When we use auto focus, by default our cameras require us to half depress the shutter button in order to lock focus.
Back button focussing is when we use a button on the back of the camera (hence the name!) instead of the shutter button to lock focus, and is activated by changing the settings within your camera (and it varies from camera to camera so you’ll need to check your user guide or google how to bbf for your specific camera model).
So Why Would We Use It?
You know how sometimes where you want to focus is not covered by a focus point?
You might just sigh in exasperation and recompose, or maybe you use focus recompose, which is where you half depress the shutter, move the camera to recompose whilst keeping the shutter button half depressed so as not to lose focus, then shoot.
Back button focussing is a method of focus recompose however it doesn’t require you to keep that button half depressed whilst you recompose.
So you can lock focus where you need it, then let release the button, recompose and shoot. Recompose again and shoot again, and again and again…all without ever having to re lock your focus.
Which is super handy if you don’t have time to toggle through all of your focus points (this is particularly an issue if you have one of the newer models with elventy nine more focus points).
What’s The Catch?
It sounds so freeing doesn’t it?
That’s because it is, and there’s no catch. But you do need to be aware that any method of focus recompose comes with the risk of losing your area of focus if you change your focal plane.
That is, if the distance between your area of focus and your lens changes, whether with your lens or your subject or both, the focal plane changes and your focus area moves with it.
The more shallow your depth of field, the more likely this is to happen. So you just need to be careful when recomposing, that you are only moving left to right.