On another note MTF numbers are only good indication as long as the measurements are compared to similar lens with the same general focal lenght:
Using an arbitrary 50 and extrapolate the results to long lenses or a wide angles is not really a kosher thing to do since their MTF curves
are totally different from one another. Someone posted a link from the luminous-lanscape site before which basically say the same thing... In another word the single number rating are subject to interpretation and doesn't tell the story or the results that you want to accomplish with a particular lens.