Underwater Photography Glossary
Browse Terms
Guide Number
Technically the Guide Number = f/stop x distance (specified in meters or feet) and is ordinarily expressed at ISO 100. So a guide number of 22 (ISO 100/meters) can expose a subject 1 meter from the camera at f/22, or 2 meters at f/11. These are guidelines and rated for shooting in air versus water, which of course yields a significantly lower effective underwater guide number due to absorption.
Sponsors
