On Tue, 12 Feb 2002, Dave Dougherty wrote:
> Fellow Zuiks,
>
> Guide numbers are usually expressed as a function of 100asa. As light is, I
> think, subject to the inverse square rule, does anyone remember the formula
> to convert a GN from 100asa to say 400asa, etc. I had it in a Shipman book
> which I sold, having failed to write down the formula before I did so.
Multiply by the square root of 2 for each stop increase in film speed.
e.g. GN(400) = GN(100)xSQR(2)xSQR(2) = GN100 x 2
--
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