From this post, I understand that the flash guide number (GN) is given by the following formula:
Guide Number = Shooting Distance × f-number ÷ ISO factor
I'm assuming that the GN calculated via the equation above will be based on a flash that is firing at full power. Am I correct to assume that if I set the flash to fire at half power, that the guide number is effectively halved? i.e. the equation will now be:
Guide Number = Shooting Distance × f-number ÷ ISO factor * Power Level
where power level can be the following 1, 1/2, 1/4, 1/8, ... 1/64, etc?
Answer
The guide number is inversely proportional to the power squared. This is due to the way that light intensity diminishes with distance, at twice the distance light is spread over four times the area, so each bit of that area receives 1/4 of the light.
So the actual formula needs to take into account the square root of the power level:
Guide Number = Shooting Distance * f-number * (Power Level)^0.5 ÷ ISO factor
The guide number has the same inverse square relationship to the sensitivity, as detailed by the ISO factor, defined as follows:
ISO factor = (ISO / 100) ^ 0.5
Substituting and bringing sensitivity onto the top of the fraction gives a formula which you can simply plug numbers into:
Guide Number = Shooting Distance * f-number * (Power Level)^0.5 * (ISO/100)^-0.5
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