Image sharpness algorithm

Anyone could recommend a good algorithm for computing the sharpness of an
image content?

The aim is to write an autofocus function.

I tried several simple algorithms by computing the grey-level differences
between a given (i,j) pixel and its close neighbors (and repeat and sum this
over the whole image pixels) but it seems to be more sensitive to noise then
the commerical autofocus functions I have tried in commercial packages. I
heard some packages use the edges in the images to do this but that would
probably slow down things a fair bit?

Any idea much appreciated,

Bob.




.

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