Best autofocus method for subsampled images

Hello, are you aware of focusing algorithms that works fine on small
portions of an image? I found an article "Autofocusing in Computer
Microscopy: Selecting the Optimal Focus Algorithm" by Y.Sun, S.Duthaler
and B.J.Nelson, with a roundup of focus algorithms: it points out that
the best way of focusig subsampled images is by Tenenbaum Gradient
(Tenengrad). Have you got any experience on the matter?
Tenengrad approach isn't illustrated in details (the article refers to
another source, "Focusing" by Krotkov, a document that I couldn't
retrieve by University), so I had perplexities coding it. Basically it
convolves the image with Sobel operators and then sums the square of
the gradient vector components. When doing such an operation I don't
know the legal values that the result can assume: in other words,
ignoring the theory behind this algorithm, I dunno if the values
returned by this operations must range between 0 and 255 (since I am
analyzing a 256 grey level image) or not.
Any help is higly appreciated and welcome. Thanks in advance.
Best regards.

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