Guide Numbers

By Marty Snyderman

This month's question is from Chris H. of Miami. Chris asks, "What is a guide number for a strobe and what do I do with that information?"

In short, a guide number describes the relative power of light output by a given strobe, and the information is used to get proper exposures when strobe lighting a subject. The number can usually be found in the specifications literature provided by the manufacturer. Be sure to note whether the guide number provided is for use when shooting on land or underwater. If you are given a topside guide number, divide by three to obtain an approximate underwater guide number.

Often you will find several guide numbers provided for a single strobe. This happens because a guide number is provided for a specific film speed. The guide number will change as film speed changes. The higher the guide number, the more powerful the strobe. Typical guide numbers for ISO 100 films range from 15 to 52.

A guide number is used in two versions of what is known as the exposure formula in order to help you determine the correct f/stop or strobe-to-subject distance. It is used to determine the exposure of a strobe-lit foreground subject, but not about the exposure of any water in the background of your shot.

Let's work through an example: a sponge on a reef at night. Assume that for the film used, the guide number for his strobe is 24. If the strobe is three feet away from the sponge, the correct f/stop is f/8. The guide number 24 divided by the strobe-to-subject distance of three feet equals eight, the correct f/stop. Easy!

In wide angle, daylight photography, when you want to properly expose water behind your subject, first take a light meter reading on the background water to determine your f/stop. Then use the second version of the formula to determine the correct strobe-to-subject distance: guide number divided by f/stop equals the correct strobe-to-subject-distance. For either formula, be sure to bracket your exposures.

If you have a photographic question for Marty, please e-mail to If your question is selected, the answer will appear in a future issue of Skin Diver.