Normally, when you press your shutter button half way down, the camera determines the exposure needed and, if autofocusing is switched on, it will focus the lens as well. You can opt to have the focusing done just once (One Shot AF) or have the camera continually refocus on whatever passes across the active autofocus points (AI Servo AF).
Most photographers get by for years without ever thinking about changing how and when the lens is focused. This is understandable – if the normal method works, why change it?
A different way to handle autofocusing
Focusing with the shutter button half-pressed can be tiring if you want to focus initially and then wait for the right moment to take the picture. Your finger must remain static in order to stop the camera re-focusing when you press the shutter button and you may find that you accidentally press the shutter release before you’re ready. Switching to manual focusing is a possible solution, but there is a way to use autofocus with a greater degree of control.
