--- Ken Durling <[EMAIL PROTECTED]> wrote:
> On Mon, 30 Jul 2001 10:51:54 -0700 (PDT), you wrote:
>
> >Now this does raise an interesting question. Dos
> the
> >aperture actually change size as you zoom, or only
> >when the lens is stopped down for picture taking?
> I
> >think it must vary in real time, because I don't
> >notice changes in brightness as I zoom my 70-200.
>
>
> Wouldn't you have to hold down the DOF preview
> button to tell?
No, I'm talking about max aperture. Suppose (just to
pick a nice round number) that the aperture is 25mm in
diameter at a zooms maximum focal length. Now zoom
out to half that focal length. If the diameter of the
aperture stays the same (if it DOESN'T compensate in
real time), your effective aperture has just
increased.
Since we're presupposing a constant aperture zoom, the
actual diameter of the iris must vary in real time as
you zoom, even when the lens is wide open.
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
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