[LargeFormat] Trouble with calculating F-stop

[Guy Glorieux]

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