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-   -   Weston U-A-C-O markings on meters (http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=14057)

egrabczewski 24th December 2021 07:38 AM

Weston U-A-C-O markings on meters
 
Recently, I was intrigued what those symbols mean on the old Weston light meters, U-A-C-O, so I did some research. I'd be interested to hear from others if they have any more information.

At first I tried guessing what they mean:

U - Underexposed
A - ?
C - Contrast
O - Overexposed

but I couldn't fathom what A stood for.

In the "Weston 617 Type 2" exposure meter manual we see the start of this system:

A means "Distant Weak Contrast"
B means "Normal"
C means "Dark Strong Contrast"

U and O were not yet used but descriptions on the meter itself state:
"Darker Objects Would be Underexposed" and
"Brighter Objects Would be Overexposed".

So it looks like ABC were just chosen arbitrarily as the first letters of the alphabet without any particular meaning.

Soon afterwards, Weston replaced the B symbol with an arrow and used U-A-C-O on all their meters.

In one of their final manuals for the Weston Master 6 exposure meter, a clever author thought up a meaning for "A":

A - Absence of Contrast.

This completes the table for curious people like me who want closure!

Merry Christmas and a Happy New Year for 2022!!

Michael 24th December 2021 09:55 AM

A and C represent respectively the lower and upper limits of colour film range. "After setting the dial for any scene, all objects whose light values fall on or between these two positions will be correctly exposed". U and O are as you thought, I presume for monochrome film.

I quote from Instruction book: Weston Master II model 735 universal exposure meter, no explicit date but I think about 1949.

egrabczewski 24th December 2021 10:22 AM

Maybe I should have explained that the issue in this post is not how to use the U-A-C-O but on what words these letters are based. This post is only for the curious of mind. I've found in the past that some people use abbreviations and acronyms without questioning their derivation or meaning. Like so much of film photography nowadays, it's a historical question :)

Michael 24th December 2021 12:08 PM

Sorry I spoke.

JOReynolds 30th December 2021 06:16 PM

Weston Photometry
 
On the subject of Weston photometry in general, I discovered that the light-sensitive cells in Weston Meters were iron discs coated with selenium and, until recently, manufactured by Centronic, now in Croydon.

egrabczewski 31st December 2021 07:12 PM

Quote:

Originally Posted by Michael (Post 142899)
A and C represent respectively the lower and upper limits of colour film range. "After setting the dial for any scene, all objects whose light values fall on or between these two positions will be correctly exposed". U and O are as you thought, I presume for monochrome film.

I quote from Instruction book: Weston Master II model 735 universal exposure meter, no explicit date but I think about 1949.

It's fascinating to note that before this Weston Master II (1945) manual was published, the manuals for the Weston Model 617 Type 2 meter (1934) and the Weston Model 650 (1935) don't mention colour film - because it wasn't used in photography until the 1940s - and so the meaning of "A" and "C" don't relate to colour photography in those early manuals. Instead, "A" and "C" are simply used to signify minor shifts from the "B" (Normal or arrow) setting to compensate for a lack of contrast or an excess of contrast respectively. It seems like only later did Weston start to use "A" and "C" to cover the range of colour film.

As a final observation, in the Weston Model 617 Type 2 manual then "A" is 1.33 stops down from "B" (Normal or arrow) and "C" is 1.66 stops up from "B". However, in the Weston Model 650 manual then "A" is 1 stop down from "Arrow" (B becomes an arrow in this manual) and "C" is 1 stop up from "Arrow". This is how it stays for the rest of Weston's history.

egrabczewski 1st January 2022 11:15 PM

Serendipitous observation
 
An excellent paper by Douglas A. Kerr entitled "The Weston Master family of photographic exposure meters" made the same observations as mine regarding "U-A-(B)-C-O" only way back in 2014 - so I can't claim any originality, even though my observations were made without any knowledge of this paper.

This paper is well worth reading, together with "Exposure Metering: Relating Subject Lighting to Film Exposure" by Jeff Conrad [1998, 2003]. Although Conrad's paper has some mathematical formulas, it's all basic algebra.


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