This page http://www.bobatkins.com/photography/technical/resolution.html
on Bob Atkins' excellent site for geeks, gives one of the most lucid explanations I've seen for why resolution tests are worse than meaningless. He has other pages that go into detail on MTF, SQF, and other information that should, after a while, drive you to turn off the computer, clear your head, and take a few pictures.
He's just addressing aerial image tests. Most folks don't even have the set-up to perform such tests and use test targets. In general, Bob tries (and succeeds !) to explain things in plain language without equations. The site truly for geeks is http://normankoren.com/
some of that only applies to aerial images, but one of the main points ... that the single number value (which tells us the point at which modulation drops to zero) tells us nothing about the shape of the mtf curve, or about the modulation at frequencies that are actually most important to the human eye for perceiving sharpness and image quality. This will vary depending on enlargement size and veiwing distance, but at least an mtf graph gives you much of the information you need to figure it all out. The single number values can be completely deceptive.
Many agree that MTF at 0 is meaningless, but MTF at 50% is not too bad for a single number, although of course is always more informative to have the whole curve.
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