[LargeFormat] Trouble with calculating F-stop

[Les Newcomer]

On Sunday, October 27, 2002, at 07:21 AM, Les Newcomer wrote:

>
>> 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.

>> I have exactly the same problem with another lens, an 19" apochromat
>> Atar F11.
Now it's a different problem with the Artar since at f11 it can't be in US 
stops (US doens't have a US11)
I'm not sure what to tell you on this one.  Is it all there?  Since the 
Artar is symetrical, you can use half of it. This gains you a 1.75x focal 
increase and a decrease of about 2 stops.  there should be cells (glass) 
both front and back.

Les
>>
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