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.)


However, a pound of gold and a pound of feathers do not.