Using the light meter

- We've talked a bit about the quality of light. Now let's talk about lighting quantity. The first step in controlling anything is to first figure out how to quantify it. Using a light meter is how we quantify lighting. How much light do you want? How much light do you have? This matters, because quality is directly related to quantity. A certain amount of light feels a certain way that's different from more or less light. Exposure is the camera's response to a given amount of light, so that's a pure measurement of quantity. There are two types of exposure meters. There's the incident light meter, and then there's the spot meter. The incident meter measures illuminance, or the amount of light falling on the subject. In the United States, a foot candle is the unit of measure of illuminance. In the international system, it's measured in lux. The incident meter promotes consistency in exposure from shot to shot and scene to scene. Now, the spot meter measures luminance, the amount of light emanating from the subject. But most often, it's related to the reflectance of the subject. How bright or dark the dye, paint, or pigment and the surface quality of the subject all contribute to its luminance. The unit of measurement of luminance used in the United States is the foot-lambert. In the international system, the unit of measurement is the candela per square meter. Now reading a meter is simple. Interpreting what the meter means is where most people run into trouble. And this is all at the core of the art of lighting. Now the incident meter can be read with either a sphere or a disk. Now the sphere allows you to integrate all the lights together, which gives you an exposure reading. However, for lighting, we use the disk. With this particular meter, we simply turn the dial, and it drops the sphere down, and becomes essentially a disk. Now that allows us to find out how much light is coming from that particular light as opposed to all of the others. That requires interpretation. For instance, in my interpretation, the, well, the key light is 90 degrees off axis from the camera. I consider that, that requires the key light to be increased by about a half a stop over what it otherwise would be if the key light were closer to the lens. However, if the key light were overhead, I'd consider the need to increase that by a full stop. Now, every cinematographer has to make a determination of what that means for him or her. Now the spot meter requires a bit more complex interpretation. The spot meter will give you a reading of whatever's in front of it as if you wanted to interpret that at 18% gray. Obviously, the world isn't uniformly 18% gray, so we have to decide where we want to place values. What the spot meter means depends on the characteristic of the particular camera, in other words, the level of contrast of the camera, and that's one of the things that we determine when we're doing our tests. Knowing how to control light can either make or break the look of your movie. Meter skills are the cinematographer's best friend, because information is power. The light meter yields an enormous amount of information to control your work with great precision.