Light Meter 101

Here is an introductory explanation of light meters for ultra low budget filmmakers.

Light meters are used to measure the intensity (brightness) of light so that a cinematographer can:

1. determine proper exposure when shooting on film
2. calculate lighting ratios.

While light meters are not normally used to set exposures when shooting on video, many DPs will still use them to calculate lighting ratios.

There are two types of light meters: incident meters and spot meters. Incident meters measure the amount of light falling onto the photosphere of the meter. Spot meters measure the amount of light reflected off an object.

Usually, an 18% gray card is used as the object light is reflected off of because most spot meters are calibrated to obtain readings from 18% gray.

Before using the light meter, the cinematographer must enter the ASA of the filmstock which he/she is using and enter that into the light meter. The ASA (also known as ISO or speed) of the film is a measure of how sensitive that film is to light.

The ASA is usually between 50 (designed for bright daylight) and 800 (designed for dark, tungsten lit interiors). For low budget filmmakers, medium and higher speed filmstocks (ASA of 250 and above) are the most common because they allow for fewer or smaller lights to be used.

After the ASA is entered and the cinematographer measures the brightness of the light is with the light meter, the meter will display the recommended exposure in the form of an F-stop.

(Please note that calculations for exposure and depth of field are made with F-stops, while lenses are calibrated with T-stops. For virtually all applications, the two measurements are interchangeable.)

Most light meters will also allow you to directly input frame rates for when you're shooting at speeds other than 24 fps - a very handy tool when it's 3 a.m., you're dead tired and can't do the mental math.

On the set, there's little difference between an incident meter and a spot meter. Cinematographers use whichever one is most appropriate for a given situation.

Incident meters are most commonly used to determine proper exposure for actors under most circumstances . Spot meters are most commonly used for objects or people which cannot be easily reached to take an incident reading. Most cinematographers and gaffers (the chief lighting technician) own both an incident meter and a spot meter.

I own a Spectra Pro IV incident meter and a Minolta Spotmeter F. These are the two I usually recommend to others. Some people like the Sekonic L-608 meter, which is a combination incident and spot meter. I prefer two separate meters - in case something happens to one of them, I still have the other as a back up.

(I also have a 3rd meter, a Sekonic footcandle meter, as a back up in case something happens to both of my meters. It doesn't use batteries, and my ASC manual has a footcandle to F-stop conversion table in it.)

Jessica Gallant
Los Angeles based Director of Photography
West Coast Systems Administrator, Cinematography Mailing List
http://www.cinematography.net

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