Back Button Focusing: How (and Why) to Separate the Focus and Shutter Release
You know the old saying, “You learn something new every day…”? Well, if you keep reading about photography, then you will. After a few years of shooting sports at my high school, I learned about something new just the other day: back button focusing.
I was browsing the forums at Digital Photography School, and I came across a suggestion to use this for shooting sports. I did a little research, figured out how to turn it on on my Canon t2i, and I’ve been experimenting with it for the past week or so.
What Is Back Button Focusing?
Let’s start by explaining what this feature is. Normally, when you press the shutter release button on your camera, two things happen. The auto-focus kicks in, and when the camera thinks it’s found a good focus it takes a picture.
You can kind of separate these actions. If you hold the shutter release button down halfway, you can “pre-focus” the camera and lock in the focus. Then, you can fully press the button when you’re ready to take the picture.
Back button focusing is a feature that separates the two actions. Once you’ve turned it on, the shutter release button won’t prompt the camera to focus; it will only take a picture. Instead, the button on the back of the camera labeled with a star (see the picture above, and the red box) will prompt the auto-focus motor to engage when it is depressed.
Now, you can press the button and focus. You can recompose as you like. And you can take a picture without waiting for the camera to focus.
How Do I Turn Back Button Focusing On?
On a Canon camera, you need to go into the “Custom Functions” part of the menu. I’ve only done this on a Canon t2i, so it may vary somewhat between camera models and between firmwares.
In the menu, there is a “Custom Function IV.” As I understand it, this contains options for several different custom functions related to the way your camera works. In the Canon t2i’s menu, option number nine allows you to toggle the back-button focus option. There are a few options to pick (and I haven’t used them enough to know the difference), but I picked the second one.
Now I can focus with the back button and shoot with the shutter release button.
Why Use Back Button Focus?
Well, good question.
The tip I initially read said that it was common practice for sports and action photographers. I can see why. There are times when I think the camera is in focus already, and I try to take a picture. As soon as I press the shutter release button, the camera starts searching for focus. This delay – even of a fraction of a second – can make me miss a shot.
This also makes it much easier to recompose if you’re in AI servo mode. In single shot AF mode, you can lock in the focus and recompose. In servo mode (which is common for sports), then re-composing will prompt the camera to re-focus. This way, I can pre-focus on something (like the basket, or the point in the air where tip off is going to occur), re-compose the picture, and click when I’m ready.
The picture at the beginning of this post is a good example. If I tried to compose it there and focus, the auto-focus may have been confused because nothing was in the center. Instead, I followed the girl with the ball (Angel), and at the last second I moved the camera a bit to catch the defender behind her.
But, this isn’t foolproof. In fact, it’s a bit weird at first. It takes practice. If you’re used to shooting without the back button focus, don’t use it for the first time at some important event. After three games, I’m still getting the hang of it, and every once in a while I catch a picture that’s slightly out of focus. Which is so aggravating! Arg!
So my advice is that if you’re shooting action, turn it on and play with it. I do like it. It just takes some getting used to. So you’ll want to practice as much as you can before you use this in a really important game or event.
Brian is a photographer and a teacher. He runs a photography and design studio with his daughter, Olinda. At his high school, he teaches social studies and advises the yearbook club.