>From: Andrew Dacey <frugal@xxxxxxxxxxxxxxx>
>
>I could be wrong here...
Might be! Or not. Or both right and wrong at the same time... :-)
>but I believe that 1.0 -> 1.2 -> 1.4 -> 1.8 -> 2.0
>are all half-stop increments.
A ratio of one stop (in terms of focal ratio) is the square root of two.
Therefore, a half-stop ratio would be the fourth root of two, or 1.1892, so it
looks like the 1.2 is pretty damn near one-half stop larger than 1.0.
But applying this ratio to the next stop (1.4 * 1.1892) yields 1.6649, so the
1.8 is more than one-half stop slower than the 1.4.
However, one-third of a stop ratio is the sixth root of two, or 1.1225.
Multiplying 1.4 by this twice yields 1.7639, which is pretty close to 1.8, so
it looks like the 1.8 is 2/3rds of a stop slower than the 1.4.
(One-third stop increments have become popularized via the ASA film speed
system.)
Overall, this means the 1.2 is 7/6ths (1/2 + 2/3) stops brighter than the 1.8.
Precisely, that would be:
1.2 * (7 * (2^1/6)) = 1.7980
--
: Jan Steinman -- nature Transography(TM): <http://www.Bytesmiths.com>
: Bytesmiths -- artists' services: <http://www.Bytesmiths.com/Services>
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