How do you know when focus is achieved when you press the shutter button halfway down?

Focus

Explanation:  Most newer digital cameras have a two step shutter button that’s designed to help you with focusing and framing your image. It also helps you get rid of shutter lag. It’s time to unravel the mystery behind your shutter button. Let’s take a more detailed look at what it does. The extra step in the two step shutter was designed to give people a little bit of preparation before ultimately snapping the photo. If taking a picture is like playing football, then pressing halfway down on the shutter button is like getting together for a huddle. Your camera figures out where you want to focus and primes itself for the shot. That way, when you finally do press the shutter button all the way down, your subject will already be in focus.