[Leica] RD-1 users: Light loss with fast lenses?

[pklein at 2alpha.net (Peter Klein)]

Folks: I've noticed an interesting phenomenon with my E-1 and fast OM Zuiko 
lenses, and I'm curious if something similar happens with RF lenses on the 
RD-1.

The widest apertures don't give me as much light on the E-1 as they do on 
film. My 50/1.4 is more like a 50/1.8 wide open.  My 50/1.8 is more like a 
50/2.

Note that I'm not talking about metering error here, nor am I talking about 
vignetting at the edges.  I'm talking about using my 50/1.4 to take a bunch 
of bracketed shots of a blank wall with manual exposure. If I get a pixel 
level of 128 near the image center at 1/30 at f/2.8, then I would expect to 
get the same level at 1/125 at f/1.4.  But I don't.  I need to slow the 
shutter to 1/80 to get the same shade of grey.  This is 2/3 of a stop more 
exposure than expected.  The same lens shows less than 1/3 stop loss with 
film.

I'm curious if the RD-1 has a similar effect with f/2 and especially f/1.4 
lenses.  Could some of you RD-1 owners who have Summiluxes and Noktons 
check this out and let us know?

I'm sure all this has to do with sensor angle of acceptance vs. lens 
characteristics like exit pupil size and angle of the cone of light.  There 
has been some talk of this on digicam forums, with the usual 
indistinguishable combination of heat and light.  I personally believe that 
something is indeed going on.  I've seen too many examples of weird DSLR 
behavior with film lenses at maximum and minimum apertures.

DSLR owners who have used the same fast lens on both a DSLR and film body, 
feel free to chime in, too.

Thanks!
--Peter