Guide Numbers—How Do They Work Again?

Started 1 month ago | Questions thread
OP rfsIII Regular Member • Posts: 348
Re: No magic involved with GNs, just math

The one part of your explanation that I don’t understand is why you would be moving the lights farther away when the goal is to put more light on the subject? The father the light, the harsher and dimmer it will be.
The point of adding speedlights is that I want to put more light onto the subject and for aesthetic reasons I don’t want to change the position of my lights (I shoot my speedlights through soft boxes and umbrellas).

And also, what is “normalizing”? I see the word get thrown around from time to time but how does it relate to photography?

“Normal” is the last term that comes to mind when I think about photographers or photography.

First, you need to normalize the guide numbers. Some ISO, same angle of coverage, same units of measure, etc. With all else being equal, a GN of 250 would be 4 times more light than that of one that is 125.

This is because when you double the distance, you make the width of the resulting beam twice as wide, AND twice as tall. Together, this makes the area four times the size. To make the whole area the same brightness, you need four times as many photons.

A one stop change in distance is 1.414 times the original distance. Or .71-ish times the original distance (in the other direction). If you note, f/7.1 and f/14 are one stop changes from f/10.

Neither the antares103 quote above or any of the other replies in this thread have proposed that 1 + 1 equals more that 2.

No magic involved with GNs, just math.

  • John
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