Flash Guide Number Calculator
A flash guide number describes how much light a flash puts out, and it links three things together: guide number equals the aperture multiplied by the distance to your subject. This calculator lets you solve for whichever one you need. Enter a guide number and a distance to find the aperture that gives a correct flash exposure, or enter a guide number and an aperture to find how far the flash will reach, or enter aperture and distance to work out the guide number of an unknown flash. Guide numbers are quoted at ISO 100, and if you raise the ISO the effective guide number grows, so the tool includes an ISO adjustment. This is invaluable for manual flash work, off camera lighting, and understanding why moving a flash closer brightens a shot so quickly. All math is done in your browser with nothing sent to a server.
How to use Flash Guide Number Calculator
- Choose which value to solve for: aperture, distance, or guide number.
- Enter the two known values.
- Select meters or feet.
- Read the calculated third value for correct flash exposure.
Frequently asked questions
What is a flash guide number?
A guide number, or GN, expresses flash power as GN equals aperture times distance at a reference ISO. The tool solves for any one of aperture, distance, or GN from the other two.
How do I use GN to set aperture?
Divide the guide number by the flash to subject distance to get the aperture for correct exposure, which the calculator does for you in meters or feet.
Does ISO change the guide number?
Yes, guide numbers are quoted at a base ISO, and raising ISO effectively increases the usable GN, extending reach or allowing a smaller aperture.
What is Flash Guide Number Calculator?
A flash guide number describes how much light a flash puts out, and it links three things together: guide number equals the aperture multiplied by the distance to your subject. This calculator lets you solve for whichever one you need.
Is Flash Guide Number Calculator free to use?
Yes. Flash Guide Number Calculator is completely free, with no sign-up and no usage limits.
Does Flash Guide Number Calculator work in a web browser?
Yes. Flash Guide Number Calculator runs in any modern web browser. There is nothing to download or install.
Is my data private with Flash Guide Number Calculator?
Yes. Flash Guide Number Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.
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