P3D Re: Film speed scales

[Rob <lilindn@xxxxxxxxxxx>]

(Bill Stickley wrote:> 
> Where can I find a conversion table for Weston/ASA/GE film speed
> ratings?
>
    The 1971 Photo Information Almanac (by Modern Photography) has one,
with 8 (!) film speed indexes!  Doubtless the same book from other years
has the same table.
    One should note that comparing film speed indexes is not a constant
simple conversion (as in metric/old english measures).  The different
indexes had different ways of determining a film's rated speed, so that
two films with the same Weston speed could have different GE speeds (the
ASA for said two films could be the same or different as well!)
    This may be more helpful than hunting down the table - Weston speeds
were about 1/3 stop (or one DIN speed) lower than ASA or ISO, whereas GE
speeds were about 1/3 stop higher than ASA or ISO - Kodachrome 64 would
likely have been Kodachrome 50 if Weston had won out, of Kodachrome 80
under GE.
    ASA ratings, BTW, for B&W films all flew upward about a stop in the
late fifties, but the color films (which had set controlled processes)
did not change.


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