Project 55

Having studied Physics to postgraduate level the inverse square law is very well know to me, but this was a fun exercise and reinforcement is a useful educational tool. .  For this exercise I placed my primary lighting tool, a 400W Elinchrom studio flash head, on a lighting stand pointing out of my patio window.  I used the naked flash gun without any device to modify the light, trying to create more or less a point source.  I then measured the distance into the communal garden of m apartment complex and using a flash meter with radio control of the flash head started to take measurements of the light.

My neighbours must be starting to wonder what I am up to, watching a guy walk backwards onto the lawn occasionally firing a large flash gun at himself.  As suggested in the text I set the exposure meter to ISO 100 and 1/25s.  The following chart shows the measurements:

The curve is a classic exponential curve, reflecting the fact that the light intensity is falling of at the square of the distance.  In effect every doubling of the distance drops the light by 2 stops.  This is only obvious with relatively close and small point sources.  In fact the exact same is true of the sun, however, as the distance from the sun is so huge, any change we can make in our distance from it is infinitesimally small and not detectable.