[rollei_list] Re: Hello (again), new Rollei TLR

----- Original Message ----- 
From: "Jerry Lehrer" <jerryleh@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, April 29, 2005 4:32 PM
Subject: [rollei_list] Re: Hello (again), new Rollei TLR


> Richard,
>
> I have been using Weston meters since before you were
> born, and I did not have under exposures when I used
> the meter correctly.  I have used Weston, ASA, DIN and
> Scheiner speeds with the corresponding dials.
>
> The first Weston meter I used was in 1936, and it was spot 
> on
> when I sold it this year.
>
> Jerry
>
   Check the Weston against another refleced light meter on 
a large flat surface. A gray card is not necessary because 
one is only comparing the two meters for a given amount of 
light. Laying them on the surface of a light table also 
works. You will find that the Weston will give you shorter 
exposures when the speed setting is made to the "equivalent" 
ISO speed. This is the next lower number on the Weston 
scale. When the original ASA system was in use this worked 
fine because that system had a 2.5 times fudge factor built 
in.
   I was somewhat surprized at this myself but have found it 
consistent on several Weston meters which are calibrated in 
Weston speeds.
   It is interesting to compare the Kodak speeds with Weston 
speeds. Kodak speeds were measured using the minimum 
gradient method of L.A.Jones and later adopted by the ASA 
but with the addition of the fudge factor. Kodak Speeds are 
approximately double the current ISO speeds and were about 4 
times the old ASA speeds (with safety factor). Here is a 
comparison of some film speeds as given in c.1944 Kodak 
literature:

Film          Kodak     Weston
Plus-X        250        50
Portrait Pan  200        40
Tri-X         800        160

    Weston speeds may not be exactly a factor of 5X from 
Kodak speeds because Weston also classified speeds by 
groups.

    The Weston speeds were determined by making a D-Log10E 
curve of film developed to a a gamma typical for the film. 
The measurement point was the log exposure point at which 
the density was equal to the gamma, i.e., for a gamma of 0.7 
the speed point is measured at a density of 0.7.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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