Autofocus
A system that measures focus error and drives the lens to correct it automatically.
In depth
Every autofocus system answers two questions: where is the subject, and how far out is the lens? Modern implementations solve the first with subject recognition and the second with on-sensor phase detection, which measures both the size and the direction of the error in a single reading. Speed depends as much on the lens's focusing motor and the mass it must move as on the camera's processing.
Examples in practice
A modern system acquires a bird's eye across the frame and drives a 600 mm lens to it in under a tenth of a second.