Image contrast theory of electronic focus

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Abstract
The degree of focus of an image may be assessed by scanning a photo sensor through the image and evaluating its response. Equations for the accuracy of electronic focus by this method are developed. Taken into account are image illuminance, image frequency bandwidth, lens f number, detector size, detector velocity, and amplifier noise. Several numerical examples are worked out to show how to compute the minimum scene luminance required for accurate focus together with the focusing error which results. Pinholes, slits, and arrays of slits are analyzed. It is shown that focusing sensitivity increases as the square root of the slit length provided that the noise remains constant. For arrays, focusing sensitivity increases as the number of elements provided that a narrowband filter can be used whose Q is equal to the number of elements; otherwise, focusing sensitivity increases as the square root of the number of elements, still assuming that the noise remains constant.

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