Phase-detect autofocus
An AF method that measures the displacement between two sub-images to compute focus distance in one step — fast and predictive, ideal for moving subjects.
Phase-detect AF (PDAF) splits incoming light into two paths and compares the phase difference. Unlike contrast-detect (which racks back and forth seeking peak contrast), PDAF computes both the direction and the distance to in-focus in a single read.
Why it matters
- Speed. Acquire focus in 0.02–0.05 s on modern bodies.
- Tracking. Predicts subject motion between frames — essential for sports, wildlife, birds in flight.
- Continuous AF. PDAF underpins AF-C / Servo modes; contrast-detect alone cannot maintain focus on a fast-moving subject.
On-sensor vs dedicated
Mirrorless cameras embed PDAF pixels into the imaging sensor. Older DSLRs used a separate AF sensor that only worked through the optical viewfinder, not in Live View.
Hybrid AF
Modern bodies combine on-sensor phase-detect with contrast detection. Phase-detect handles acquisition and tracking; contrast-detect provides micro-adjustment for final accuracy. Canon's Dual Pixel AF and Sony's Fast Hybrid AF are vendor names for the same idea.