Why does the guide number equation work?
Remember the guide number equation: GN = aperture * distance. Does it seem completely mysterious that this simple equation should work? And how is it that aperture and distance have any relationship, let alone so simple a relationship? First off, if you’re rusty on aperture numbers and what they represent, brush up on the topic with digression #1. If you’re up to speed, then skip it. Now to remind you of something you may have encountered in a high school physics class. Light intensity falls off like the square of the distance from the source. That is, I = I0 / r2, where I is the intensity measured at any point, I0 is the intensity of the light measured at the source, and r2 is the distance between the point in question and the source of the light. All this means is that if you double your distance from the strobe, the light is 1/4 as intense. Triple the distance, and you’re down to 1/9 the brightness. Let’s say you measure the distance between the strobe and the subject to be 2 feet a
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