"Icoz, Evrim" wrote:
> What's the advantage of having a constant aperture zoom then? Aren't we
> losing useful aperture at the shorter end if we are limiting ourselves to
> the center?
?? If you have a constant f-stop zoom, then the diaphragm opening is always as
large as it needs to be (at maximum f-stop) across the zoom range to maintain
that constant f-stop. The size of the front elements has nothing to do with it
as far as I can see, other than to allow for proper correction and light
transmission when the diaphragm opening is at its largest (at the longest focal
length).
If you're suggesting that the maximum diaphragm diameter be held at the at the
size it is at the longest focal length across the entire zoom range, consider
the kinds of optical difficulties this would present: taking the 28-70/2.8L as
an example, the diameter of the diaphragm at 70 mm and f/2.8 is (theoretically)
25 mm. If you then keep that same 25 mm diameter at a focal length of 28 mm you
end up with an aperture of f/1.12. Now imagine what would be necessary to
correct all the optical aberrations you'd have in a 28-70/1.1~2.8 lens . . . and
what it would cost. :-)
fcc
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