How do I work out the guide number of an old flash at ISO 100?

Question

I have an older Consul 400 flash distributed by Braun North America and want to estimate its guide number at ISO 100. If a correct exposure at 10 feet requires f/4, what would the guide number be, and how is it calculated?

user2719 · Answer

The guide number of a flash is simply the aperture times the distance (for a proper exposure at a given ISO). With the figures, you've given, the guide number of the flash would be given as:

GN 40 (ft)/12 (m) at ISO 100

That would be consistent with the model number (which, for a surprisingly large number of flash models from various manufacturers, is often ten times the claimed ISO 100 guide number in feet).

You would use the guide number to figure out the aperture required for a correct exposure. For instance, with the camera set to ISO 100 and a subject 5 feet away, you'd divide the guide number (40) by the distance in feet (5 - you can usually get that information from the distance scale on the lens or from the LCD panel on the camera if the flash is camera-mounted; you'd need to measure the distance for off-camera flash) to come up with the aperture you need to set (f/8).

How do I work out the guide number of an old flash at ISO 100?

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