Phase-Detection Autofocus
Phase-detection autofocus works out, from a single look at the scene, which way and how far the lens must move to make the subject sharp.
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Phase-detection autofocus works out, from a single look at the scene, which way and how far the lens must move to make the subject sharp.
How it works
Special pixels on the sensor, or a separate focusing module in a DSLR, view the subject through two slightly different positions of the lens. If the two views do not line up, the image is out of focus. The size and direction of the mismatch tell the camera exactly how far to move the lens, so it can go straight to the right spot instead of hunting back and forth.
How it differs from contrast detection
Contrast detection moves the lens and watches for the point where edges look sharpest. It can be accurate, but it needs several steps, so it is slower and tends to wobble in video. Many cameras use both methods together, called hybrid autofocus.
Common forms
- On-sensor phase detection places the focusing pixels on the image sensor itself.
- Dual-pixel designs let nearly every pixel take part in focusing.
What it does not promise
Phase detection is fast, but it does not guarantee sharp pictures. It can struggle in very dim light, with low-contrast subjects, or when the subject moves unpredictably. Subject recognition is a separate software layer on top of it.