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If you are talking about extended objects, ie anything but stars, the most of the above logic holds true.
Stars however are different, since depite their apparen brightness and size, they are truly only pinpoints of light of different brightness. In this case F ratio has no affect, only aperture.
Why does your F2.8 produce apparently brighter stars than when at F4.. because of another optical function, and which your eye pays part to as well. Its PDF, point dispersion function. As bright pinpoints of light pass through any different medium, glass or eyeball, they tend to be scattered, and appear bigger than they actually are. The scattering is proportional to brightness.
An 8" F4 optical system will record a higher nebula: star brightness than an 8" F10. In both cases though, the star brightness should be pretty much the same. ( Notwithstanding the difference in field of view).
Took me quite a while to get that concept into my tiny brain, as it goes against conventional photographic theory.
HTH
BC
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