Join Tim Grey for an in-depth discussion in this video Back button autofocusing for faster framing, part of Photo Gear Weekly.
- When it comes to taking a picture, of course, the shutter release button is your primary button. And this little button does a lot. At least normally it does a lot. By default, of course, is that you take the picture you can activate automatic exposure, metering. You can even activate auto-focus. That's all by default with most cameras. But I find that that's asking too much of that one little button, and I would rather separate out the auto-focus. And so I'm a huge fan of employing back-button auto-focus. I'm able to then separate the exposure and the taking of the picture with actually focusing on the scene.
And that's especially helpful when I want to re-frame. I want to focus on one thing and then re-frame. Let's assume, for example, there's some wonderful reflections on the water here. Perhaps I'm photographing this little railing. I could set my focus, and then re-compose to get more of the water in the frame, for example. Or, with this larger, beautiful scene behind me, if I want to make sure that as much of this scene is in focus as possible, I would want to focus on the land over toward the left. It's getting some wonderful sunlight on it. And then re-compose, re-frame the scene, so I've got the water, and the city in the background.
And so that becomes very easy. I simply point at the area that I want to focus on, activate that auto-focus, and then I can re-compose (camera clicks) and capture my image. Now, that essentially is a one-shot type of of focus. In other words, I'm establishing focus, and then discontinuing that focusing so that the focus is locked. I just then need to make sure when I'm using that back button focus in that way that I'm not changing my distance to the subject. So if I were focusing on this railing, and then I recompose by getting closer to the railing, that distance has changed, and therefore I'll need to re-focus.
To enable me to enable the process of both one shot plus continuous auto-focus, I'm actually setting my camera to the continuous auto-focus option. So as long as I'm holding my back button auto-focus button on the back of the camera, the camera is actively trying to re-focus on whatever subject is within that focusing point. And so I can press and hold and keep that button down if, for example, I'm photographing a moving subject. Perhaps one of the geese on the water flies by, and I decide to get a shot of just that goose, then I can press and hold with continuous auto-focus active as long as I'm holding that back button auto-focus button, I'll be continuously focusing.
But then in a situation like this where I want to focus on one area and recompose, I can press that back button, focus button on the back of the camera, focus on the subject that I want to focus on, release that button so that I'm no longer adjusting focus, and then re-compose. Once again, making sure that I'm maintaining the same distance between my camera, the lens, and the subject that I've focused on. So I find that that back button focus can be tremendously helpful. Separating the taking of the picture, the metering off of that shutter release button, and onto that back button focus. The key thing, though, is to make sure that you practice.
When you first change your camera settings to employ back button focus, disabling focus on the shutter release button, you do need to make sure that you're actually focusing. If you're not in the habit of pressing that back button, then you'll be taking pictures thinking you're focusing, when in fact you're not activating auto-focus. But once it becomes a habit where you're pressing that back button, I think you'll find that it gives you a lot more flexibility, a lot more control, and you'll still be able to employ that automatic focus capability of your camera with greater results.