You don’t need to constantly refocus
Say you’re photographing a stationary subject (e.g. a portrait sitter, a landscape, or a plate of food) and want to create a few different compositions. Each time you adjust your framing you’ll need to also adjust your focus – either by moving your focus point, using the focus and recompose method (i.e. half pressing the shutter button to lock focus before reframing the shot), or pressing down the AF-L (autofocus lock) or AE-L (auto exposure lock) button.
While these methods work, they can be tedious and time-consuming.
With stationary subjects, back button focus locks focus with a single press of the focus button. As long as your subject remains within the plane of focus (i.e. the distance between it and your camera stays the same), your subject will stay sharp and you can freely change your composition without needing to press other buttons. (Care should be taken when using shallow depth of field, however.)
Even in this age of sophisticated autofocus systems, however, achieving sharp focus can be a challenge for many.
The good news is, there’s a simple trick that can help make the job of focusing much less error-prone and much more efficient. It’s called back button focus, and it could forever transform the way you shoot.