Single AF vs continuous AF
When single autofocus mode is activated, the lens will lock on to your subject when the shutter is half pressed. As long as the shutter remains half pressed, it will keep its focus allowing you to re-compose before firing the shot. With continuous autofocus selected, the lens will constantly adjust as the shutter is half pressed, hunting and tracking your subject as it moves. When photographing a static subject, single AF is the best option as it will allow you to lock onto your subject and then move the camera to fine-tune your composition. Continuous focus will allow you to track a moving subject such as a bird in flight, or a mammal running, enabling you to keep it in focus at all times, and vastly increasing your chances of a sharp shot.
Benefits of back-button focus
Setting your focus to the back button will enable you to combine both single and continuous AF modes in one.
