[bjorke]

Uh ... I don't want to start anything, but ... Do you mean to suggest that the light from an "ordinary" (non-retrofocus) lens DOESN'T "end up in the correct place on the film"?
When you speak of "diffusion", do you mean "dispersion"? - With more glass, there should be more, not less...

Yes, that is indeed what I mean, though SLR lenses are generally retrofocus lenses, which is just why they have problems versus RF lenses (or LF lenses).

Some of the light ends up in the correct place, but some photons are scattered inappropriately, due to more glass, more surfaces, etc. SLRs == Retrofocus designs == more surface, more glass, and more diffusion of the irradiance ("dispersion" is the same thing). Non-retrofocal designs, such as SLR tele lenses, are on a more even footing with their RF cousins. Of course, for those of us 35mm shooters who treat anything longer than 45mm as a rarely-used exotic lens, I guess that particular SLR benefit goes largely unrealized

(None of this means you should change exposure when shooting with a rangefinder or LF camera as opposed to an SLR, though)