PDAF (Phase Detection Auto Focus): what it is, how it works and what it does

PDAF (Phase Detection Auto Focus) is an autofocus method used in digital imaging systems that calculates focus distance from the phase difference between two optically displaced signals. Unlike contrast-detection autofocus, which iteratively searches for the point of maximum sharpness, PDAF directly determines the direction and magnitude of the required lens movement, allowing faster and lower-power focusing.

Operating principle

The principle relies on two-beam interferometry: a portion of the light reaching the sensor is split toward two photodetectors (or pixel regions) separated by a known distance. The positional offset of the image on the two detectors — the phase shift — is proportional to the focus error. The image processor resolves this difference and sends a correction signal to the lens actuator, displacing the optical elements until the phases align.

Architecture: dual-pixel variant

The dual-pixel PDAF implementation integrates phase detection into the image sensor matrix itself. Each pixel is divided into two adjacent photodiodes: one receives light deflected by an internal prism or micro-mirror, the other by the opposite one. As a result, the entire sensor surface participates simultaneously in phase measurement, without requiring a dedicated strip of pixels exclusively for focusing.

• Broad coverage: phase detection is distributed across the full sensitive area, facilitating subject tracking during motion.
• Coexistence with OIS: in systems where it appears, dual-pixel PDAF operates complementarily to Optical Image Stabilization (OIS), which corrects camera shake, while PDAF corrects focal-distance error.

Documented applications

Available product specifications indicate the presence of dual-pixel PDAF in both the rear camera and the front camera of mobile devices:

• Main (rear) camera: 50 MP sensor, f/1.8 aperture, 24 mm equivalent focal length, 1/1.56" sensor size, 1.0 µm pixel pitch, with dual-pixel PDAF and OIS.
• Front camera: 12 MP sensor, f/2.2 aperture, 26 mm equivalent focal length, with dual-pixel PDAF.

The presence of PDAF in the front camera is notable, as many devices limit self-portrait autofocus to contrast methods or fixed focus; incorporating phase detection in that position speeds up focus lock in low-light or close-subject conditions.

Scope and limitations

• Scope: PDAF is effective within a distance range determined by the separation of the two beams and the lens focal length. Outside that range, the system may fall back to a contrast method or lose lock capability.
• Local-contrast dependence: in very low-texture scenes or under uniform illumination, the phase signal can be weak, reducing calculation accuracy.
• Sensor integration: splitting a pixel into two photodiodes implies a slight reduction in per-pixel light sensitivity compared to a single photodiode, though in sensors of 1/1.56" size and 1.0 µm pitch this effect is compensable through signal processing.

Interpreting its presence in a product

When a device's specifications list dual-pixel PDAF, it indicates that the manufacturer has chosen a phase-detection autofocus architecture distributed across the entire sensor matrix, rather than a dedicated pixel strip or exclusively contrast-based focusing. Its coexistence with OIS signals a redundant stabilization-and-focus system: OIS compensates for mechanical hand movement while PDAF corrects focal distance. The explicit mention in the front camera further suggests the manufacturer prioritizes autofocus speed for self-portraits, a differentiator over fixed-focus or contrast-only solutions in that position.

In summary, dual-pixel PDAF is a focusing strategy that turns phase measurement into an operation distributed across the full sensor, offering speed and coverage that contrast detection alone cannot achieve, and whose presence in product specifications is an indicator of the opto-electronic architecture chosen by the manufacturer.