Edwin,
I use the formula 1800 / f/stop to estimate diffraction limited resolution. That comes to 28 lp/mm for f/64, which is indeed closer to 20 lp/mm than my figure of 40 lp/mm.
That allows approximately a 2X magnification to stay within the limits of the threshold of human resolution when viewing a print at ten to fifteen inches from the eye.
Sandy
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