How to use a flash guide number
Guide number divided by distance gives the aperture. That one relationship is all manual flash requires.
A guide number expresses a flash's power as a relationship between distance and aperture. Divide the guide number by the distance and you have the aperture for correct exposure.
Step-by-step
- Enter the guide number and the ISO it was quoted at.
- Enter distance or aperture.
- Read the other.
Read the conditions attached to the number
Guide numbers are quoted at a stated ISO, in stated units, and usually at the flash head's most zoomed-in setting. Manufacturers naturally quote whichever combination produces the largest figure, so two flashes with the same headline number are frequently not equivalent.
Check the ISO, check metres versus feet, and check the zoom position before comparing.
The inverse square law
Light falls off with the square of distance. Doubling the distance loses two stops, not one.
The practical consequence appears in group photographs: if the front row is at two metres and the back at four, the back row is two stops darker. Moving the flash further back reduces the relative difference and evens the light — which is why a flash close to a group gives such uneven results.
Shutter speed does not affect flash exposure
The flash is far shorter than any usable shutter speed, so the whole burst lands whatever the shutter does. Shutter speed controls the ambient exposure only.
This is the key to balancing flash with daylight: set the aperture for the flash and the shutter for the background, and they are independent. The only limit is the sync speed, above which the shutter is never fully open.
Frequently asked questions
How do I compare two flashes' guide numbers?
Check they are quoted at the same ISO, the same units and the same zoom setting. Manufacturers quote the most flattering combination, so headline numbers are often not comparable.
Why is the back row of my group shot dark?
The inverse square law. Doubling the distance costs two stops. Moving the flash further from the group reduces the difference between front and back.
Does shutter speed change flash exposure?
No, only ambient exposure. The flash burst is far shorter than the shutter. That independence is what lets you balance flash against daylight.
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