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This is "true" 50mm, bacause it's adopted to Canon 5D. Shot made wide open @f2.8, handheld (they don't allow to use tripods in their museum), ISO 800. This might seem to be overkill to attach MF glass on 35mm cameras, but imho results show that this old glass has its own advantages.
I would believe C-Y 50 1.4 on 5D is better than 2.8/50 Hassy on 5D. I agree that the shot above is good, but if you use C-Y 50 1.4, you would get even better image quality.
I agree that most medium format cameras outperform 24X36 only because the format is larger and not because the lenses are optically better. Indeed, most medium formats lenses are optically not as good as the small format lenses.
2.8/50 Hassy lenses might be better in the way that it is heavier and gives a better handheld shot, but certainly not better in the optics.
I would not worry too much, as soon as it is handholded quality cannot be challenged/scratched anyway.
at least the 6x6 2.8/50 gives round bokeh rings...
There was a whitepaper on the Markins Ballhead website, actually comparing heads with and w/o Mirror Lockup MLU. If you have to handhold you have 10x more shake/movement than with a tripod (20 mikrometer vs 200 mikrometer)
"sharpness" of the lenses under ideal conditions would be 5 mikrometer... (100 LP/mm) which cannot nor need to be reached in real world
another idea off topic to this contaxinfo forum would be to adapt the hassy lens to a Minolta/Sony Alpha 100 body with super steady shot...