Back button focusing is taking focusing away from the shutter button and assigns it to one of the buttons on the back of the DSLR. Focusing using the back button of the DSLR is a very useful feature. It can be used to shoot moving subjects as well as still subjects. With the shutter button half pressed in AI Servo mode, it will refocus each time as the camera tracks the moving subject and this results in a lot of wasted out of focus images and one will need to switch from “One Shot” mode to “AI Servo” and vice versa (Canon cameras) when subject stops and starts moving randomly. There is no point in the camera having a high frame rate if it simply cannot lock focus using the shutter button and this reason is enough to use the back button to lock focus.
Focusing using the back button locks focus on the moving subject without having the camera to refocus and the shutter button is used to simply takes the photo. With moving subjects, the back button needs to be pressed continuously. For still subjects, the back button needs to be pressed once to lock focus and take as many photos as one would like without having to refocus using the back button unless the subject moves or change in distance. One could also focus and recompose the shot only if subject does not move and distance between subject and camera is the same. What this means is you could use the back button to focus on the subject with centre point and then slightly move camera to position the subject off-centre either on the far left or far right and take the photo. Even though the focus point is not on subject, they will still be in focus since the back button locks focus as mentioned earlier.
The back button maintains and locks focus on the selected subject whether still or moving even if something or someone else moves in front of the camera. However I have gone a step further and I not only lock focus with the AF-ON button but also able to lock focus using the * button on my Canon 5D Mark III and use the shutter to just take the photo. Once the AF-ON button is set to AF and metering using the custom controls on your camera then by pressing the INFO button on the camera, you select the HP or home point option. Now set the AF selection mode to single point AF and select one of the left most or right most points and set that point as the registered home point. The registered point will blink even when the AF point is moved somewhere else and pressing the AF-ON button will bring it back to that registered point. Now here I set the AF selection mode to something like Zone AF. Similarly I go back into the menu and assign the * button to AF and metering but for this button I do not register a home point but instead set it to manual selection. So here I can use the AF-ON button to focus and meter anything left of the view and the * button to focus and meter on anything on the centre or right of the view.
All the bird in flight photos shown on this page have been shot using back button focusing. Now try out back button focusing on your camera and see how it works for you.