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I have an old Consul 400 flash. It was distributed by Braun North America. I would like to figure what is the GN number at ISO 100, f/4, 10 feet.

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Can you clarify something for me? Does it say that the guide number is 10 feet at ISO and F/4.0 and you're trying to figure out what that means? Or, are you trying to determine the power of the flash? –  mattdm Apr 1 '12 at 0:55
    
@MikeW — your edit actually makes my question about the question more ambiguous. I think it's meant to be what does "GN at ISO 100, f/4, 10 feet" mean, not what is the guide number of the flash with these settings. Putting the "is" where you did (and where I did in my initial edit, which I then rolled back) tilts the interpretation towards the latter, and I'm not sure that's right. gjw, can you explain a little more? Thanks! –  mattdm Apr 1 '12 at 1:49
    
I rolled back that change - yeah not sure –  MikeW Apr 1 '12 at 9:00

2 Answers 2

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).

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With you camera set to ISO 100, take pictures of a subject 10 feet away, adjusting the f/stop until the exposure is good.

Then multiply 10 (feet) by that f/stop. If f/5.6, then the guide number is roughly 56. If it's f/4, then the guide number is 40.

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