What is Back Button Focus (BBF)?
When you press the shutter button half way down on a DSLR camera the metering activates along with the cameras autofocus system. When you press the shutter button a further halfway the picture is taken. Back button focus will take the focus function away from the shutter button and assigns it to another button on the back of the camera. The new back button (AF) will then activate the autofocus system independently of the shutter button which activates metering and fires the shutter to take the picture.
With back button focus you can set focus and it will stay set until you decide to change it. The benefit of this for Wedding, Portrait, Lifestyle, Fashion, Newborn, Event, Press & Media, Action & Sport, Wildlife, Bird, Animal & Pet, and Equine photographers is that you can fire the shutter as many times as you want with the subject in focus as long as the subject hasn’t moved. You won’t miss that unrepeatable split second ‘moment’ due to the camera re-focusing.
AI Servo / AF- Continuous
The three main types of autofocus on a Canon camera are One shot, AI Focus and AI Servo and AF-Single (AF-S) and AF-Continuous (AF-C) on a Nikon.
One shot is when you press the shutter button half way or press your assigned back button focus button, your focus will be set once on the subject you are aiming at. If you have a moving subject or you move you will have to refocus. So this method of focusing is inadequate for a moving subject e.g. Sports, Children, Wildlife, Wedding couple coming down the aisle.
Another way of focusing called AI Servo (Canon) or AF-Continuous (Nikon) can be used instead. This focusing method actually tracks focusing on moving subjects. With a fast moving subject coming straight at you, using one or more focus points kept on the subject in AI Servo/ AF-Continuous mode and a fast shutter speed, a high percentage of shots will be sharply in focus.
AI Servo / AF-Continuous when combined with back button focus can significantly increase the amount of usable images in focus and also help to reduce post production time spent trying to salvaging out of focus images.