Now if only i knew the GN of the YN460.

This is hear-say, I have no clue myself, but I have heard mention of it being 33 (meters), which the person felt was more accurately used as 24 (meters). 33 m would be 108 (feet), and 24 m would be 79 (feet). That is about 1 stop difference.

It is easy to determine Guide Number yourself. Your own number will probably be more accurate in your situation anyway, than the spec number which may sometimes be optimistic. The Nikon charts might be a stop optimistic in places too, whenever you have to compensate them.

Measure off ten feet between flash and a subject (or 6 or 8 or 12 feet, it does not matter, but know your distance carefully). Shutter speed is not a factor, use any shutter speed you normally use with flash. Experiment taking test pictures to find the correct exposure aperture there. Like at full power level, determine whatever aperture actually gives the correct exposure at the measured distance, like 10 feet. Maybe it turns out to be f/11 (your call what is a "correct" exposure, however you want your pictures to look).

Then the Guide Number is 10 feet x f/11 = GN 110, in this example.

Could not be simpler. :smile: If you use feet, your GN will be for feet. If you use meters, your GN will be for meters. There are 3.28 feet in one meter, so the GN in feet will be 3.28x more than the GN in meters.

This is all GN is, distance x aperture number which actually will give the correct exposure. Next time you are at 10 feet, you can compute GN 110 / 10 feet = f/11 again. This is a constant, and then for that flash power, this GN constant automatically handles the inverse square law. So that if some new distance was going to be 15 feet, then the correct aperture for correct exposure at this flash power level will be GN 110 / 15 feet = f/7.3.

The difficult tricky mathematical part is knowing what f/7.3 means. It happens to be about 2/3 stop past f/5.6, towards f/8. But we can only set the camera lens to f/5.6, f/6.3, f/7.1, or f/8 (third stops), so this is not much issue, it should be clear enough, and it is not a concern about measuring the Guide Number.

This is how the original guide numbers are determined, experimentally to see what it actually is. That is all there is to it, for any flash. This is demonstrated to be the actual guide number which gives the correct exposure (aperture and distance) for the amount of light that the flash creates.

The one issue is that if the flash has zoom or power level settings, it will have a new GN for every combination of those settings.

One other issue that could be good to know in this case is that the f/5.6 that we know is actually f/5.7, and f/11 is actually f/11.3. We just round them off and call them 5.6 and 11, but the precise number in calculations is 5.7 and 11.3. So, maybe plan things out to compute GN using f/4, f/8, or f/16, which are more exact numbers.