guide numbers

I'm looking for a flash and could use a little help understand what the guide numbers mean exactly. Like 73ft 100 ISO. Maybe a stupid question but I don't use flash often. I prefer available light.

Assuming you have a fully automated modern flash, all the computations where Guide Numbers come in are done for you. ...but when manually doing things..

The guide number will tell you how far the flash will reach in usually feet or meters depending on the f-stope. This is always given in terms of ISO 100. If you double the ISO...say from 100 to 200, you can assume that the GN can be multipled by 1.4...if you quadruple the ISO..say from 100 to 400, you can multiply the GN by 2.

GN are a result of:
GN = f-stop x flash-to-subject distance.

so...if you want to calculate the f-stop needed it would be:
f-stop = GN / flash-to-subject distance

or...if you want to calculate the max flash range:
flash-to-subject distance = GN / f-stop

All the dirty details including calculators, ISO charts, etc... can be gotten from multiple places on the net. Just google "Guide Number" & Flash"

hope this helps

thank you for all the info.

coleygm got you started well. Here's additional information...

Modern flashes, particularly those with ETTL and motorized zoom heads, make the GN sound wonderfully high simply by telling you the rating when the zoom head is set to its longest focal length and illuminating the smallest area of the scene. So a Canon 580EX (GN190) has a GN which sounds higher than a Metz 45CL (GN148 )...but you need to understand the Canon GN is zoomed to 105mm coverage area (about 23 degrees), while the Metz 45 has a coverage angle for 35mm lens on 35mm format (about 63 degrees). So be careful if shopping for flash by specification!

Additionally the GN expressed for distances in Feet is much larger than the GN expressed for distances in meters...example GN148 (feet) or GN45 (meters) for the Metz 45, is the same power output.

More info here, if you need it.