--- Don Feinberg <donf@cybernex.net> wrote:
> ....The small end stop becomes smaller by the same factor as
the
> large-end stop
> (as do all the numerical apertures; the lens mfr expects the
> camera's meter
> to compensate.
>
> So, assume a 100 to 300mm f/3.5 to 5.6 zoom which has a
> diaphragm
> numerically scaled from f/3.5 to f/22 -- very typical. These
> lenses tend to
> test out at f/4 to f/6.3 in the real world. At the 300 end,
> instead of
> f/22, the aperture will be closer to about f/37 to f/45. (You
> can calculate
> for yourself the diffraction effects...) Of course, Joe
> VacationPhotographer will be blithely using the lens stopped
> down to try to
> increase "depth of field"... Not nice!
>
You raise some interesting points. I've long known that the
actual apertures of lenses is often somewhat different from the
"published" ones. But I have 3 questions.
1. How would an individual, without any formal technical
training, actually test this with a particular lens?
2. How did you arrive at the values "f/37 to f/45" minimum
aperture from an actual f/6.3 maximum aperture?
3. How would one "calculate for diffraction"?
Math is not my strong suit but I'll try to absorb any
explanation you care to offer. This subject (variable f/stops)
has long intrigued me.
Richard
=====
Richard Martin specializes in Cityscape and Waterscape stock photography as well as Children's Portraiture. E-mail: marphoto@yahoo.com
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