Back Button Autofocus
One of the least used techniques in photography to lock focus quickly and to switch between single and continuous auto-focusing is to use back button focusing. In very simple terms, back button focusing is the technique where you focus with a button other than the shutter release.
Normally, when you take a picture in auto-focusing (AF) mode, pressing the shutter release first locks focus, then the mirror flips out and finally the shutter curtains open to expose the sensor to light. This way the exposure is made. This whole process cycle has some advantages (I am not going to get into any details here) but on the flip side it also has some serious drawbacks.
If you only use your camera to shoot everyday photos you are probably not pushing the limits of this system, so chances are that you will not notice it. Ask a sports photographer and he will immediately tell you why he hates this system of auto-focusing.
Let’s take a few scenarios – scenario 1
The kids are playing soccer. A player pauses at a spot for a moment. You decide to shoot a few portraits using your tele-lens. You set the camera to single-shot auto-focusing and choose a single AF point and fire away. Suddenly, he makes a dash towards the goal. You frantically try to switch the AF mode to AF-C (continuous auto-focusing) to track him. By the time you manage to set the camera on AF-C the action is over.
You are shooting portraits outdoors and you ask the subject to stand under the shade while you click away. Standard stuff. However, at one point you decide to compose while leaving some negative space. There are two ways to accomplish this using auto-focusing. You could either use the AE-L/AF-L (Auto-exposure Lock / Auto Focus Lock button) or you can recompose by selecting an AF point that is either to the left or right edge of the viewfinder and coinciding the point on the eye that is closest to the camera. Both cumbersome procedures.
The solution is back button focusing. This technique separates the focusing and exposure functions of the shutter button and assigns one of the buttons at the back of the camera to be the focus button instead. The exact process is different in different make and model so you will need to consult the booklet that came with your camera. I shall be jotting down the process as per my D7000. The steps are somewhat similar for all cameras though.
Go to Menu > Custom Setting Menu > Select ‘f’ Controls > Go to ‘5’ Assign AE-L/AF-L button > Select AF-ON > Press OK.
What you just did was to take away the focusing function of the camera from the shutter button and assigned it to the AE-L/AF-L button. Next, set your camera to AF-C mode and select a single AF point. This is because if you set your camera to AF-S (Single Shot auto-focus) your camera will not allow you to take a picture after you have locked focus, let go off the shutter button and recomposed. You will only be able to take a picture if you keep the shutter button or the AE-L/AF-L button pressed while recomposing, which kind of defeats the whole purpose. In AF-C mode the camera will allow you to lock focus and recompose freely. To re-acquire focus simply press the AE-L/AF-L button again.
Second advantage of selecting AF-C mode is that when you press down the AE-L/AF-L button it continuously keeps the subject in focus when it’s moving about. This is ideal for sports photography where you need to not only be able to shoot when the subject is stationary but also when it is moving, without having to fiddle with the buttons on your camera.
The third advantage is, if there is any obstruction in the image frame, normal AF mode will try to correct by shifting focus on the obstruction. In this mode focus will remain on the subject. This is ideal for sports photography where many photographers are vying for the same shot and there is bound to be a situation where one photographer gets into the way of another one.