There's an interesting article in the May issue of the British Journal of Photography that compares the low-light capabilities of the 5D mk3 and Nikon D4. Both cameras use their own 24mm f1.4 lenses in the article. As I've just bought a mk3 and Sigma 85mm f1.4 I thought the article would be pretty informative but was quite surprised to read:-
"Fast lenses are only part of the solution, as the microlens structure of sensors has the effect of limiting the maximum effective f stop - you get the reduced depth of field, but not the true lens speed. To compensate for this, camera firmware detects apertures faster than f2.8 and applies additional gain. You must add the gain mapping applied at all apertures to compensate for vignetting."
It goes on to say that shooting at f1.4 and ISO 6400 will probably give you ISO 25600 in the corners. Yikes!
Well, you learn something new every day.
I guess those shooting Leica with a Noctilux f0.95 must feel robbed!