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:
- getting the most out of your flash / speedlite / speedlight
- practical tutorial: controls for manual flash exposure
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?
