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# Guide Numbers—How Do They Work Again?

Started 1 month ago | Questions thread
Re: Guide Numbers—How Do They Work Again?

In other words you don’t just add up guide numbers to get the total light output. It looks like you are adding the GNs and then multiplying by 1 over the square root of 2 or about 0.7071 (sorry, my phone doesn’t do fractions).
Or is there a different route to the correct answer.
And does this work when combining speedlights with non-matching GNs?

Gato Amarillo 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.

Correct that doubling the power is a one-stop difference. But the guide number increases by a factor of 1.4 (actually by the square root of 2, but 1.4 is close enough for photography). The combined GN is 175/53.

If you want to dig deeper research aperture numbers. The f number series is based on doubling or halving the area of a circle (the diaphragm opening), thus it is based on square root of 2.

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?

What fotowbert said above

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