Phase-detection autofocus relies on measuring the optical phase difference between two light paths to determine both the direction and magnitude of focus error. A sensor or module receives light from opposite sides of the lens aperture and compares the images; if the two images are out of phase the system computes how far and which way the lens must move to reach focus. This direct measurement allows the camera to perform one-shot and continuous focus corrections rapidly without hunting.
Basic principle
In practical terms the camera forms two partial images of the subject and computes their relative shift. The shift corresponds to a phase offset that translates to a lens displacement through known optics geometry. This is fundamentally different from contrast-detection methods that search for peak image contrast; phase detection gives an explicit correction vector, so the lens can move directly to the in-focus position. Richard Szeliski Microsoft Research has described the underlying image registration and phase techniques that underpin many vision systems, which are conceptually linked to phase-detection approaches used in cameras.
Implementation in DSLRs and mirrorless bodies
Traditional single-lens reflex cameras use a secondary mirror to direct light to a dedicated AF module containing micro-lenses and a linear sensor array. Camera manufacturers including Canon Inc. documented how these modules measure phase differences to enable fast subject tracking and predictive focus in action photography. Modern mirrorless and smartphone sensors integrate phase-detection pixels across the imaging sensor itself. Sony Corporation and other sensor makers place paired photodiodes within or beside pixels so each pair samples light from different pupil regions; on-sensor circuitry computes phase shifts while exposure data is captured by surrounding pixels. This integration reduces optical complexity and improves focus during live view and video.
Relevance, causes, and consequences
Phase-detection AF transformed genres that require speed and reliability such as sports, wildlife, and photojournalism because it minimizes lag and false hunting. Nuanced trade-offs include slight reductions in sensitivity under low light or with small-aperture lenses, addressed by hybrid systems that combine phase and contrast cues. Culturally and commercially, the technique accelerated smartphone photography adoption by enabling instantaneous autofocus in compact devices, reshaping how people document everyday life across regions. Environmentally, faster focus reduces wasted frames and power use during continuous shooting, contributing modestly to efficiency in battery-limited devices.