Archived posting to the Leica Users Group, 1999/07/12

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Subject: [Leica] Old and New ASA Speeds
From: Marc James Small <msmall@roanoke.infi.net>
Date: Mon, 12 Jul 1999 09:07:02 -0400

Two points.

First, the ASA scale was reworked in 1960 when the "safety factor" was
re-evaluated.  The actual means of computing the ASA speed remained
unchanged, but a safety factor was reduced, effectively raising ASA speeds
for the same film.  As I recall, Tri-X went from 320 to 400 and Plus-X from
80 to 125, but I'd have to look up the specifics to be certain.  It was not
a linear transformation:  each film had a different increase.  See the
GEVAERT MANUAL for discussion.

Second, ASA ceased to be used as the index for film speeds a decade back
and was replaced by the "ISO" index.  This is a two-part figure separated
by a slash, the first part being the ASA index and the second the DIN
index.  Hence, Tri-X and 400 Delta and the like are now "400/27" and
Kodachrome 64 is "64/19".  So, those camera store clerks actually do have
to "translate" ASA speeds into the modern ISO index, though they shouldn't
have to struggle too manfully with the problem.

Marc

msmall@roanoke.infi.net  FAX:  +540/343-7315
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