Autofocus: A History Worth Understanding

Good autofocus is invisible until it isn't. Understanding how the system actually works, from passive contrast detection to phase-detect on-sensor arrays to today's subject-tracking, makes you a sharper troubleshooter when it fails on a job. This is a history piece, not a buying guide, but if you've ever wondered why your camera hunts in low contrast or why eye-detect locks differently across bodies, this gives you the foundation.