Archived posting to the Leica Users Group, 1999/07/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Marc, Thanks for the clarification on the ASA change. While I may be an old f*rt to some people, I started serious photography in 1958, and I was fairly sure the change happened after that. I seem to remember Tri-X going from 200 to 400 but I am not sure. Ken > -----Original Message----- > From: Marc James Small [SMTP:msmall@roanoke.infi.net] > Sent: Monday, July 12, 1999 9:07 AM > To: leica-users@mejac.palo-alto.ca.us > Subject: [Leica] Old and New ASA Speeds > > 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 > Cha robh bas fir gun ghras fir!