Ive seen around the traps these things called MTF Charts (Modulation Transfer Function Charts), which relate somehow toand the quality of their image.
I never understood a thing about them! So found this:
And now they make (sort of) sense. Just got to remember it now...
p.s. For a 1.6x camera such as the 20/350D, you're only concerned with the horizontal scale up to 13, past that it doesn't matter. (It will make sense after reading the link above )