On my 645AFD/II after calibrating the lightmeter on 18% gray the difference are in the 0.7 and 0.5 stops (depending on ISO) for some lenses there is a 0.2 difference.
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I am sure there are people more expert in the science of F stops and light transmission that I am as my experience just comes from practical use . . .though . . . .
Without knowing the specifics, light souce, distance to subject, etc. I think what Andre is seeing is probably bellows factors and lens light transmission differernces.
An RZ really is a 6x7 camera reagardless of the capture device and like a 4x5 took approx. another stop of light compared to a 6x7, a 6x7can require another stop of light than a 645.
Unlike cinema lenses, still photography lenses are rarely accurate or better put the F stop rating is subject to the transmission value of the lens elements. In other words a lens may have a 2.8 aperture value but loose 9% of light in transmission, which once again is the reason most cinema lenses are rated at T stops vs. F stops.
Back in the film days I've compared Bronica, Contax, mamiya and Hasselblad all with the approx. same focal lengths and could see close to a full stop difference on transparency film.
Also back in the film days I use to have one client that wanted some sessions shot 6x7 instead of my standard 645 so when we moved from the Bronicas to the RZ we would just bump the packs up another 1 1/2 stops of power, depedning on the distance to subject.
With digital we usually don't notice the differences because we don't meter, factor in bellows factors or transmission loss . . . we just shoot to the computer or camera lcd and adjust, not really knowing that the light on the suject is F8 though the capture is F4..5
This is one of the reasons that comparisions between different formats is so difficult. Comparing a medium format back with a Canon would sound easy, but so much of it depends on the lens, the exposure at the capture device and the conversion process.
This is also one of the reasons that if your going to invest in medium format it's important you test the exact camera and back in the situations you work in.
JR