# Guide Numbers—How Do They Work Again?

Started 1 month ago | Questions thread
Re: Math to combine Guide Numbers
1

That makes much more sense, I knew we’d get there eventually.

Thanks for explaining.
And as a side benefitnI now I have a use for the square root function on my phone.

fotowbert wrote:

rfsIII wrote:

I used to believe that Guide Numbers were normal—double the Guide Number, double the power. But after reading an article on flash I'm not sure anymore. (And yes, we know that guide numbers are usually optimistic.)

For instance if I am shooting at full power with an SB800 (GN 125/38) and add another SB800 at full power in my mind that means I have doubled my power and bumped my Guide Number to 250/76. In other words I only gained one stop of exposure.

Perhaps I should have included the math in my 1st reply when I said here Guide Numbers follow the same relationship as F Numbers, so here now is the math.

You are correct in saying that doubling the power is a one stop change.

To express combining two SB800s using GNs you don't sum the GNs. Instead you compute the square root of the sum of the squares of the GNs.

Combined GN = sqrt ( GN1^2 + GN2^2 ) = 177. (This is also what you get when multiplying 125 times the square root of 2 which others have correctly said represents a 1 stop increase.)

If you had 4 SB800s then sqrt ( GN1^2 + GN2^2 + GN3^2 + GN4^2 ) = 250 and you have four times the power or light which is two stops.

You can also use the same square root of the sum of the squares math process with F Numbers for determining the exposure when combining multiple lights.

For example two SB800 each giving F/8 light on subject.

Combined F = sqrt ( 8^2 + 8^2 ) = 11.3

This also works when combining flash with ambient light. Suppose the ambient light measures F/11.3 when shooting with only one SB800.

Combined F = sqrt ( 8^2 + 11.3^2 ) = 13.8

• John
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