One of the most frequently asked questions is about the scales on Weston meters and the Weston film speed, and how to tell if the meter is accurate.
With modern films and cameras, setting the film speed is something you don't have to worry about - you just buy an ISO 400 film for general use, or perhaps ISO 200 or even 100. The speed is encoded onto the film canister, the camera picks it up, and all the rest is automatic. Back in the 1940s, film manufacturers were optimistic in varing degrees about their film and their published speeds were just not trustworthy. Weston were making very accurate exposure meters and they didn't want to jeopardise things by relying on inaccurate speed ratings. So, they came up with their own and published their Weston Speed for all the popular films of the day. How do the Weston speeds translate to todays ISO or even DIN ratings?
Then we have the numbers on the Weston dials - what do they mean?
Firstly the dials.
Up to the Master IV the scale on the dial is in cd/ft2 - foot candles, now known as Lumens. After the IV, Weston started to use arbitrary numbers. With the baffle open, the scale is from 0.2 to 50 and with the baffle closed, 25 - 1600. Note the overlap - it's usually best to use the baffle-open in this overlapping area as the meter is more accurate there.
Now the Film Speeds.
Meters up to and including the Weston III used the Weston Speed rating. From then on they used ASA which is now better known as ISO. DIN was a European rating until recently and GE was General Electric's own system used for their meters.
How to check if the meter is reasonably accurate.
Make sure that the pointer is reading zero with the cell covered. If not, adjust with the screw on the front. Point the meter to a clear north sky at any time between 10 A.M and 2 P.M. There must not be any clouds or haze. An accurate meter will give a reading of approximately 320 Lumens - the scale used before the Euromaster series. That's just over half way across the dial with the baffle closed. This should translate to f/16 at 1/100, with film speed set to ASA/ISO 100.