How autofocus works
Camera manufacturers might use different brand names for their AF systems, but they are largely based on two distinct methods for driving the autofocus: phase detection and contrast detection. Some cameras use a combination of the two for ‘hybrid’ autofocus when shooting in Live View.
Phase detection AF uses distance information to determine whether a subject covered by the AF points is sharply focused. SLRs use a dedicated autofocus module for phase detection. As a result, it’s a rapid-response system, although it can be prone to errors, as it relies on the lens, the AF module and the AF points working in harmony. This is not always the case, which is why some cameras include an autofocus calibration option. Contrast detection isn’t affected by focusing errors in the same way: it’s the final picture captured on the imaging sensor that’s used to set the focus. You also have more freedom for positioning the point of focus, as you’re not relying on fixed phase-detection AF points.