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.