In article <4ci0c1$e10@earth.njcc.com>, csink@pluto.njcc.com says...
>I have 2 lenses which both have a maximum aperature of f3.5. One is a
>28-70 Tamron and the other is a 75-150 Nikon E. In the viewfinder when
>focusing, one of them (the Tamron) looks much darker/dimmer than the
>other. Why is this? I thought f3.5 was a solid, actual, real value.
>Sorry if this is a really ignorant question.

No, it is an excellent question.
Oh, you wanted an answer to the other part, too, I suppose......?
On film, f3.5 is f3.5 (assuming no optical problems intervene, which
they often do), but in viewfinders, f3.5 is not necessarily f3.5
(for a good example of this, look through a 28mm f3.5 early Nikkor,
then a 28mm f3.5 AI - you would swear the 2nd lens was about 2 stops faster than the 1st, but on film you would get about the same
density at f3.5 (though not the same sharpness...). Nikon fairly
early on began to design lenses to not only perform well on film,
but to give a good viewing image. If you look at the rear of Nikkor
wides, you will find that they have unusually large rear elements,
which seems to aid in viewing brightness (even my 15mm f5.6 lens,
with its tiny diaphram, has a fairly large rear element). In
addition to the above, there is also the simple fact that long
lens images strike the viewing screen at more direct angles than
short lens images, so the viewing optics can bend more of the light
toward your eye. Also, is the speed of your zoom still f3.5 at 70mm?
If not, try comparing both lenses at 70-75mm at f5.6 (true, not
marked, aperture - there is probably a second aperture index mark on
the barrel for the long end of the zoom range), if your camera has
DOF preview.
Hope This Helps