Re: Got a flash, don't know how to establish it's GN?

Method A
Get 10 feet from the flash and measure the output with a flashmeter.
The required f-stop multiplied by 10 give you the GN in feet.
Multiply by roughly 2.85 to get the GN in meters.

Method B
Set an object 10 feet from the flash.
Use a test roll of b&w film and shoot the object across the aperture range.
Pick the best exposure/aperture and multiply by 10 for GN in feet.
Multiply by roughly 2.85 to get the GN in meters.

Method C
If the flash has a scale on the back with intersecting feet/f-stop
metrics, just multiply any two matching values.  This will give you
the guide number.