Actually, light can be different through different lenses. While it's true that f8 should give the same amount of light, that's only correct if the lens was perfectly transparent. I compared the exact same scene through a 50/f1.4 and a 90/f2.0. Instead of 1 stop exactly, the difference was 1 stop and 2/3 (if I remember correctly, it could have been more). The 90mm is a much more complex design and it seems that all the glass is absorbing some of the light. When one thinks of the "signature" of a lens, it's easy to understand some light must be absorbed.
There were two terms for the mathematical f-stop (lens diameter / focal length) and the f-stop with light absorption calculated in, but I forgot them. You can be certain that lenses show mathematical f-stop.