http://thephotographerwithin.com/2015/05/back-button-focus/
What are the advantages of BBF?
Faster shooting with static subjects: you can lock focus with the AF-ON button once then let go of the button for subjects that don’t move. Great for landscapes, macro, even sleepy newborns. No need to refocus every time you press the shutter.
Easier to focus and recompose: you can lock focus using your center focal point with the AF-ON button then let go of the button and recompose your image very easily without having to re-do your focus each and every time you press the shutter. Useful for Canon cameras with limited focal points or if your initial composition isn’t optimal.
Better focus for moving subjects: you can shoot continuously with the AF-ON button depressed and your focus will track your moving subject.
Override and fine-tune autofocus: you can ensure tack sharp images when using Live View and zooming your focal point in to 100%, then locking in that focus by using the AF-ON button. You can even adjust your focus manually by turning your focus ring on the lens without turning the switch to MF on the front of the camera (if required).
And finally, and this is the HUGEST advantage I’ve found, you never, ever need to change your auto focus mode again. Setting it to AF-C/AI Servo AF and using BBF allows you to access all of the focus modes with this one button. BBF eliminates the need for switching focus modes between AF-S/One Shot, AF-C/AI Servo, and AF-Auto. BBF makes AF-C/AI Servo AF usable for both moving and stationary subjects. Wow, right?!
