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My read is that the contrast auto focus is using edge detection algorithms like laplace or sobel.

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Since autofocus quality and speed are very important selling points, the algorithms are almost certainly proprietary (though they may be based on well-known ones) and kept secret by the camera makers.

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There's no standard method, some form of local gradient estimation (such as Lapace or Sobel filters) will be involved. The best ones probably use a form of deconvolution (mathematical reversal of the blurring effect you get when the lens is defocussed) in order to estimate how far away the image is from being in focus. For more information on deconvolution, see:

http://en.wikipedia.org/wiki/Deconvolution#Optics_and_other_imaging

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