Obsolete Film Exposure Indeces Compared
The rating of film "speeds" has gone through several eras (as defined by me): the "laboratory" era when scientists like Jules Scheiner and Hurter & Driffield were studying sensitometry and recording emulsion response in a scientific method; the "meter-maker" era when the manufacturers of light meters (e.g. Weston and General Electric) made their own scales; the "national standard" era when various countries or regions settled on a single scale that could be "universally" used (e.g. ASA, DIN and GOST); and the current era is "ISO," which set a world standard scale.
Due to the world-wide manufacture of meters and the long span of time, it's possible to find meters that are calibrated in any number of different scales. I've gathered as many as I could here, drawn primarily from my 1939 edition of the Photo Lab Index, my Lifetime Photo Lab Index (1971 suppliment), and anything else I could pull together. The idea is that the user of a meter with an obsolete scale (e.g. American Scheiner) might be able to find a reasonable starting point by comparing it to a more familiar scale like ASA, DIN or ISO.
I cannot overstate the fact that film speeds are a starting point from which the photographer needs to determine what's best. Film speeds vary not only from film to film, but by the way they're processed. Kodak T-MAX 100 may be rated at ISO 100/21, but the speed changes depending on the developer used, how long it's developed, the temperature of the chemistry, and the technique used. Moreover, meter manufactures (both on- and off-camera) have their own ideas on how their meters should be calibrated, and don't necessarily conform to "standard." That's why many photographers test their films and determine the speed which works best for them, which they call the "exposure index." Thus a photographer may rate T-100 as E.I. 64, or E.I. 125, or whatever, and process it in a way that yeilds the results he wants.
This was particularly common among manufacters during the "meter-maker" era. Companies like Weston and GE would test and assign speed ratings to film based on their own trials, and would publish the results in pamphlets they sold to their customers for a nominal fee. These speed ratings were revised often, and the speed rating of a popular film (e.g. Kodak Pan-X) could change an entire stop from one rating to the next.
The point that I'm trying to make is that none of this is written in stone.
Some comments on the scales:
ASA is from the American Standards Association; it was the long-time popular speed scale in the USA. Together with DIN, it became the ISO speed scale in 1987. Many people still refer to a film speed as "ASA 100" or "ASA 400," etc.
ASA and ISO ratings are interchangable. e.g. ASA 100 and ISO 100 are equivalent. So you can use a meter calibrated in ASA with a modern digital camera, and vice-versa.
Weston comes from Weston Electrical Instrument, the leading light-meter manufacturer in the US. This and the General Electric scales were made before standards were established, and were eventually replaced by ASA. Weston is around 1/3rd of a stop lower than ASA; GE is around 1/3rd stop higher. If you want to use a meter calibrated on GE or Weston scales, adjust accordingly as your starting point and experiment for best results.
Scheiner was named after the German scientist Jules Scheiner, who was a pioneer of sensitometry. The logrithmic Scheiner scales were very popular in Europe and were the basis of the DIN scale. Scheiner scales are commonly found on pre-WW2 German meters. After the war they're replaced by DIN. There are two versions of Scheiner: European and American. Why—I don't know. If your meter is marked in Scheiner, you need to try to determine which version it actually is.
DIN stands for Deutsche Industrie Norm. Together with ASA, this became the ISO rating after 1987. Most light meters manufactured in the US in the 1950s and later are marked in both ASA and DIN. Cine meters are typically rated in DIN. German meters started using DIN in the late 1930s
H&D is from Hurter & Driffield, two pioneer scientists who measured film speeds and plotted response curves. Unfortunately there are a number of different H&D scales depending on who published them. This is most commonly seen on British meters prior to World War II.
GOST is a Russian scale (it means "State Standard") and is found on Soviet-era meters from Iron Curtain countries, but was dropped in favor of ISO in 1987. Because it's Russian and not common in the US, I've had a lot of trouble finding an equivalency scale. It also appears that GOST went through various revisions over the years, so an early GOST scale may not be compatible with a later one. The scale here is from a site about Russian cameras, and says that GOST is rated at about 90% ASA. I'll update it if I get better information.
|ASA||Weston||American Scheiner||European Scheiner||Din||H&D||GOST||General Electric|
Britain & Europe
|Weston||American Scheiner||European Scheiner||Din||H&D (Brit)||H&D (Ilford)||H&D (Euro)|