A handy feature on your camera you may not be familiar with is back button focus, often referred to as back button AF. When using this feature, a button on the back of your camera (hence the name, back button!) is used to lock in the focus of your shot, rather than having to continuously press the shutter button halfway, then shoot. Basically, the feature removes Autofocus from the shutter button and transfers it to a button on the back of your camera.
All of the current DSLRs being manufactured by Canon with the EOS system have this feature. In this tutorial, we will be looking at how to activate back button AF on the 5D Mark ii, however, activating back button AF is similar on all Canon EOS cameras, just consult your user manual.
Now, you may be asking yourself why it would be necessary to take an extra step by activating this feature. First, we will look at some examples of when to use back button AF, and then we will look at how to activate this feature on your camera.
Let’s say you are photographing a wedding, and, during the ceremony, the bride and groom are going to be in the same spot for an extended period of time. By selecting the focus only once, and having the ability to continuously press the shutter without the camera refocusing for every shot, you will be able to more easily capture facial expressions and gestures that happen within a fraction of a second. If you are continuously refocusing during the ceremony, you might miss some great moments here and there.
Back button focus is great for portraits, as it gives you the ability to have your subject off-center, without losing the focus on your subject when you recompose your shot. To accomplish this, you would first point the camera at your subject (with your subject in the middle of the frame), press the AF button on the back of the camera, recompose the shot with your subject off-center, and then press the shutter. Since you initially pressed the AF back button, your subject will be off-center, in focus, and the rest of your composition will be out of focus, drawing the viewer’s eye to your subject.
While back button focus may seem like it is best-suited for relatively stationary subjects, it can be very useful for moving subjects, as well. For example, if you are shooting a sporting event and you have a particular subject you want to be in focus, you can lock in the focus on where your subject will be, without worrying about the camera autofocusing on someone else (other players, referees, spectators, etc.)
Activating Back Button AF on a Canon 5D Mark II
Now, to activate this feature on your Canon 5D Mark II, here are the steps. In order to activate this feature, you must be either in M, Av, Tv or P modes. Back button AF will not work in full auto mode.
1) Press the Menu button on the top left back of your camera
2) Scroll over to the second tab from the right (it is an orange camera icon with metering lines underneath)
3) Scroll down and select C.Fn IV: Operation/Others
4) Select either option 2: Metering start/Meter+AF start or option 3: AE lock/Metering + AF start
Here are the differences between options 2 and 3
Option 2: Metering start/Meter+AF start
Option 2 activates the back button AF, removing AF control from the shutter button. The shutter button will still fire the shutter, though. With this option, the metering is updated every time you take a shot, so if you are shooting in Shutter or Aperture priority, the camera will still automatically adjust these values to compensate for any changes in lighting. Option 3: AE lock/Metering + AF start
Just like option 2, this setting shifts the AF from the shutter button to the back button, however, it also locks the metering. So, the shutter/aperture values that you have when you lock the focus will be applied to all subsequent shots. This feature is useful if you are shooting in lighting conditions that will not change, such as a studio. Just like any new technique you learn, back button AF takes a little bit of practice, but once you get used to it, it’s a very useful feature. Give it a try before your next shoot, and you may find that it is one of those cannot-live-without features on your camera!