Focus Lock SX 50
Focus lock is easiest to use when it is assigned to the Shortcut button. When you want to focus, just press the "S" button till you get the confirm beep and "AFL" appears at the top of the screen. It will hold that focus until you take the image, or until you touch the zoom lever to disengage it. Remember, the Shortcut button can be assigned with different tasks for different modes on the camera.
Hope this helps.