using the guide number of your flash to determine flash exposure
GN = distance * f-stop
Your flash’s Guide Number (GN) is determined at 100 ISO, when it gives correct exposure at a certain distance, multiplied by the f-stop
The idea that we can figure out the manual flash exposure by the combination of distance and aperture (for a given ISO setting), was covered in these recent topics:
In these articles, we relied on the display on the back of the speedlight to show us the distance we need to hold the flash from our subject. A flashmeter / lightmeter would’ve given us a similar answer. (There might be a discrepancy, since the manufacturers tend to be a little bit optimistic about what the flashgun is capable of.)
Now, the question is, what do we do if our speedlights don’t show the distance / aperture relationship on its display, or if we don’t have a lightmeter on hand?
Well, if we want to be more methodical than just looking at the back of our camera display, we can use the Guide Number of our flash.
Keep in mind that the GN is given for a specific zoom setting on the flash-head. Zooming from wide to tele on our flash, changes the Guide Number!
The specs for the Nikon SB-910 speedlight (B&H) is given as:
Guide number of 34/111.5 (at ISO 100, m/ft., 35-mm zoom head position, in FX format, standard illumination pattern) for high flash output volume
This means that our GN (in feet), is 111.5
So if we are using our flash at full power, at say 10 ft from our subject, then we have to use an aperture of 111.5 / 10 … which gives us an aperture of f11
The specs for the Canon 600EX-RT speedlite (B&H)is given as:
Guide Number: 197′ (60 m) at ISO 100
(None of the websites that I’ve seen, currently says at which zoom setting this would be. I’d guess this would be with the zoom head at maximum focal length. It makes the flash sound so much more powerful on the spec sheet, than if quoted at a wider focal length. Marketing people!)
Working through the example again, if we are 20 feet away from our subject, then our aperture is 197/20 = f10
Simple as that. The math isn’t scary. We all have smart phones with us, and they all include a calculator.
If you want all of this in a handy form on a website, here is the Flash Power Calculator on Canon Europe’s site.
And if you want to read up more about Guide Numbers for flash, the Wikipedia entry is thorough.
Now the question remains … what is the Guide Number of your flash?
Time to find that instruction manual again! But firing up Google might just be easier.