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.)
- Will the prime be sharper wide open than the zoom at 28mm?
Probably, particuarly if both lenses are in the same price range. On
the other hand, there are lots of expensive zoom lenses that are
sharper than cheap fixed lenses.
-Joel
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