guide numbers

[jri]

I have assumed that guide numbers were additive, whereby if I am using 2 strobes (ufl 1's) with a guide number underwater of 7, I have light output equivalent to a single strobe with a guide number of 14. Is this true? and if not please discuss. Thanks, JRI

[bvanant]

If you understand what a GN is then you can skip this paragraph but for beginners this can act as a primer. The GN of a strobe is a measure of how much light it puts out. Be careful since GNs can come in two flavors, one in meters and one in feet. For the UFL-1 the GN (at full power) is 7 feet (UW). This means for ISO 100, at 1 foot distance from your subject, the correct exposure will be at f/7 since distance to subject x F stop = GN. At two feet away from your subject you will need to expose at f3.5. Simple.
So if you have two strobes you get twice as much light but remember that twice as much light is NOT two stops but rather only 1. So if your GN is 7 then with two strobes the new GN is 7 x 1.4 (square root of two) for a new GN of 10 (more or less).

Hope this helps

Bill