MTF tests from Nikon, Canon, Leitz, Zeiss , Hasselblad are real MTF tests and of the lens only
Actually, most of those published MTF curves are not real MTF tests
, i.e. the manufacturers are not actually measuring
the resolution of a real
lens. Canon's and Nikon's published MTF curves are theoretical
MTFs (as are Sigma's, Tamron's, etc.) - they are calculated curves, generated by a computer algorithm based on the optical design of the lens. AFAIK, Zeiss is the only lens manufacturer that published MTF curves that are empirically measured on a real production lens (not sure about Hasselblad).
Since neither Canon nor Nikon make public their algorithms for generation of theoretical MTF curves from the lens design, it's not really valid to compare them to one another, nor to Zeiss' real measurements. Comparing within
a brand is fine.