I was talking with a director about a moonlit scene along a beach here in Southern California. He said the producer wanted a scene where he was walking down the beach after dark. The question was how to light it and how much would it cost? That's what leading me to this particular blog post this week.
First, some basics. Light from a light fixture falls off in as the inverse squared. Confused? Let me explain. Let's say you are standing 10 feet from a standard Arri Fresnel lamp and you take a meter reading. The meter says T5.6 (no f-stops!...we're filmmakers here, not still photographers). Then you move to 20 feet from the lamp. The meter should now say T2.8. The reason? You moved 2x the distance from the lamp therefore the comparative amount of light is 1/(2squared)=1/4. That's 2 stops less light. Now let's say you moved out to 40 feet (2x again) from the lamp and take another reading. The light meter should read T1.4, i.e. 2 stops less again. The main point I want you to take away is that every time you double your distance from the lamp you have 2 stops less light.
The other point is that as you get further and further from the lamp the light fall off becomes more and more gradual. Just look at the way the graph is trending. Think about the sun and moon. It's a large lamp a long, long distance away. Have you ever noticed an exposure difference by moving 10, 50, or 100 feet back from the sun? How about a mile?
The other basic concept for any cinematographer is how to calculate what size of lamp you need give the essential exposure parameters of ISO, shutter speed, and lens T-stop. I've wrote about this before, but here is the summary again...
foot-candles = 25 x (T-stop)^2 / (ISO * shutter time)
A quick way to calculate the required lamp size is to remember that T2.8 at ISO 100 and 1/48th shutter (i.e. 180-degree shutter at 24p) takes 100 foot-candles. You can see how to quickly do the math in your head from there if you know the formula.
So back to the problem at hand...
I figured along this particular beach we want to light about 1/10th mile (528 ft) of 100 foot wide beach to sell the light as moonlight. As a starting point I'm going to assume a modern camera with ISO 800, a T2.8 aperture, and 24p. Moonlight is never fully lit like sunlight, so let's assume that the person is exposed 2 stops under. This means I need enough light to properly exposure at T1.4, i.e. 2 stops less than T2.8.
This gives the following table of required foot-candles from our fixture...
Distance (feet) T-Stop Foot-Candles (assume ISO 800, 1/48th shutter, T1.4)
528 T1.4 3.125
264 T2.8 12.5
132 T5.6 50
66 T11 200
33 T22 800
16.5 T45 3200
8.25 T90 12800
Now let's look at the photometric chart for a Mole-Richardson 4k HMI PAR fixture.
The medium setting on the beam focus seems to give us just over 3200 foot-candles at 20 feet. This roughly matches our requirement at 16.5 feet. Also the narrow beam focus greatly increases the output, however you need to keep in mind that we need to illuminate a 100 foot wide section of beach so the background doesn't quickly go to black and ruin the effect.
What this doesn't take into account is atmospheric haze, the light position, and gels. There's a diffusion effect from the atmosphere, so you may want to consider moving up to a 6k PAR from a production margin standpoint. I want the light to be about 528 feet back from the subject we are lighting. The light will require a crane and generator, which needs to be parked somewhere and you won't necessarily have a spot exactly 528 feet away to park! Also, moonlight tends to be very blue so you may want to gel the light with a CTB of some sort. That will significantly drop the light output. You may then want a 10k or 12k PAR.
The main point of this exercise is to show why you may want to consider a "big" light sometimes. Cinematographers commonly use 12k and 18k HMI lights and it's not because they want to be Hollywood fancy-pants blowing through those multi-million dollar budgets. Sometimes you just need a big light, especially when you need even lighting over a large production set. A "Wendy" light is a good example.
If you have questions, feel free to leave a comment.
HOW WE DID IT: Doc Style
2 years ago