how does camera auto focus work?
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More expensive cameras use “phase detection”. This has the advantage that the system can tell not only if you’re out of focus, but also whether you’re too near or too far, and even how much you’re out. This means that the focussing motor can start off moving in the right direction at the right speed to quickly focus.
The principle is that the camera has some special sensors that see a small, thin strip of the image as seen by only the left and right edges of the lens (or sometimes the top and bottom edges). In-focus images will have the strips matching; strips misaligned indicate poor focus. The system relies on the presence of some contrast in the image; it won’t work on a totally blank wall or a clear blue sky.
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