Archived posting to the Leica Users Group, 2006/12/08
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I think I'll start shooting all my Tri-X at 666 ISO. BTW has anyone noticed the change (years ago) from ASA to ISO appears useless? Henning Wulff wrote: >> Doesn't it have something to do with logging rhythms. in .3 increments? >> >> Henning Wulff wrote: >> >>> At 10:29 PM +0100 12/6/06, Philippe Orlent wrote: >>> >>>> I was just remembering my ISO scale on the back of my MP: >>>> 50-100-200-400-800- etc. >>>> >>>> But the zones in between are divided in 3 parts. >>>> >>>> So between 50-100: 50/3=16,7 >>>> Between: 100-200: 100/3=33,33, which would put 160 at 100 and 2/3ds >>>> Two full stops under brings us at >>>> 400 and 2/3ds >>>> Which is 400 + (800-400)x2/3= 666,7 >>>> >>>> I may be wrong, but it looks like Leica logics to me. >>>> :-) >>>> Philippe >>>> >>> >>> ISO is not continuous. It's only defined for the discrete >>> progression (from 100 to 3200) for 100, 125, 160, 200, 250, 320, >>> 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200. >>> >>> No numbers in between, ie, there is not 'ISO 300' or 'ISO 666'. >>> > > ISO combines the old ASA and DIN scales, and makes the measurement > methods and ratings equivalent. DIN was logarithmic while ASA was > arithmetic, with 400 ASA = 27DIN, 320 ASA = 26 DIN. For every step the > ASA took an arithmetic step, and DIN took a logarithmic step. > Different measurement methods meant that there wasn't a complete > equivalency, but then they got together and came out with the ISO > method and scaling, which allows for both an arithmetic and > logarithmic scale. So now the old 400 ASA is approximately ISO 400/27. > > Both systems jump in discrete, defined steps with intermediate values > undefined. >