Using ‘back button focusing’ is a technique which often comes up as a subject during my wildlife photography workshops. So, what is it all about and why is it used?
Many cameras have a dedicated button labelled ‘AF-On’ on the rear of the camera. When holding the camera, you’ll find it somewhere within the reach of your thumb! The technique of back button focusing is using this AF-On button to activate the autofocus instead of the more traditional half-squeeze on the shutter button method.
If your camera does not have a dedicated AF-On button you will probably be able to assign a different button to operate in this way. Even if your camera does have a dedicated AF-On button, the chances are you will also be able to assign something else to do this should you find an alternative to be more ergonomic.
To understand why back button focusing can be useful we first need to understand the differences between single and continuous autofocus modes.
With the default shutter button autofocus and our camera in a single shot autofocus mode, when we half-squeeze on the shutter button our camera acquires autofocus. No matter how much we move, or our subject moves, this focus distance is ‘locked in’ as long as our finger remains half-squeezed on the shutter button. This is the first important bit…. it means we can focus and recompose before taking the photograph.
In continuous focus mode (C, AF-C, AI servo etc. depending on your camera), when we half squeeze on the shutter button our camera acquires autofocus. But, if we move, or our subject moves, the autofocus will continuously adjust to maintain focus. This is the second important bit…. Focus and recomposing is therefore not possible because as we recompose, the autofocus continuously updates focus, thus rendering our subject out of focus.
By disengaging the autofocus from the shutter button and using AF-On or another customised function button instead, we can focus and recompose like we would in a single shot autofocus mode but while we remain in continuous focus mode. This is because the camera will not refocus when we half-squeeze on the shutter button before taking the photograph.
A popular misconception is that back button focus is the holy grail of tracking fast moving subjects. There are no cameras (certainly none I know of anyway!) which have some secret super autofocus settings which are only enabled in back button focus. All autofocus tracking settings can be equally applied to shutter button focus as they can to a back button / AF-On method.
Personally, I always used to use back button focusing. However, cameras have changed a lot since I orginally published this blog post in 2014. On older cameras, autofocus points would usually be grouped tightly around the centre of the frame. Therefore, focus and recompose was be more necessary for certain compositions. Modern cameras may now have the luxury of huge numbers of autofocus points which cover the entire frame and as a result, I rarely need to focus and recompose. Instead, I prefer to move the active autofocus point. Like everything though, it’s personal preference. Cameras are highly customisable and they allow us to work pretty much how we wish.
This blog post was originally written in September 2014 and updated in October 2020.