I have an old Weston Master II light meter, probably 1950's vintage. It came with some instructions and a pamphlet listing Weston exposure numbers for various films. For some of the films I'm familiar with, these Weston numbers (which are used to set the dials on the meter) are about half or less of the modern film speed numbers for these films. For example, Tri-X Pan film, rated at 400 is listed with a Weston number of 200. Verichrome Pan which is now rated at 125 has a Weston number of 50.

Since I'd like to use this old meter with some of my vintage cameras, should I use the Weston number of 50 for the VP film or its modern rating of 125? If there's someone who is familiar with these meters and can give me some guidance, I'd appreciate it very much.

-- Greg Denson (gcdenson@aol.com), August 22, 1999

Answers

Greg-

The Focal Encyclopedia of Photography says "...some meter manufacturers concerned issued their own film speed systems. These were based not on any sensitometric calculations, but on practical tests and experiments."

They then say "The older speed systems are not strictly comparable since they are all based on different properties of the characteristic curve.", and present a table of conversions.

Here are the ASA and Weston numbers at selected points, ASA:Weston, 25:20, 50:40, 100:80, 125:100, 200:160, 320:250, 400:320, 800:640. As you can see, Weston is slightly more than half the ASA (EI) value.

There is also a system called Weston-Scheiner (a logarithmic scale) and this is equal to 10 log Weston + 9.

"another factoid from the dusty Hoffman library of obsolete, hard to find, and generally weird information!" Good luck.

-- Conrad Hoffman (choffman@rpa.net), August 22, 1999.

Conrad; Thanks a million for the information! I think I'll start out with the Weston numbers you supplied and test a roll or two of film to see what kind of results I get from the meter's recommended exposure. Then, if need be I'll make adjustments accordingly. Again, thanks. Greg

-- Greg Denson (gcdenson@aol.com), August 22, 1999.