Archived posting to the Leica Users Group, 2006/12/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>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.
--
* Henning J. Wulff
/|\ Wulff Photography & Design
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