Quote:
Originally Posted by giordano
I've got a 1942 book on miniature photography which cites the following examples (not Agfa or Perutz, alas). I've added the approximate ISO equivalents, and it gives an idea of how things looked then:
Slow (e.g. Kodak EF Pan): 12/10 DIN, 23 degrees Scheiner (ISO 16)
Medium fast (eg. Panatomic X): 15/10 DIN, 26 Scheiner (ISO 32)
Fast (e.g. Kodak Plus X, Ilford HP2): 18/10 DIN, 29 Scheiner (ISO 64)
Ultra (e.g. Kodak Super XX): 21/10 DIN, 32 Scheiner (ISO 125).
One reason the ISO values are approximate is that the three systems use different methods to measure film speed. Another is that in those days speed ratings tended to include a generous safety margin (towards overexposure, which made sense at the time, at least for medium and large B&W negatives which made up the vast bulk of the market). This was lowered by 1 stop some time after WW2, i.e. the published speed ratings of B&W films doubled with no change in the emulsion or processing. So in modern terms it may be appropriate to double the ISO numbers above.
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A 1937 Leica Manual gives 'Fast Panchromatic' as 23 Scheiner and 'Medium Fast Panchromatic' as 20-21 Scheiner, so a bit slower than the 1942 figures, the list of manufacturers of such films includes Agfa and Perutz as well as others.
Earlier films might be slower still.
Gerry