Well, I'm learning that the in-camera meter won't help you as much with lighting balance, especially if you're using multiple sources.
The following was written by Randy C. from Houstonfilm.org. I found it very useful:
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Any light meter that can read incident and reflective light will work when lighting for video. It can even be a photography meter, but choose a digital meter that displays the f-stops. This will allow you to read the meter quickly. I use a Sekonic 558 meter because it reads incident light and has a spot attachment built-in to read reflective light.
When using a light meter, it doesn't matter what format camera you are shooting with. To calibrate the meter accordingly, you must refer to your owner's manual to get the lux rating of your camera. With most new cameras, the sensitivity will be f-11 @ 2000 lux. If your camera sensitivity is f-11@2000 lux, set your meter at ISO 800 when shooting indoors and at ISO 500 when changing the color wheel filter for shooting outdoors. If you have an older camera that has a lux rating of f-8 @ 2000 lux, then your ISO will be 400 indoors and ISO 250 out doors. You will need to enter the ISO information into the meter along with the shutter speed (or _frame_ rate if the meter is made for cine or Motion picture). The shutter speed is normally 1/60. ISO + Shutter Speed = f-stop
You will get a difference between the incident and reflective because the incident measures the light coming from a source and renders it as 18% gray. So what you are reading, when using an incident meter, is the uninterrupted light output that is reaching your subject at a specific distance (which is also foot-candles). Reflective readings render the amount of light that the camera will be sensitive to AFTER it is altered by the color or density of whatever it reflects off of. Darker tones will render a reading that will indicate that you should use a wider aperture setting while lighter tones will render a reading that will indicate that you use a smaller aperture setting. This is because the meter assumes you want, whatever you point it at to be 18% gray or the same as your key or subject light.
You are also able to use a spot meter to read luminance on a set. Once you establish, with your incident meter, what f-stop your key light will be set at, in order to render our subject at 18% gray or let's say 80% on a waveform monitor, all we need to do to be able to control other reflective light in a scene is to measure it with a spot meter and adjust it's output until it matches that of the key light or slightly over or under as desired.
By understanding the contrast range and capabilities of the format camera that you are using, and then using a light meter to measure the ratios of a scene, you will be able to control the high lights and shadows scene to scene so that little adjustment will be needed to be done in post.
• CONVERTING "MINIMUM LUX" TO ISO.
Trying to fiqure the ISO for a mini DV camera when all the manual gives as a reference is a minimum lux can be a bit confusing, so here is some math to help fiqure out what the ISO may be.
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Using my camera's sensitivity of f-11@ 2000 lux and it's established ISO of 800 as a reference, we can illustrate how to find the ISO from a minimum lux rated camera.
If we already know that f-11@ 2000 lux is the maximum illumination for an 800 ISO camera, and we want to find out at what f-stop and lux rating my minimum illumination is for that same camera, all we need to do is run down the math as follows:
2000 lux = f-11
1000 lux = f-8
500 lux = f-5.6
250 lux = f-4
125 lux = f-2.8
60 lux = f-2
30 lux = 1.6
15 lux = 1.6 with 6db of gain (every 6 db of gain = 1 stop)
8 lux = 1.6 with 12 db of gain
4 lux = 1.6 with 18 db of gain
It seems to me that If I were to include my maximum gain setting of 18db as part of my minimum illumination, my 800 ISO speed camera would have a minimum illumination of approximately 4 lux which would put a minimum illumination of 3 -5 lux. Our example is pretty darn close, if not exact.
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