Paper and Associated Computer Files re Software-based autofocus
suitable for oil immersion lenses:

2006 Methods in Enzymology, Shen et al : "Digital Autofocus Methods for
Automated Microscopy." [.pdf]

To download software-based autofocus computer files accompanying the
above paper, right click on the file links and save the link targets to your
computer. Read the Readme.txt document to know what to do with the
other files.

Readme.txt
Autofocus.h
Autofocus.cpp