I am sure that many readers out there are aware of the Inverse Square Law already but it’s extremely useful and saved mine and Dave’s butts the other day.
Inverse Square Law: when an object that is twice the distance from a point source of light, the object will receive a quarter of the illumination.
The reason why the strength of the light diminishes so rapidly is not because the light runs out of energy, but because the beam of light spreads out. As it spreads, a smaller and smaller portion of the light hits the object.
So, David and I were working on a shoot (Dave, the DP and I was the Gaffer) where we needed about 25 feet of fairly consistent light – in the stop or a stop and a third range, in terms of how consistent it had to be – and we had a Senior (5K tungsten Mole Richardson) head shooting through an 8by8 frame of LT Grid. We had about a 2 and 2/3 stop difference from the beginning of the shot to the end it. It was just too drastic and we couldn’t live with that so (very luckily, I might add) we pulled out the Tenor (10K tungsten MR head) and put it about 25 feet farther back. We got what we wanted, only 1 stop difference, maybe a stop and a third, from the beginning to the end.
Basically, if you are fortunate enough to have the budget to rent bigger fixtures to accomplish these kind of situations, you will see the benefits immediately, especially from night shoots where you need a consistent single source.
Cheers mates!
– Peter Mosiman