing and holding the shutter button to lock focus shifts the weight balance in your hand. Simply pressing the shutter button down fully to take the photo reduces the effect of hand and camera shake in photos.

Where do I find it on my dSLR?

Depending on your camera model, back button focus either needs to be turned on in the menus and you then assign the function to a dedicated button, or it is already active by default. Unfortunately, some entry-level models may not have the ability to use back button focus. If in doubt, check the manual.

The most important thing you need to know about setting up back button focus: you must turn on the continuous AF mode (AF-C) on your dSLR for it to work on Nikon models. You don't have to shoot on the equivalent mode on Canon models, called AI Servo, but it will make it easier. Here are some brand-specific tips on finding back button focus.

Canon

Higher-end models like the 5D Mark III will have back button focus already assigned to a button by default. Look for the "AF-On" button near the viewfinder. It should fall naturally near your right thumb which makes it easy to find without looking once muscle memory kicks in.

To fully enable back button focus using AF-On only, you will need to disable focusing when pressing the shutter button. In the menus, scroll to find Custom Function 2, then go to the Custom Controls section. Here you can change the shutter button function from Metering and AF start to Metering start only. Now, back button focus is enabled when using the AF-On button.

For Canon dSLRs without an AF-On button, you can usually set the asterisk or AE Lock button to be your focus button. In the Custom Controls section within the menu, find an option called "Shutter/AE lock button". The options from here in will vary from camera to camera, but note that the number before the slash will control the shutter and the number after the slash will control the asterisk or AE lock button. Generally you will want to choose Metering start/Meter+AF Start or similar. Again, consult your manual for a full rundown of each option to choose the right one for your situation.

Remember that to access the custom functions menus, you have to be in one of the PASM modes, not full automatic.

Nikon

In the camera menu, head to the custom setting menu (the option with a pencil icon). Then, find the autofocus section. Look for AF activation and select AF-On. Press OK to enable it. This means that rather than letting the shutter button act as the focus and shot-taking button, the AF-On button will look after focus, and the shutter button will be used only for taking the image.

For Nikon dSLRs without an AF-On button, you will need to reassign another button at the back of the camera to act as your makeshift AF-On. Find the back button focus option under the Custom Settings menu and then the Controls section. Choose Assign AE-L/AF-L button and scroll down to AF-On. Press OK and then the AE-L/AF-L button at the back of the camera now acts like an AF-On button for back button focusing.

For other camera brands, consult your manual. Back button focus is sometimes referred to as autofocus lock.

This feels weird. Help?

Getting out of the habit of half-pressing the shutter button to focus is really hard. Give yourself at least a few days to get to know the back button focus method and try it out in lots of different situations before throwing in the towel.

It will take time and effort to get used to back button focus, but once muscle memory kicks in you will find it hard to go back to the old half-press shutter button method. The good news is that once you teach yourself this new method, you don't have to go back!