I see and hear lots of questions about “guide numbers.”
First off, guide number aren’t used for studio flashes (packs and monoblocks) because which modifiers (reflectors, snoots, soft boxes, umbrellas, …) you use affects it enough to render any guide number useless . Read on and you’ll understand.
As you read, remember that shutter speed has almost no effect on flash exposure. Shutter speed can control the amount of ambient light that contributes to your image, but a flash is generally around 1/1000 of a second and I doubt your sync speed is faster than that (sync speeds are usually more like 1/250).
Learning to balance flash power with ambient light helps preserve the mood in a shot instead of just blasting the whole scene with flash.
So, with shutter speed out of the equation, what’s left? f-stop and ISO. Guide numbers are usually listed at ISO 100. Lately, Nikon has been publishing their guide numbers for ISO 200 because that’s the native sensitivity of their sensors. More on ISO later.
Also, be careful and check your units. guide numbers may be listed measured in feet, meters, or both. The units will be listed along with the ISO that the number is valid for. You’ll probably notice if you use a guid number for the wrong units because your numbers will be so far off you’ll think “Now that CAN’T be right).
The guide number of a flash is simply f-stop times distance (GN = f*d). This means that I can take a Vivitar 285HV which has a guide number of 140 (feet @ ISO 100) place it 13 feet from my subject and shoot at f/11 (GN/d=f, 110/10=10.77). If I’d rather shoot at f/16, I can move my light to just under 9 feet (GN/f=d, 140/16=8.75).
To convert the numbers for different power settings and different ISOs, first determine your flash and camera settings for the guide number, then convert for the settings you want to use (refer to my last post for that). If you’re going to move the flash around to change the setting, you’ll need to know about the “inverse-square law.” I’ll do a post on that later.