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And some...

WESTON
Very Well-known are the Weston meters, very nice collector items also, there are about 40 different models.We focus on the Master series and descendants only.
These meters were designed to measure reflected light (luminance) expressed in cd/ft2, however with the invercone attached they were able to measure Incident light.
I will give a resume here which are very usefull today:

Weston Master III (see image) the only drawback: you can't lock the needle.You must have extremely good eye-sight as the numbers are very tiny. Not valid when you know how to work with cd/ft² but that's another story.
Weston Master IV
Weston Master V
Weston Euro Master
Euromaster II
What's under the bonnet of the Weston Master IV?
What's under the bonnet of the Weston EURO Master?
Parts List Weston Euro Master.
Weston scales.

Exposure Formula
Explained by JohnD esq
A formula exists which, once learned, facilitates rapid exposure calculation mentally, without the need for setting dials on meters. It requires knowing luminance values in candles per square-foot, and unfortunately few, if any meters today are calibrated in these units. Since the early Weston Master meters read directly in cd/ft2, very rapid exposure determination was possible. Current meters can usually be calibrated in these units to facilitate the use of the Exposure-formula, as described below.(See Converting to candles per square foot above) To use the Exposure Formula, take the film speed number of the film you're using in ISO or ASA and determine its approximate square root. This number is remembered as the Key Stop for that particular E.I. For example, a film rated as ASA 125 has a key stop of f/11.
At the key stop, the correct shutter speed in seconds to expose a given luminance on Zone V [Kodak 18 % Neutral Grey Card], is the reciprocal of the luminance expressed in Cd/ft².
Thus for a surface that measures 60 cd/ft², we would use a shutter speed of 1/60 second at the key- stop. In a typical situation, you might visualize a certain area as a Value III in the print and find that its luminance is 30 cd/ft². If this value is placed on Zone III, then 60 Cd/ft² falls on Zone IV and thus 120 cd/ft² on Zone V. Your exposure then is 1/125 at the key-stop, or any equivalent combination. Resuming, the Exposure Formula thus permits very rapid calculation of exposure without reference to the meter's rotating dial. Hence, you will have to compute a little more , when you don't have a scale calibrated in cd/ft². An example.... You're using a film rated at 400 ISO, thus the keystop will be f/22, which you will set accordingly to your Camera lens. For a surface that measures 13 and placed on Zone V, this will give you a luminance of 200 cd/ft² ( 10=25, 11=50, 12=100, 13=200). Your final exposure time will be thus 1/250 second at the key-stop, which we defined as f/22.

More models...
Successors of the Weston 650 (1935)

WestonMaster

WestonMaster Cine

WestonMaster II

WestonMaster II Grey

WestonMaster II Grey Black Scale

WestonMaster II Cine

WestonMaster III Sangamo

WestonMaster Chrome Newark

WestonMaster IV

WestonMaster V

Weston EURO Master Last in Production till demise in 1974.
Euromaster Killbride

Euromaster II Latest descandant

Weston Ranger 9 Ansel Adams & Minor White favorite

Invercones - Domes
All types on stage....

**Exposure Formula**
Explained by JohnD esq
A formula exists which, once learned, facilitates rapid exposure calculation mentally, without the need for setting dials on meters. It requires knowing luminance values in candles per square-foot, and unfortunately few, if any meters today are calibrated in these units. Since the early Weston Master meters read directly in cd/ft2, very rapid exposure determination was possible. Current meters can usually be calibrated in these units to facilitate the use of the Exposure-formula, as described below.(See Converting to candles per square foot above) To use the Exposure Formula, take the film speed number of the film you're using in ISO or ASA and determine its approximate square root. This number is remembered as the Key Stop for that particular E.I. For example, a film rated as ASA 125 has a key stop of f/11. At the key stop, the correct shutter speed in seconds to expose a given luminance on Zone V [Kodak 18 % Neutral Grey Card], is the reciprocal of the luminance expressed in Cd/ft². Thus for a surface that measures 60 cd/ft², we would use a shutter speed of 1/60 second at the key- stop. In a typical situation, you might visualize a certain area as a Value III in the print and find that its luminance is 30 cd/ft². If this value is placed on Zone III, then 60 Cd/ft² falls on Zone IV and thus 120 cd/ft² on Zone V. Your exposure then is 1/125 at the key-stop, or any equivalent combination. Resuming, the Exposure Formula thus permits very rapid calculation of exposure without reference to the meter's rotating dial. Hence, you will have to compute a little more , when you don't have a scale calibrated in cd/ft². An example.... You're using a film rated at 400 ISO, thus the keystop will be f/22, which you will set accordingly to your Camera lens. For a surface that measures 13 and placed on Zone V, this will give you a luminance of 200 cd/ft² ( 10=25, 11=50, 12=100, 13=200). Your final exposure time will be thus 1/250 second at the key-stop, which we defined as f/22.

Moonrise, Hernandez, New Mexico
(Ansel Adams)

I came across this extraordinary scene when returning to Santa Fe from an excursion to the Chama Valley.
The sun was edging a fast-moving bank of clouds in the west. I set up the 8x10 camera as fast as I could while visualizing the image.
I had to exchange the front and back elements from my Cooke lens, attaching the 23-inch element in front, with a glass G filter(#15) behind the shutter.
I focused and composed the Image at full aperture, but I knew that because of the focus shift of the single lens component , I had to advance the focus about 3/32 inch when i used f32.
These mechanical processes and the visualization were intuitively accomplished.
Then to my dismay I could not find my Weston.
I remembered that the luminance of the Moon at that position was about 250 cd/ft²; placing this luminance on zone VII, I could calculate that 60 cd/ft² would fall on Zone V. With a film of ASA , the exposure would be 1/60 second at f8(key stop) Allowing a x3 exposure compensating for the #15 filter, the basic exposure was 1/20 at f8, or about one second at f32, the exposure given! I had no idea what the foreground values were, but knowing they were quite low, I indicated water-bath development. The distant cloudes were at least twice as luminous as the moon itself. The foreground density of the developed negative was about a value II and was strenghtened by intensification in the KODAK IN-5 formula (quite permanent and colorless) It was of utmost importance to preserve texture in the moon itself. We all have seen the blank white circle that represents the moon in many photographs, primarly caused by overexposure...!