What is a Guide Number (GN)
Let’s start by stating what Guide Number is not. A Guide Number is not a measure of the power of a Flash.
A Guide Number is an indication of the power of a flash. The formula for calculating Guide Number is:
Guide Number = f-Stop x Distance
Seems simple enough, but there are other factors to be taken into account.
- Zoom setting on the Flash Gun.
Some speedlites have a zoom head for use with different focal length lens. This zoom function can send the light from the speedlite further than a non-zoom head. - Camera ISO setting.
The guide number for a Speedlite is usually quoted at ISO100. This is a camera setting, and if your camera is set to a different ISO or is on Auto ISO, then the GN you are using is wrong. - Light modifiers.
Are you using flash bounce? Are you using diffusers, softboxes, umbrellas, etc.? The GN for a flash assumes you only have the bare flash with no modifiers. If you are modifying the light in any way then your GN is wrong.
The GN of the Yongnuo YN-460 is 38m at ISO100. Let’s say you want to shoot a subject at 3 metres distance using the YN-460 at full power. What f-stop is required?
f-stop=GN/dist
f-stop=38/3 = 12.6 (let’s use 11.0 as the closest whole f-stop). When you calculate the distance using this f-stop with the GN you get…
dist=GN/f-stop
dist=38/11=3.45m
If you carry out the same calculation for the pop-up flash in a Canon 750D (GN=13)
dist=GN/f-stop
dist=13/11=1.2m
The guide number system has become nearly superfluous due to the ubiquity of electronic flashguns featuring variable flash output and automatic exposure control, as well as digital cameras, which make it trivially easy, quick, and inexpensive to adjust exposures and try again.[3] Still, guide numbers in combination with flash devices set to manual exposure mode remain valuable in a variety of circumstances, such as when unusual or exacting results are required and when shooting non-average scenery.