The Breakdown on Back Button Focus
The Secret to Sharp Focus on Fast-Moving Subjects
By Todd Winner
Back button focus first appeared on Canon SLR film cameras over 20 years ago. Pro photographers wanted a way to separate the Auto focus function that is typically connected to the shutter release. Every Canon DSLR body is capable of back button focus either through a dedicated AF button or another button on the back that can be programmed. Nikon also has dedicated AF buttons on the back of some of their models, and the AF lock can be programmed for focus on most bodies. In the simplest terms, back button focus allows us to auto focus by pushing a button on the back of the camera body with our thumb and taking the picture by depressing the shutter release button with our index finger. So let's take a closer look and see if back button focus is for you.
Setting up your Camera
First, if you're planning to use back button focus underwater, make sure your housing gives you easy access to back focus and the shutter with thumb and forefinger at the same time. Next, we need to disable the focus function (depressing the shutter halfway) from the shutter release. This will vary slightly depending on camera make and model, but the setting should be found in your custom control menu.
After focus is disabled, the camera will meter with a half press of the shutter button and take an image with a full press. If you have a dedicated back focus button, you're ready to start shooting. If not, you will need to program one of the buttons on the back to meter and focus. This is also typically found in the custom control menu.
You can use single or continuous focusing, but one of the real advantages I find is with continuous. As long as I keep my thumb depressed I can continue to track and focus on a subject. If I want to stop focus, I just release my thumb and I can shoot images at any time by depressing the shutter. This setup essentially gives me the benefits of single and continuous autofocus without having to switch back and forth.
When is it Useful?
Let's take a look at a few situations where back button focus could be useful. If you have ever shot something like a small jaw fish popping in and out of a hole you know how frustrating it can be. You wait and wait only to have the auto focus hunt and fail as the fish finally emerges. No matter how hard you push the shutter button the camera won't take a picture because it can't lock focus. You can pre-focus, and either hold focus with a half press or focus lock, but this can get tedious after a few minutes. On the other hand, if I use back button focus, I can pre-focus and as long as my distance from my subject does not change I can fire off a shot every time the fish shows itself.
Taking images with your subject dead center in the frame is usually not the most flattering. Most of us are aware that we can move the auto focus points around in the viewfinder, but this method is usually slower than focusing with the center focus point and then recomposing while holding down the shutter half way. The problem is that once I take the shot using the above method, I have to preform the whole procedure of focusing and recomposing for each addition shot. If I'm using back button focus I only need to focus once. As long as my distance from the subject does not change I can fire away at will.
A Tip for Supermacro
When shooting supermacro, one very useful technique is to pre-focus for the basic composition and magnification and then rock a little back and forth to achieve the sharpest focus. When the important area becomes sharp you take the picture. If using the half press method, you must repeat the whole process after the first photo. If using the auto focus lock, you can continue to shoot as long as you hold in the AF lock button, but this can be strenuous on your hand. Once again, back button focus is the smart choice for this type of shooting. You can concentrate on composition, rock back and fourth to achieve sharp focus and take a shot whenever you like.
If you have shot for a long time using the typical index finger focus and shutter release, then back button focus will take some time to get used to. That being said, I don't know one photographer that has given back button focus a real try and switched back to using a single button focus and release. Once you start using it it, I'm sure you will start to see the advantages for yourself. So why not give it a try? It may change the way you shoot forever.
Back Button Focus In-Action
Check out Todd Winner's Story Behind the Shot Article on Hatching Octopus, shot with back button focus.
About the Author
Todd Winner is the technique editor for the Underwater Photography Guide and an instructor and trip leader for Bluewater Photo Store in Santa Monica, CA. You can see more of his work at www.toddwinner.com.
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