how does camera auto focus work?
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There are a few systems, but the easiest to understand is contrast detection. This is very common on any digital camera showing a “live view” of what the sensor sees. The camera searches for parts of the image where color or brightness differ and adjusts the optics until the edges of colors and brightness get sharper. This is why if you try to focus on a featureless area, e.g. snow or a cloud, it will keep hunting as it’s difficult to find any well-defined edges.
Since all this requires is firmware (you need the image sensor already…), this is the cheapest. Other systems use specialized, dedicated sensors so the system can compute how unfocused the light hitting the sensors is, and *predict* how to move the optics; this is why the autofocus on a nice dSLR camera will be very fast. This is called phase detection.
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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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A guy I used to know worked on one of the weirdest auto focus schemes for Polaroid. When you press the button the shutter doesn’t immediately open. It simultaneously sends an ultrasonic ping and a solenoid flips a curved rectangular lens up. The lens has a gradient focal length, and once the lens starts falling, the shutter opens when the reflected ultrasonic ping is detected. The whole thing was designed so that the focal length where the lens is falling corresponds to the distance determined by the ping. There is also a correction lens because the top of the picture will have a different focal length than the bottom due to the gradient lens. Whole thing takes a small fraction of a second so it seems instantaneous. So that’s a mechanical engineering approach to it…
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