>>2915219At the given distance (the GN digit), it assumes f/1 (yes really), typically 100 ISO. The rest is simple math.
Spend a while looking at a GN table and find the pattern, when you start to see that aperture and distance start to be relatively interchangeable, and that ISO 100, 400, 1600, and 6400 are whole stops, eveyhting else is in between and therefore confuses thngs a bit.... then you are beginning to see how it works.
Then google it
but as an example:
GN56 (metres)
At 56m and f/1 this flash will expose correctly on ISO 100
At 56m and f/2 this flash will expose correctly on ISO 400
At 56m and f/4 this flash will expose correctly on ISO 1600
At 56m and f/8 this flash will expose correctly on ISO 6400
Keep in mind that light (and ISO) have inverse square falloff.
eg: at constant flash power and arc of spread:
1m from subject flash on wall is 1mx1m - 1sqm - 1:1 intensity reaches wall
2m from subject flash on wall is 2mx2m - 4sqm - 1:4...
4m from subject flash on wall is 4mx4m - 16sqm - 1:16...
8m from subject flash on wall is 8mx8m - 64sqm - 1:64...
etc
I made these for myself to be easier to spot the patterns. (and therefore easier to do calculations)
http://pseudosticky.wikia.com/wiki/File:RangeGuide@50mm_YongnuoYN560IV.pnghttp://pseudosticky.wikia.com/wiki/File:Pentax_AF160FC_ringFlash-rangeGuides.pngthis last one shows flash zoom... another minor complication that basically means you have to have a table to look at, or memorise the flash power at all it's relevant 'zoom's... as if!
http://pseudosticky.wikia.com/wiki/File:RangeGuide-metric_YongnuoYN560IV.png 
