Yeah, I noticed somebody else mentioned the "funnel effect."
The myth about 30mm tubes allowing scopes to be brighter, by comparing them to a larger funnel allowing more water to pass through, was invented by an American advertising agency used by a European scope maker in the early 1990's. But the tube isn't what controls the amount of light that makes it through a scope. Instead it's the lens system. If the tube diameter actually made a difference, the exit pupil would be larger than predicted by the formula of dividing the objective lens diameter by magnification. And it isn't.
