> -----Original Message-----
> Hi, gang.
>
> If the 550EX has a guide number of 180 at ISO 100 in feet, how do
> I properly
> increase the guide number for ISO 400?
Think of it this way: with a GN of 180 with ISO 100, the f-stop at ten feet
would be f18. ISO 400 is two stops faster than 100, so with the same light
it would be properly exposed at f36. Thirty-six times ten is 360, so the ISO
400 guide number would be 360.
The GN increases by the square root of the increase in ISO speed.
When the ISO quadruples, the GN is multiplied by the square root of 4, which
is 2. When the ISO doubles, the GN is multiplied by the square root of 2,
which is 1.4 (or close enough). Thus the ISO 200 guide number for this flash
would be 252.
Chuck Skinner
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