Dear all
I've been wrestling with a bit of a riddle ever since I read something on
one of the photo newsgroups, and try as I might I can't figure it out in my
head.
Somebody was asked which kind of zoom lens is better: fixed or variable
aperture?
The answer is easy for me. Using as I do a manual camera, a fixed aperture
lens is certainly more convenient as it saves you having to adjust exposure
after zooming (aside from if the light had changed etc.) if you're shooting
wide open.
But, and here's the rub.....as one of the answers pointed out,*all* zoom
lenses, by definition *should* be variable aperture, shouldn't they? After
all, the focal length changes without the physical size of the opening by
which light leaves the lens altering one way or the other. At least, it
doesn't seem to on my 65-200/4. Bafflingly, the 35-70/3.5-4.5 I used to
have did change size, as the aperture was stopped down a tad at the 70mm
end. (If you've got one, try it.) To me this means that any lens with a
fixed aperture must be a compromised design in some way, and also why isn't
my 65-200/4 a 65-200/2.8-4 or something like that. The only thing I could
think of was that the lens is actually a 65-200/3.7674757 - 4.3323343, and
for simplicity's sake it gets corrupted to 4. I've been kept awake trying
to figure this one out. Can somebody explain this to me in non baffling
terms?
Cheers
Dave
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