Very nice piece. On top of being sturdy and reliable what I like the most is that is it calibrated in candles per square foot. That means you can play you are Alsel Adams with it (before the day he met a spot meter). In most of the meters we read directly the camera settings for certain ASA film number. Some measure the EV (exposure value) which are related also to equivalent pairs of camera settings (speed and aperture) for a given ASA speed film. Many classic cameras permit to set this exposure value directly on them and the pairs are automatically locked together.

Candles are real light units independent from photography. One candle is described in the International System of measurements as: light intensity in a perpendicular direction from a flat surface of 1/600000 square meters of a "black body" at the platina solidification temperature under a pressure of 101325 Newtons per square meter (that is one atmosphere). Being pragmatic: take a small cavity with more or less 1.5 millimeters in diameter. Do that in a beach to be sure you have 1 atm over you. Heat it up to 1774ºC. The intensity of the light coming from is 1 candle. The small cavity stands for the "black body" because in physics we call black body something that can absorb all light falling on it. Normally cavities do it: have you noticed that holes are normally black ? that is because the light falling on them is trapped in multiple reflections and never comes out again: total absorption. Being good absorbers they are good emitters as well when heated up. That is why we need 1774ºC to get one candle. This definition is complicated but it ensures that one person doing that in any part of the globe will get the same amount of light.

Ok, nobody does it but somebody did and then reference meters were calibrated like that. The readings in a Weston Master III show though candles per square feet. It means, finally that the intensity of light reflected by the scene arrives to the meter in a way that a surface measuring one square feet at the point of measurement receives x candles. Being x of course the number you read in the gauge.

The part Anselm Adams liked about that is that it is easy to convert candles per square foot (c/ft²) into camera setting with the following procedure: take the square root of the ASA number of your film. For a 400 ASA it is 20. For 125 it is ~11. So that is your Key f Stop. Now you get your reading with a Weston Master. The speed is simply the reciprocal of the luminance ! You don't even need to know what is a reciprocal because the speeds are already shown as "reciprocals". It means for 60 c/ft² you just set 60 as your speed combined with your Key f Stop. (the square root of your ASA number). Simple!

Somebody asked me the following: What is candle per square foot? What is EV? Why Weston used the first one?

I prepared an answer that repeats part of the explanation above but in other words and starting from the very beginning: the nature of light. Here it is case you want to go deeper:

Light is energy.
It is energy emitted by atoms when electrons jump from one orbit to another. More precisely when they jump from one orbit where they have high energy to another that demands less energy because it is closer to the center.
The difference is emitted in form of radiation that can be visible light.




Of course they need also to go from less energetic orbits to the higher ones. They need to gain before being able to give. Normally they get this energy when heated up by an electric current (a lamp) or a chemical reaction (a flame).
So the atoms drain energy from the heat and give energy as light. That happens in a lamp or when you lit a match and also in the sun or other stars you see at night. So that is LIGHT.

Imagine now that one needs to measure light. What does it mean: to measure?

To measure a distance you need a standard that can be one feet, one meter, one mile. One point in time everybody agreed that a specific "size" would be called "one meter". One metal bar was constructed to be THE original meter, copies were made, copies of this copies were made and sold and now if I tell you I am 1,83 m tall you can reproduce exactly how tall I am using a rule or tape graded in meters and its subdivisions.
What could be the standard to measure light ?
Well , as light comes from heated bodies (seen above) one decided to describe an experience that could be reproduced anywhere at anytime and that would give the same standard quantity of light. He called this "one candle". The same way a specific metal bar was once called one meter.

The experience consists of heating up a surface of 1,5 millimeters to 1774ºC. Normally this surface is just a hole and it doesn't matter what it is made of, it is called 'black body'. The light coming from this small hole in the perpendicular direction is one candle.





To keep our analogy with the meter I must say that to make copies of candles is much harder than to copy a metal bar and say it is a meter.

I don't believe that people working in Weston company used to deal with heated holes to adjust their light meters. Most probably new standard experiences easier to reproduce, were defined but yet giving the same quantity of light kept for continuity reasons.

The same happened with the meter. Nobody nowadays go to the original metal bar to reproduce directly from it. It is in a museum. The meter is now defined based on atomic measurements because we know atoms are quite stable and can be used as reliable standards.

So a candle is nothing more than this: a standard quantity of light, historically defined based on the emmition of a heated black body under well controlled conditions (size, temperature and some others)

Now why the square feet ?

In photography we are basically getting light from "surfaces" (objects) and "spreading it'' onto other surfaces (film, paper, screens). If you take 800 candles and converge it to one square inch it is a quite intense light we are talking about. If it is distributed onto a square feet it is not that much anymore. If you take a square meter it is of course even less. In a large sense there is no strong or weak light it depends on how much surface is emitting or receiving it. Because our eye is also a surface receiving light. It is more or less like pression: If somebody step on your feet using high heels it is much more painful than with normal shoes. The same weight over a smaller surface.

That is why in photography the "per square feet" is important.

Finally the EV.

The candle per square feet was introduced in photography borrowed from science. At the end, nobody needs them in photography.

While shooting we only need to decide on speed and apperture based on the film sensibility (Exposure Index or ASA)
So one decided to create a specific scale for photographers and many camera makers adopted it. That is the EV or Exposure Value.
If you have a light meter that shows EV and a camera that has also an EV scale you only need to match them and shoot.
You don't need to read apperture and speed on the meter and transport it to the camera. You read the EV and set EV on the camera as well. That is easier.
Normally, cameras using EV scales were constructed in a way that once the EV was set you could turn just one ring an the pairs apperture and speed would pass through all possible combinations: increasing apperture one stop would automatically decrease the speed also one stop.
That is the case of Contaflexes, Retinas Reflex, Vitessas and many others.
The coupled meters killed the EV usage. If you have an indication that you need more or less light regarding the actual settings on your camera and you can turn you speed or apperture to match the correct setting you don't bother with EVs anymore.

I think Weston produced meters using candles per square feet just because that is a real measure of light and on those times photography didn't have its own standards.