It feels much more than a third of a stop, though. According to DXO the Sigma has a transmission of 1.5 and the Tamron is 2.8. If the 5D mark II's screen caps out at 2.5 then the Sigma should only look a third of a stop brighter, but it's much brighter. If the Tamron's transmission was much further from its f-stop then I'd understand it, but it cant be that far away. Even if I have a particularly bad copy, it's not like it can be a whole stop slower than others.
If you said the 5D mark II's screen has a brightness limit of f/2 then that I would believe because my Canon f/1.8 and f/2 lenses (don't know their transmission) produce almost the same bright viewfinder as the Sigma, but still not quite a total match.
If you said the 5D mark II's screen has a brightness limit of f/2 then that I would believe because my Canon f/1.8 and f/2 lenses (don't know their transmission) produce almost the same bright viewfinder as the Sigma, but still not quite a total match.
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