It’s one of those unshakable, unquestioned truths of life, as certain as the tax man and the fact that the sun is going to make a reappearance when the night is over. To focus your camera, you push the shutter button down halfway.
But here’s the thing: that’s not the only way to focus a DSLR. It might not even be the best way to focus a DSLR.
Wait, what? Did the moon stop orbiting the earth? I promise you, it’s still there. But it is true that that one unshakable tenet of photography is not actually as unshakable as you thought it was.
What is back button focus?
Back button focus is an alternative focusing method that takes the job away from your shutter button, freeing it up to do what shutter buttons are supposed to do, and that’s, you know, releasing the shutter. With back button focusing (also known as BBF), you use a different button to focus and you leave the shutter button in charge of that one important task: actually taking the photo.
OK, so why would I want to do this?
Back button focus makes focusing simpler and more precise. When you remove the job of focusing from your shutter button, you leave it to do the thing that it does best: making the exposure. The reason this matters is because when your shutter has the dual job of both focusing and actually taking the picture, it can be difficult to coordinate both of those things. Let’s look at some examples of what I mean.
Autofocus works a little differently depending on your camera. Canons have a feature called “one shot,” which is a focus lock mode. In this mode, when you push the shutter button down halfway, your focus locks and stays put. Unfortunately the focus you get when you use one shot is only good for a single exposure (or set of exposures, if you’re using burst mode). Once you’ve pressed the shutter button down all the way and then lifted your finger again, you have to refocus in order to take a second exposure.
With back button focus, you don’t need to keep refocusing. You focus one time, and then you’re free to take as many photos of that subject as you want without having to refocus. When you remove the focusing job from your shutter button, there’s no danger that the focus point will change just because you hit that button. This is going to make it a lot easier to time your shots. If your subject does something interesting and you know it’s only going to last for a split second, you don’t want to have to wait for your camera to refocus because once it does, the moment might already be over. If you already have your focus locked through your back button, you can just take the shot, no refocusing necessary. Without BBF, your only other option is to sit there with your finger halfway down on the button, waiting for something to happen.
Other cameras are designed to refocus on whatever happens to be under the assigned focus point. This isn’t a problem as long as whatever is under that focus point isn’t moving, but as soon as it does—or if something moves in front of it—the autofocus system will refocus on the object that has replaced the original one. For example, let’s say your subject is the head on a stalk of wheat. You can place your focus point right on the end of that stalk, but you’re so close that you may not really able to keep the focus point exactly where it needs to be. If your subject moves just enough in the frame to slip out from under that focus point, your autofocus system will go, “Whoops! Better refocus.” Or maybe the culprit is a slight breeze, which is moving that stalk from side to side just enough so that your autofocus system thinks that you’ve changed your mind about what your subject is. The result is autofocus hunting, which can be really, really annoying. You can’t take the shot because your camera won’t stop zipping back and forth between that stalk of wheat and the other stalk of wheat that’s just behind it. You can’t stop this from happening because you can’t take your finger off the halfway point until you make the exposure.
With BBF, you simply tap your focus button once to lock focus on your subject, and then focus remains exactly the same until the next time you tap the AF-ON button. Your camera will stop hunting and you’re free to take the photo—in fact now you can take a series of images of the same subject, as long as it remains on the same focal plane, and you don’t have to worry about all that hunting or about the need to refocus each time you take a shot.
How to use BBF
Back button focus can be accessed via (you guessed it), a button on the back of your camera. Now, using BBF is different depending on which camera you own. More advanced Canons and Nikons offer BBF through a button labeled AF-ON, which is close to the top of your camera, just under the shutter button. If your camera doesn’t have an AF-ON button, you can still use back button focus but you will need to program the AF-L, AE-L button to act as AF-ON. This is done through the camera’s menu—on a Canon, you’re looking for the “Shutter/AE Lock Button” option. The setting you want is “Metering Start/Meter + AF Start,” which is not exactly intuitive. On a Nikon, you need to go into the custom setting menu (that’s the pencil), then to Controls, and then choose “Assign AE-L/AF-L button.” Now, manufacturers love to change the way they do things so if that’s not getting you where you need to be on your camera model, check your camera’s manual.
If you do have an AF-ON button, you still need to do some tweaking before you can use it effectively. Most cameras default to letting you use either the AF-ON button or the shutter button to focus, but that’s not really going to help you. What you want is to remove the focusing power from your shutter button altogether, so AF-ON is the only button on your camera capable of focusing That means you’re going cold turkey, and quite frankly that’s the best way to get used to using BBF. Again, you do this through your camera’s menu, which should give you the option to either share focus between buttons or to use AF-ON alone.
Once you’ve got the back button set up, you just need to press it once to focus on a non-moving subject; press and hold to track a moving subject. It really is that simple, once you get used to it.
In case you’re still not convinced …
There are a lot of situations where you probably find yourself switching over to manual focus. Dark rooms, for example, low contrast situations or when you need to shoot through something such as glass or wire. Now under ordinary conditions (depending on your lens) if you want to focus manually, you can focus manually using your focusing ring, but if you don’t also flip that little switch to “M,” you may find all that manual focusing to be for naught because as soon as you touch that shutter button your camera goes, “Oh! Time to refocus!” In back button focus mode you don’t need to flip that switch anymore, because your shutter button doesn’t control your autofocus anymore. So even though you don’t use that AF-ON button when you’re manually focusing, you’re preventing your shutter button from taking over that job after you’ve so carefully focused manually on your subject.
Rooster (through the cage) by Flickr user menteblu61
Here’s another reason why BBF just makes more sense: that whole “halfway down” thing is, when you think about it, a little bit awkward. To focus your camera, you push the button down halfway and then you have to keep your finger there until you’re ready to make the exposure. That’s kind of dumb, really, because what happens when your finger slips or you have to let go because you’ve just realized that you need to turn your ISO up a click or change your white balance settings? Now you have to start all over again, because your camera will refocus as soon as you put your finger back on that halfway point. Or how about this one—you press the shutter button just a little too hard and you end up taking a photo before you’re ready—and then the moment passes while you’re in the midst of refocusing so you can try again.
Photobomb! by Flickr user adamjackson1984
But wait, I’m not finished. One final reason why you need to switch over to back button focus is because of the unpredictability of everything in, around or near your scene. If your camera is one that will refocus whenever the object under the focus point changes, that halfway-down option puts your photos at risk whenever you’re in a busy place or a situation where that’s likely to happen. This is also a problem on any camera, any time you’re using focus tracking to try to keep a moving subject tack-sharp. Let’s look at one more example: you’re at the park with your kids, and your son is in one of the play structures. You have him in the frame, you’ve pressed that shutter button down halfway to lock focus and you’re just waiting for the right moment—for that perfect smile, or for a wave, or whatever—and then someone else’s kid steps between your son and the camera. Your autofocus system goes, “Oh! Time to refocus.” And suddenly your son is no longer the subject, that stranger’s child is. You’ve lost the shot because your camera decided that it knows better than you do.
With BBF, you focus on your subject one time and as long as he remains on the same focal plane, you don’t have to refocus—even if someone else steps between you and your subject. You can take a series of images without having to refocus for each and every one. This also makes the focus/recompose method infinitely more usable, because now you can focus/recompose one time and take as many shots as you like without having to go back and focus/recompose in between each one of them. On the other hand, if you’re using your shutter button to focus, your camera will attempt to refocus on whatever is under the selected focus point as soon as you go for the second exposure, which is why you have to go back and reframe/refocus all over again if you’re going to take multiple shots.
Sandwich Tern by Flickr user Paul:Ritchie
BBF works in any focusing mode, even focus tracking. The principle is the same as when you’re using the halfway-down shutter button method—just keep your thumb on that AF-ON button and the camera will track your moving subject.
Now, I don’t want you to think that when you switch to BBF light will spill down from the heavens and birds will sing and your life will be infinitely better. BBF is a tool that you have to get used to, and at first it’s going to seem really awkward. At first it’s going to drive you nuts, because your shutter finger is programmed to think that it needs to push that shutter button down halfway in order to focus on something. So you will need to retrain your hands to automatically go to that AF-ON button instead, and that’s something that takes some time. Maybe even a lot of time. You know, like when your spouse moves the silverware drawer and you end up spending the next six months opening up the old drawer and swearing every time you need to get a fork. Muscle memory is hard to undo. So please, don’t get frustrated in the first hour and switch immediately back to the old way. Definitely don’t make this switch when you have something important and once-in-a-lifetime you need to capture, because at first it’s going to be really slow going and you will end up missing some shots. But the really great news is that after a few days of nothing but back button focus, your brain will start to reprogram your fingers, and eventually you’re going to see what you’ve been missing all this time.
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