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To some, the science of cinematography is like that pile of peas on your plate. You know they're healthy - but you rarely go there first. Blame it on teachers. It’s an art to make complicated concepts fun, accessible and useful. In workshops or in the classroom, I teach the inverse square law in reverse by attempting to light a room with several low wattage instruments. In short order, it’s obvious that the lights are too close for a wide shot, plus the fall off and multiple shadows are unacceptable. Solution: larger lights placed farther away, preferably off the set completely. Suddenly, there’s more room on the set for the actors to roam about and many more options for camera placement, grip nets and flags. Here’s a basic example:
Lets say that this single candle is a 250 watt movie light - 12’ away from an actor. It provides sufficient illumination, F 5.6 - but is way too close, completely in the way of camera and also severely limiting the range of actor action. We need more room, so lets move the light farther back.
Now the light is 24’ feet away - twice as far from the actor. And look how much more room we have for camera and performance! We’ve solved one problem but created another. At twice the distance, this lamp is now 4 times less intense which means the actor is now 2 stops under, reading now at a 2.8. If we want to shoot at a 5.6 for depth of field considerations, we’re going to need a bigger light, in fact, one that’s 4 times brighter.
So with 4 candles, we have equaled the illumination needed for our desired exposure. In other words, if we swap out our 250 watt unit with a 1000 watt or 1K fixture, we’re back in business! Going with a light that’s four times brighter has given us twice as much room to work with.
For night exteriors, I’ll order the largest lights the budget will allow and put them as far away as is practical to create a vast shooting area with very little falloff. Remember though, the farther away you go, the higher the light must be to avoid camera flares and glow from the unit. Once you start going higher than an 80 foot lift things start to become real expensive. If I had my choice I would rent150’ lifts but then, I wouldn’t be ever be hired again. Knowing this Law can help you plan your next shoot by maybe renting a few larger units than usual. Besides if you don’t use them you can always return them to the rental house.
The definition of the Inverse Square Law is: Double the distance from the source, lose four times the intensity. This universal law also applies to sound, force, radioactivity and electromagnetism.