A theory is presented to account for the fact that a photoconductive cell can detect when an image projected on it is in sharpest focus. Each of the small particles in the photoconductive surface is treated as an individual photoconductor in series-parallel connection with all the other particles. When the distribution of light on the surface of such a photoconductive cell is varied (total incident flux being constant) the total conductance across the whole surface of the cell depends upon detail contrast, acutance, and other factors of the projected image. The observed dependence of conductance on sharpness of focus can be explained by this theory.
PAUL PARGAS, "Phenomena of Image Sharpness Recognition of CdS and CdSe Photoconductors," J. Opt. Soc. Am. 54, 516-517 (1964)