----- Original Message -----
From: "Shel Belinkoff"
Subject: Re: lens brightness
> Ivan Prenosil wrote, re effective lens aperture:
>
> > It can't be (as people sometimes say on this list) as simple
as
> > lens diameter divided by focal length; otherwise my
FA28-70/4
> > could not be constant aperture zoom :-)
>
> Hi ...
>
> Like you, I don't believe it's as simple as diameter/focal
length, and
> somewhere i have the details of that. I've noticed, for
example, that
> my Leica lenses are often quite a bit smaller in diameter than
the
> same focal length/aperture combination in Pentax and other SLR
lenses.
Let me clarify, it is not quite as simple as focal
length/aperture diameter, as I first stated.
My encyclopedia of photography says:
"The f number of the lens equals the focal length divided by the
diameter of the entrance pupil of the aperture".
I don't know if this makes a huge difference in terms of this
discussion, though it is important when measuring to measure at
the front of the lens rather than the back of the lens, as exit
pupil diameters are different from entrance pupil diameter on
non symmetrical lenses (thats what we use on reflex cameras)
William Robb
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