Mauro, Of course, that IS the brightness we see--the "perceived" brightness. At the same power, the larger scope will produce a larger exit pupil. At the same surface brightness, the larger scope will produce a larger image. In both cases, a larger area of retina receives light. Because I was involved in nearly every one of the earlier discussions, I was playing l'agent provocateur here. Color perception does vary significantly because of genetics. I learned that a long time ago by comparing what I saw through the eyepiece with several other observers. Alas, age results in brunification of the lens as well, skewing color perception away from the blue and reducing overall sensitivity to light at night as the 500nm peak sensitivity gradually is reduced by the lens filter.
-------------------- Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie