According to an established tradition, autofocus is activated by pressing the shutter button halfway, and pressing it fully releases the shutter. However, it is often much more convenient to separate the shutter release and autofocus. Back-button focusing or back-button focusing is an autofocus control scheme that is not very popular among amateurs, in which autofocus is activated not by the shutter button, but by an independent button on the back of the camera.
Some cameras have a special focus button AF-ON, and some allow you to reprogram the AE-L / AF-L button so that it is responsible for autofocus, and not for exposure lock. Continue reading