A light meter objectively measures the light and breaks it down in to a quantifiable measurement. It measures the light we see and converts it to functions in a camera.
Having a great eye for lighting is a good talent to have, but being able to back up that great eye with solid data will not only increase the quality of your judgements, but also make communication on set better and easier. Knowing the quality of the light in your shot helps with exposure choices, but also allows you to choose how to light a scene more deliberately. This will help when making the choice of how to light for the look and feel you want. Lighting your scenes deliberately will create consistency between scenes, not only in exposure but also in color temperature.
Let’s start with how it works. A light meter objectively measures the light and breaks it down according to the functions of a camera. There are two kinds of light meters: reflected light and incident light. The main difference between the two is the location of the reading. Reflective is measured from the camera’s position, whereas incident is measured from the subject. These measurements, consisting of f-stop, iso and shutter speed, will help you mentally connect the perceived light in a scene to the correct exposure settings on your camera, thus helping you produce the desired look and feel of the shot. Using the data provided by a light meter to control the look and feel of your image allows for better consistency between shots. Overall, this will make lighting easier and more efficient.
The light meter built in to your camera is a nice place to start, but it only takes a reflected light reading from where the camera sits. Using only this limited, reflected light meter to light your scene doesn’t give you all of the data. This is where a remote light meter comes in. With a remote light meter, you get data from each spot within your shot, rather than a general overview of the whole scene. Although many reflected light meters have a spot option, it’s the perspective of the meter that allows for more specific data. Knowing the exposure of your foreground versus the background will allow you to assess where you’re drawing the viewer’s eyes. This can be used to create a mood with your lighting and that mood can be used to increase or decrease the drama of the shot.
The light meter allows you to be deliberate in how you control your camera functions to achieve the specific goal.
Have you ever heard someone describe a shot as flat? A flatly lit shot is one with even light throughout. An evenly lit scene lacks the dynamics of light that create texture and depth; therefore, it is flat. The subject doesn’t pop out from the background and the scene lacks interest.
Adding contrast using light creates more depth in the scene and makes it dramatic. A light meter is the perfect tool to help create this drama. By using an incident light meter, you can create contrast between your lighting sources, thus boosting the contrast ratio. For example, imagine you have two lights lighting up an interview of one person — one lighting up the left side of the face and the other the right side of the face. If you start with both lights having the same power there would be no contrast, no difference between them. Decreasing one of the lights to half their power makes a contrast ratio of two to one.
That ratio is defined by the difference in stops of light. Remember, increasing the amount of light by one stop of light means you are effectively doubling the amount of light in the scene. Reducing the light by one stop halves the amount of light. In our example interview set up, one side has one stop less light than the other, or half as much light, thus giving us our two-to-one contrast ratio. If the light was two times — two stops — less, you would have a contrast ratio of four to one.
Adding or subtracting a stop of light to your exposure can be achieved by adjusting one of three adjustments on your camera. Those three adjustments are f-stop — controlled by the aperture or iris setting — shutter speed and ISO. The increments of a stop of light using the f-stop are f1.0, f1.4, f2.0, f2.8, f4.0, 5.6, f8.0, f11, f16, f22 and on. The increments via shutter speed are 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 and so on. And lastly the ISO increments are 100, 200, 400, 800, 1600, 3200. Depending on the lens and camera, this range can be smaller or larger, but the increments remain the same.
Using a light meter to plan a scene allows you to make choices based on the light data of the space, not just feeling. It’s important to know what information the light meter will give you and how to use it. Digital light meters need one of three data points: iso, f-stop or shutter. Here is an example of how you would find exposure manually. So, you’re shooting a fast-moving subject, and want to shutter up two stops to have more detail and less motion blur in the subjects movement while keeping the same exposure. Let’s say you are starting at an exposure of f2.8, 1/60 and ISO 400. To achieve this you would need to compensate for the change in shutter speed by increasing the light by two stops, or increasing the ISO, aperture or a combination of both. In this example, you would be increasing the shutter to 1/250, meaning that you can increase your ISO to 1600 to make up those two stops, but if your camera introduces too much noise at ISO 1600 you can make it 800 to make up one stop and then gain the other stop with the f-stop going to f2.0. The final product would be f2.0 1/250 ISO800. Doing that would keep your same exposure, but through those adjustments you are able to change how smooth your subject appears when shooting.
Now if you needed to make these same decisions, but have a digital light meter, then you would first put in the most important measurement you want for the scene. For example, if depth of field is your goal, input the desired f-stop, or if, like in the example above, you need detail in fast movement, you would adjust the shutter speed. Once you put in one of the three points of data, the light meter will do the rest for you, giving you the appropriate settings for the other two.
The light meter allows you to be deliberate in how you control your camera functions to achieve the specific goal. Employing a light meter when location shooting lets you know if you have enough light to shoot or how your different lights balance each other out.
When on a shoot, it’s very important to clearly express your ideas to other team members. Doing this using subjective language instead of specific language creates confusion and might not achieve your lighting goal. Because exposure can be adjusted in many different ways, expressing, in subjective terms, how you’d like to achieve that exposure leads to trial and error lighting, and that wastes time. Being able to communicate your desired outcome via specific data takes out the subjective and creates a direct goal that is easily communicated and understood.
Say you are producing a documentary and are conducting many different interviews over a span of weeks or months. By collecting the data from your light meter on your first shoot, you will be able to set up the same lighting in a different location or at a different time of day. That data will create confidence in your ability to get the right lighting for your desired goal, making the process go faster and giving you more consistency. By using a light meter, you will gain better control of your scene and set, thus allowing you to produce a more compelling story. To get that data, you would take the light meter around and measure each area. Collect the light reading from the background, the foreground and your subject. Then, using the data you collect, you can easily replicate the same lighting setup on your next shoot, thus creating consistency between scenes.
To sum it up, it is critical to use a light meter if you desire a professional outcome. Expressing your desire clearly to all those on set working with you through deliberate choices and with measurements from the light meter will make communication easier, and you will see the benefit in the resulting shot.
SideBar - The history of the light meter
In the beginning there were actinometers. Actinometers used printing-out-paper or pop, which turns dark when exposed to light without needing chemicals. It looked like a pocket watch and pop disks were placed inside the case. To use, one would rotate the pop to expose an unexposed area. From start to finish, the user would time how long it took for that area to turn gray. Then based on that time, they would refer to a guide that gave time-aperture combinations. A good option when there was none. The two big drawbacks were that it was subjective on what shade of gray indicated completion and the pop needed to be loaded in the dark.
Next came extinction meters. To use, one would look through the meter to view a gradually darkened set of numbers. The set of number that was hardest to view gave the shooter a number to look up on a table for exposure. It worked better than nothing, but because the human eye has a live iris, the results would be subjective, and thus not very accurate.
Lastly are analog and digital meters. They both use the same method to work with the main difference being how they display the results. The analog meter came first. The analog meter functioned by exposing a white ball in front of the subject, then a needle or dial would move based on what the reflecting light levels were. That needle or dial would have a table offering all of the exposure options for that light level. A digital meter, on the other hand, requires a shutter speed or f-stop to be inputted before giving a reading.
Chris Monlux is Videomaker’s associate multimedia editor and video producer.