Autofocus

Autofocus is a mechanism used in cameras to automatically adjust the lens so that a subject appears sharp in the final image. It relies on sensors and actuators to detect focus errors and to move optical elements accordingly, reducing or eliminating the need for manual focusing.

The core methods are phase-detection and contrast-detection. Phase-detection compares light paths to estimate the required adjustment quickly, while contrast-detection evaluates the sharpness of the image on the sensor and iterates to maximize contrast. Hybrid systems combine both approaches to balance speed and accuracy. Autofocus systems typically rely on a motor or another actuator inside the lens or camera body to shift focus elements.

Common autofocus modes include single-shot or one-shot AF, where focus is locked after acquisition for stationary subjects; continuous or tracking AF, which adjusts focus to moving subjects; and automatic or flexible-zone AF, which selects focus points automatically. Many cameras also offer face or eye detection, as well as configurable focus areas and manual override.

Autofocus systems vary by platform. DSLRs often use an external phase-detection sensor in the viewfinder path, with the lens providing the focusing power. Mirrorless cameras generally use on-sensor phase-detection or contrast-detection, enabling smoother operation with smaller lenses and in video. In addition to photography, autofocus concepts underpin some machine-vision and microscopy systems.

History and development trace autofocus from early research to widespread consumer use in the 1980s and 1990s, with ongoing improvements in speed, accuracy, subject tracking, and low-light performance. Limitations remain in very low contrast scenes, extremely low light, or subjects that move rapidly or unpredictably, though advances continue to reduce these issues.