Dr. Joel M. Hoffman wrote:

Given two lenses, one a prime (say 28mm) and the other a zoom (say
28-75mm) and both with an aperture of f2.8 -
- Will the prime be brighter than the zoom because it has fewer lens
elements?


No. f/2.8 tells you exactly how bright the lens is. (It's almost
like the old question about a pound of lead and a pound of feathers -
they do both weigh the same.)


Well, no. The F-stop is the ratio of the aperature to the focal length,
which is constant regardless of the actual material(s) of the glass. In
the extreme case of spraying black paint on the lens, you could have an
F:2.8 lens with 0% light transmission.