> hey, wait a minute.....ISO 400 is two stops faster than ISO 100, which
> makes for a GN of 128. If he used ISO 200 film, the GN doubles to 64,
> for ISO 400, it doubles again to 128. Isn't this correct?
>
What's important is power per surface _area_, and area grows as length
squared. If you double (effective) power, you only multiply the
distance at which there is the same power/area by sqrt(2). Or, to
look at it more in line with this problem, if you double the distance,
you have 1/4 the light returning, requiring two stops' compensation.
joey
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