Flash Power Output and Distance
by mudanomanFlash Power Output and Distance
By: Ivan Wong (ibwphotography.com)
I think its time to go beyond referring to the flash guide table and to understand the fundamentals of what the Guide Number (GN) means and also the distance to flash power output relationship. Any photographer using manual flash/strobes should have these general ideas in mind, especially in action sports when adjustments are made on the fly.
The first point to be discussed is the Guide Number or GN, which is a measurement of a flash unit’s light output. For instance, the Canon 580EX has a GN of 191 whereas a Vivitar 285HV has a GN of 120. The higher the guide number, the greater intensity of the flash. So how can I benefit knowing this? Well the guide number comes from this equation (assuming ISO 100 film):
Guide Number = F/stop X Flash-Subject Distance
Guide numbers are usually given for ISO 100 speed film. Faster ISO settings will require less light so you can shoot at a greater distance. For this reason, you need a different guide number for each ISO setting. Increasing the ISO film speed by a factor of 4 will double the guide number. When film speed doubles, the guide number changes by a factor of 1.4X; when film speed is halved, the guide number changes by a factor of 0.7X.
Based on the equation the following can be known:
-To calculate the f/stop needed, divide the guide number by the distance to the subject.
-To calculate the maximum flash range, divide the guide number by the selected or largest f/stop.
So for example, I have a guide number of 160 and a flash-subject distance of 20ft, my aperture will need to be f/8. Move the flash-subject distance to 10ft and the aperture will need to be stopped down to f/16.
Another interesting lighting fundamental is the distance to flash power relationship. This is simply: “Doubling the flash-subject distance will reduce the amount of light on the subject to a quarter of its original amount”. This is due to the inverse square relationship of distance vs. light output, but we wont get into that.
I hope this makes some sense. I will be following up on more lighting discussion as there is much to be discussed and learned.
Best,
Ivan
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