I'm considering replacing my Tamron 28-75, with a Canon 17-55, as I could use the shorter focal length some and faster AF. Wanting to compare sharpness I went to Photozone.DE brought up the analysis of both. They only have the Tamron tested on a 5D2, while the 17-55 is tested on a 50D. Because of that the total MTF number is reduced.
Is it fair to compare the two numbers are percentage of perfection, i.e. Tam MTF was 80% of upper limit Canon was 85% of upper limit so it's better even though the absolute MTF number is lower? I'm confused.