Paul -

Ctein's paper-resolution experiments were conducted by contact-printing a glass bar target, using sufficient pressure - "equivalent to contact-printing an 8x10 negative under several hundred pounds of pressure" - to insure good contact with the paper, and exposure tuned for optimum resolution. He describes the bar target as having "nearly 100% contrast out to about 100 lp/mm; thereafter, contrast falls off until it reaches zero at about 500 lp/mm". The resulting prints were examined under a microscope to determine the resolution limits of the papers. He doesn't specify in the article the exact standard used to visually judge extinction of the bar pattern. However, I doubt very much whether he's fallen into any methodological traps here - Ctein is a first-rate amateur scientist and understands well the very legitimate issues you raise, with respect to both experimental design in general and psychophysics in particular. And his conclusion is that paper resolutions are *far* beyond the point where it makes a difference visually, a finding that should be robust to reasonable variation in judgment thresholds among experienced observers.