Learn how to use an incident and spot meter to determine the proper brightness value of your subject.
Light meters are handheld devices that measure either the amount of light falling on the light meter (called an incident light meter) or the amount of light reflected from a subject (called a spot meter). Cinematographers use light meters to determine the brightness of a particular light so they can determine the ratio of brightness between light fixtures, set the proper exposure on the camera, and ensure the light falls within the latitude of the camera.
Light meters have two basic settings that allow you to configure the light meter to the camera settings
• Film Speed/CCD sensitivity – You can set the ASA or ISO of the film on which you're shooting, or if you're shooting digitally, set the ASA of the camera chip. You can find the ASA on the film canister, or by looking up your digital camera specifications online.
• Frame Rate - You can set the number of frames per second the camera is shooting.
Types of Light Meters
There are two types of light meters, incident light meters and spot meters. Although you can purchase these separately, manufactures combine both features into one handheld instrument.
• Incident Light Meter (left) – Incident meters measure the amount of light that falls on the light meter sensor, averages the brightness value, and gives you an f-stop setting. Incident meters work by averaging all the light values together and giving you a reading that will deliver 18% gray.
• You can use an incident light meter in front of your subject to determine the average exposure of all the combined lights, or you can use your hand to shield the light meter sensor to get a reading of one light at a time. This technique is beneficial to determine contrast ratios from one light source to the next.
• Spot Meter (right) – The problem with incident meters is that they do not compensate for the reflectivity of your subject. For example, if you have a fair skinned actor standing next to a dark skinned actor, the amount of light reflected off each of their faces can differ by several stops, even though the amount of light falling on each of them is the same. Spot meters feature a viewfinder that enables you to focus on a specific part of your subject to measure the brightness of reflected light. Although a more accurate instrument when determining exposure, a spot meter requires a little more skill when translating the reading to the proper exposure.
• When using a spot meter, always meter in line with the camera lens. Because a spot meter is reading reflected light, it’s important to take readings from the camera’s perspective so you get a true reading of the light being reflected toward the lens.
• Combo Light Meters - Today's light meters incorporate both an incident meter and a spot meter into one handheld unit, allowing the user to quickly switch between functions. With costs ranging between $400-$900, the light meters are an expensive but necessary tool on set.
Incident light meters (left) read the brightness of light that falls on the meter, whereas spot meters (right) read light reflected off a subject.
How Light Meters Work
Light meters are designed to read light and deliver an f-stop setting that will render the brightness of that light at 18% gray. 18% gray is actually middle gray on Ansel Adam's Zone System (to learn more about this, watch the module on The Zone System). If you're working with an incident meter that averages the light that falls on the sensor, and expose for the resulting f-stop, more often than not, you will achieve proper exposure.
Metering with a spot meter requires a bit more calculations to compensate for the actual brightness of an object or subject. For example, if metering a pale caucasian actress, a spot meter reading of her flesh tones will result in 18% gray, so by setting the iris exactly to the meter reading, her flesh tones will appear too dark. You'll need to determine by how many stops you need to open the iris to compensate. Conversely, an actor with dark flesh tones may appear too bright if the iris is set to the resulting meter reading of his face, so it may be necessary to close the iris by half a stop or more.
In general, using an incident light meter will provide virtually fool-proof results, whereas a spot meter requires a bit more skill and practice to interpret the readings.
Whenever I begin a new feature film, I will meet with the actors to take a light meter reading of each person to gauge how far their flesh tones vary from zone V. The way I do this is by setting up a vey flat, even light and spot metering an 18% grey card to give me the base exposure. From there, I will use a spot meter to take a reading of each actor’s forehead, recording the value. So if my 18% grey card reads f8, and my leading lady’s face reads an f11, then I know that each time I take a light meter reading of her face on set, I need to open the iris up one f-stop to properly expose her flesh tones. Once finished, I have a handy guide I can reference to help me compensate the exposure for each actor on set.
Light Meter Care
Light meters are precision measuring tools that periodically need to be calibrated. Prior to beginning a new production always the gaffer and cinematographer's light meters match. While some light meters can be manually calibrated, most require manufacturer calibration.