The Fujifilm EF-X500, for instance, has a guide number of 164 when measured in feet (the guide number would be 50 when measured in meters). Does this mean the flash works at 164 feet from the subject? Not really. What it means is that if you divide the guide number by the distance from the subject, you’ll get the appropriate aperture to use for a normal exposure at ISO 100. So with the EF-X500 at full power and zoomed to 105mm, with the subject standing 10 feet from the flash, the correct exposure would be f/16.4 at ISO 100. Move that subject out to 20 feet and you’ll need to shoot at ƒ/8 for an accurate exposure. Move them in to 7.5 feet and the exposure will be ƒ/22.
You see, the guide number effectively is a measurement of exposure at a given distance. The equation is as follows: GN = f/ x D
In this equation, GN is the guide number, f/ is the aperture and D is distance from flash to subject.
So not only does a guide number tell you how powerful one flash is when compared to another, it tells you exactly what exposure it will produce. A guide number of 164 produces f/16.4 at 10 feet, while a guide number of 196 would need f/19.6 for a precise exposure at 10 feet. In this way, photographers can more easily gauge output so they’re comparing apples to apples when shopping for a new flash.
The other way guide numbers can be incredibly helpful is in determining the exact output of a given flash in a specific circumstance. To test this, with a camera mounted to a tripod and a flash mounted to the camera’s hot-shoe and dialed up to full power in manual mode, have a subject stand 10 feet from the front of the flash with a handheld incident light meter on their nose. Fire the flash and measure the output. If the meter reports f/11 at ISO 100, the flash has a guide number of 110.
Keep in mind that changing the internal focus of the flash output changes the guide number. By focusing the light into a narrower beam, it travels farther with greater intensity. So most manufacturers tout their flashes’ guide numbers when zoomed to their maximum focal length. If a flash has a guide number of 196 when zoomed to 200mm coverage, be sure to test it zoomed to 50mm, 28mm or whatever other focal lengths you plan to use with the flash. You can even make note of the resulting guide numbers at various zooms in order to quickly and efficiently dial in manual exposures and make adjustments without the need for further tests or metering.