Guide Numbers—How Do They Work Again?

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rfsIII Regular Member • Posts: 344
Guide Numbers—How Do They Work Again?

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.

But then I was reading a book on flash and the author wrote that guide numbers are related in the same way as f/stops—the multiplier is 1.4—so the difference between a GN100 speedlight and a GN200 speedlight is not one stop, it's two stops. (1.4 ×100=140 and 140×1.4=196).

So for instance, what is the difference between an SB800 GN125/38 and a Metz 76 GN249/76. Is this 1 stop or 2 stops?

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