A good prime should be sharper and exhibit fewer optical aberrations
compared to a zoom, with the aperture set wide open. Stopping the zoom down
should improve sharpness, but then you may lose the desirable effect of
limited depth of field, if that's what you wanted. Zooms typically have
different geometric distortion throughout the zoom range, usually barrell at
the wide end and pincushion at the tele end, being neutral somewhere in the
middle. A good prime should be able to combine better sharpness, contrast
and distortion characteristics compared to a zoom.

But there are some outstanding zooms out there these days and the advantage
of primes has been lessened as a result.

As the apertures are the same, there should be no difference in brightness.
The glass doesn't lose enough light in the way you fear to be a major
factor.

Ian

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"Siddhartha Jain" wrote in message
ups.com...
Hi,

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?
- Will the prime be sharper wide open than the zoom at 28mm?

Ofcourse, a lot will depend on the particular makes and models being
compared but is there a rule of thumb?

- Siddhartha