A relatively simple autofocus system that measures the contrast around the edges of objects and then adjusts the focus to see if the contrast goes up or down. When the contrast is highest the subject is in focus. Contrast AF is accurate because it uses the image being captured by the sensor itself, but because it uses trial and error it’s not as fast as phase detection autofocus, the system used by digital SLRs and an increasing number of mirrorless compact system cameras (CSCs).
The digital photography A-Z