How the Flash Guide Number works
f-number from a flash unit's Guide Number, distance, and ISO.
Where to find it
Tools tab Flash Guide Number
Summary
Calculator for flash exposure via Guide Number (GN). Combines flash power (GN), flash-to-subject distance, and ISO to give the f-number to set on the lens.
Detail
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How it works
A flash unit has a Guide Number (GN) printed on it that tells you, at ISO 100, how powerful it is. With the GN, the distance to your subject, and your ISO, you can work out the f-stop you need. This modal does that calculation.
Guide Number
Marked in metres or feet (be careful, the two are not interchangeable). A small on-camera flash might have GN 12 in metres at ISO 100. A studio strobe might have GN 60. Some flashes give different GNs at different zoom settings.
Distance
How far the flash is from the subject (not the camera, the flash). For on-camera flash this is roughly the same as camera-to-subject distance. For off-camera flash, measure the flash-to-subject distance.
ISO
Pulled from the main wheel. Doubling ISO doubles effective flash power, which means you can stop down by one stop at the same distance.
The math
f-number equals Guide Number divided by distance, scaled for ISO. The modal shows the exact f-number to set on the lens so the flash exposes the subject correctly.
Watch out for
GN assumes a normal indoor reflective environment. In a black studio or outdoors at night you need extra power. In a small white room you can stop down further. Adjust by half a stop or so based on the room.