[LargeFormat] Trouble with calculating F-stop

[Richard Knoppow]

----- Original Message -----
From: "Guy Glorieux" <guy.glorieux@sympatico.ca>
To: "LargeFormat" <largeformat@f32.net>
Sent: Sunday, October 27, 2002 6:47 AM
Subject: [LargeFormat] Trouble with calculating F-stop


> Greetings colleagues,
>
> I've been away for a while from the list but I'm back now
and apologize
> for cutting into an interesting discussion on photography
as art.
>
> I'm running into a frustrating problem with determining
the actual
> F-stop aperture on  the lenses for my 11x14 camera.
>
> One lens is an old Wollensack 18.5" triple convertible,
with the F-stop
> running from F 4.0 to F-256 at the 18.5" focal.  However,
when I try to
> use the indicated F-stop with my meter, I get an image
that is way
> over-exposed.  In fact, if I calculate the diameter of the
aperture at
> 18.5" focal at, say F-128, it gives me F-45 instead of
F-128.  On the
> other hand, if I use the exposure reading for F-45 from my
meter and set
> the lens at F-128 (as indicated on the lens), I get a
perfect picture.
> (The meter is not the source of the problem.)
>
> I have exactly the same problem with another lens, an 19"
apochromat
> Atar F11.
>
> Anybody can explain to me what is happening?
>
> Thanks in advance,
>
> Guy Glorieux
>
>
  The Wollensak lens may be calibrated in US stops. US means
"Uniform System", adopted by the Royal Photographic Society
around 1890. Its found on some old lenses, particularly
Rapid-Rectilinears.
  US stops are calibrated in terms of relative exposure.
They are equivalent to US = N^2/16, where N is the relative
stop, i.e., the focal length devided by the stop size.
  US 16 is equal to f/16.
  US 1 = f/4, US 2 = f/5.6, US 4 = f/8, US 16 = f/16, US 32
= f22 etc.
  I can't answer for the Apo Artar, I don't think any were
ever calibrated to the US scale. Apochromatic Artars came in
three maximum speeds, depending on focal length. The
shortest were f/9; from about 12" to about 19", f/11; longer
were f/16. Generally, they are marked for the infinity focus
speeds.  At 1:1 the effective stop is two stops slower than
marked (one fourth times the exposure). Some process lenses
had external scales showing effective stop at various
degrees of image to object size ratio.
  You can measure the effective size of the aperture
(entrance pupil size) by placing a point source of light at
the exact infinity focal plane of the lens and measuring the
diameter of the projected circle of light at the front
element using some sort of translucent screen.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com