Note that I find Canon's criterea too lax. My own cutoff of 70% contrast at 30 lp/mm is much stricter and is more likely to satisfy a very fastidious user, e.g. in astrophotography. Click "original" below right to see all curves at a reasonably large size.

Wide angle lenses have a special problem, the wider the worse. Check out the MTF at f8 (blue curves). Do all the curves stay >70% out to the radius of your camera's format? I.e. 13mm for the 1.6x crop DSLRs and 21mm for the full 35mm format DSLRs. If "yes" then at least that lens would be excellent at f8. If "no" then expect problems, even at f8. You may then sort of make a judgement as to how wide you can open the lens by comparing the curves at f8 with those wide open (the black curves).