It’s the unsung hero of your photos, and it’s always been improving. It’s that little switch on the lens. We’ve become accustomed to AF, and for good reason. It works, and it works very well.
This is a basic explanation, and it’s not going to show you how the camera can see eyes and keep its focus while the subject is moving around. Just think for a moment where where we are today with all the various types of focusing you can use. You can even set your camera to detect animal eyes. We consider autofocusing to just be something that’s always been there, just like many can’t imagine a world without a fiber internet line, a smartphone, or Google Maps.
When comparing these two methods of automatic focusing, the advantage of phase detection is the speed. Contrast detection has to search for the contrast in the image to detect what needs to be in focus and takes a bit longer to do so.
The next time you switch from manual focus to autofocus on your lens, just remember what’s actually happening inside your camera for it to do what you’re asking from it.