Method to determine power of light in a space

[David Edward Keen]

I will capture an actor walking down the street in Brooklyn, NY, his walk ending at a beautiful old theater.

 

I need key him along his route as he walks....

 

I've read about the inverse square law, and I know in my experiences so far just how quickly the light seems to fall off.

 

Must be battery powered LED

 

Is there a way to figure this out now mathematically?

 

Camera is just a Rebel T3i. I could use a 50mm (80mm on my sensor)1.8, a 85 mm (136 on my sensor) 1.6 or the kit lens 18-55 3.5-5.6

 

 

Cheers & Happy Holidays

Edited December 21, 2016 by David Edward Keen

[David Edward Keen]

i guess there is an edit-your-post-limit? Talent will start his walking towards a stationary camera from 30 ft back.

[David Mullen ASC]

You're not going to cover someone with a 30' walk from far to near with anything close to an even source with a single battery-powered LED, but you probably shouldn't anyway because on a street at night, people move naturally from being lit by one source to another -- an even key light over a 30' distance is fairly uncommon unless there is a huge source of light in the location.

 

Certainly it's possible to cover a 30' walk evenly, there are a number of ways but most require starting out with a bigger light from farther away to reduce the rate of fall-off, and then if necessary you can use net flags and and a half-scrim in the light to bring down the intensity gradually as the actor gets closer to the source.

 

Instead of an even key, you could, for example, have him start out in one key light from an LED, walk out from that and into a second key as he stands in front of the theater. A third light could provide a backlight / edge (or a dimmer side light so that he never goes dark) mainly for the middle of the walk when he passes between the two sources.

 

Or of there is enough natural light on the street, you can just have him walk from a general underexposed available light and into your key as if the theater is lighting him.

[David Edward Keen]

ok thanks a lot!

[David Mullen ASC]

The traditional way to fight fall-off so that an actor is evenly exposed over a walk is to shine the light at a 3/4 front angle, let's say, and then figure out how to reduce the brightness as they get closer to the light. Usually you use a series of single-scrim net flags so to feather off the intensity.