Back-button focus (AF-On) is a feature that I started using a few years ago, and one that I find is too often overlooked by photographers starting out. Using this correctly could mean be the difference between getting the image you need or getting a blurred image. So what is "back-button focus"? It’s quite simple; with its default settings, the camera focuses when the shutter button is half-pressed. Back-button focus disables the autofocus when pressing the shutter button and assigns the focusing to an independent button. If you're used to the traditional focusing method, this can be quite a change, but it is definitely worth it. Knowing how to use back-button focus can help you to take pictures faster and help you get more action shots in focus!
In the video, Tony from "Tony & Chelsea Northrup" explains back-button focus and gives us a quick demonstration on how it works.