Flash guide number calculator
GN = aperture × distance. Enter two, get the third for manual flash.
About this tool
A flash guide number links aperture and distance so you can set correct manual flash exposure. This tool takes the guide number plus one known value, either distance or aperture, and returns the other. It is the key to nailing manual flash on film where you cannot chimp the result on a screen.
How to use it
- Enter the guide number of your flash at the film ISO in use.
- Enter the value you know, either the flash-to-subject distance or the aperture.
- Read the missing value the tool calculates.
- Set that aperture on the lens, or reposition to the calculated distance.
FAQ
What is the guide number formula?
Guide number equals aperture multiplied by distance, so aperture equals the guide number divided by the distance. Doubling distance requires opening up two stops.
Does the guide number change with ISO?
Yes. Guide numbers are quoted for a specific ISO, usually 100, and rise as film speed increases. Confirm the ISO your guide number is rated for.