Measuring Light in Ratios: Everything You Need to Know

Cinematographers handle the bulk of creative visual decisions during filmmaking. As well as managing shots, framing, and use of technical equipment, they oversee the lighting on set. The way your light is set up for a film has a huge impact on the end result, as lighting affects the overall mood and feeling of a shot. You can measure light in different ways to create dramatic cinematic effects, and one of the methods you can use is lighting in ratios.

Modern cinematographers most popularly light by eye, simply adjusting lights and angles until the scene “looks right”. However, there is a more technical method you can use to light film sets, one that reaches back to the earliest days of cinematography. Measuring light in ratios can be a more specific way to light film scenes, using specific numbers to determine brightness and shadow. 

A man holding a camera on a film set.

What are Light Ratios?

Lighting ratios are an entirely relative way to measure the amount of light in a scene. Rather than being measured by the number or type of lights, ratios compare how light falls in two different areas. Understanding this element of cinematography will make you a much better storyteller, as light ratios can be used to convey emotion in a film like no other single element. 

Instead of using a numerical measurement like an F stop, light ratios only compare how one part of a scene is lit in relation to another. Light ratios are almost written as two numbers, like 2:1; in this example, one light measurement is twice as bright as the other. These can be used to compare different elements of an image (for example key light vs background light) or simply two sides of an actor’s face. 

When measuring the light in an image or scene, the key light, fill light, backlight, and background are the most important elements. It can be complicated trying to maintain the right balance between these measures by eye, however, using ratios can seriously simplify it. When lighting by ratios, you first decide on the numerical ratio and its appliance to a single one of these elements (usually the key light) and then from there you can adjust the remaining lights to suit. 

Common Light Ratios in Film 

Depending on what element you decide to measure, there are hundreds of different light ratios you can use in a single film scene. The key to fill ratio is the most commonly used measure among cinematographers. The key light refers to the light used on the main subject of the scene, for example, an actor. This provides the main illumination for a given shot and is generally the measure used for setting exposure on your camera lens. The fill light is the general light level in a scene and doesn’t have to come from an actual studio light.

Fill light is made up of the bounce of other lights, general ambient light, or negative fill. The term fill light as used here is simply the opposite of key light: the key illuminates your main subject, where the fill is simply the general light level. In film, the key to fill light ratio is most popularly utilized when lighting people. The key light will provide good illumination of one side of your actor’s face, where the fill determines how well we see the other. To determine the light ratio, in this case, you would simply measure each side of your subject's face. 

The key to background ratio is the second major ratio which cinematographers use. The key light is your main scene light as explained previously, however, the second measure is much broader than fill light. The key to background light ratio works in the same way as key to fill, however, it has a more noticeable impact on the scene.

Instead of measuring just the fill light, you measure the background as a whole. Usually, dramatic cinema uses the key to background light ratio to set the mood of the scene, and also to focus the attention of the audience. The background light can be dimmed so all eyes are on the subject, or the light levels raised to elevate the general feeling of a scene. 

Light Ratios in Genre 

Light ratios are incredibly important in genre, as they have a huge effect on the feeling and emotions of a scene. Lighting ratios can be manipulated to make the same scene look bright and happy, or dramatic and foreboding. Different film genres use light ratios to convey their message, carefully measured to allow the perfect levels of detail and shadow. 

Lighthearted comedy shows often use a key to fill ratio of around 2:1, meaning the key light shed upon your subject is twice as bright as the fill, or the side of your actor’s face in shadow. A light ratio of 2:1 is bright and shows color well, without much shadow or darkness. This lighting ratio is ideal if you need to reveal a lot of detail in your shots, and want to maintain a bright and happy effect. 

A light ratio such as 8:1 is used in very different genres, such as film noir. In an 8:1 key to fill ratio, one side of the actor's face is dark and almost completely in shadow, with the other side brightly lit in white. Higher light ratios can make shots much more dramatic, and give more depth to your image. 

People working on film set with a red sheet in the background.


How to Measure Light Ratios

There are two different methods that most filmmakers use to measure light ratios. If you want to maintain a specific ratio throughout, then getting accurate measurements is important. A light meter is the simplest way to measure light readings, you simply use it to record different light levels around the set and actors, and from these, you can figure out the ratio. This is the most cost-effective method as a light meter can be obtained for around $50, however, it can be time-consuming to collect all the readings you need.

To use a light meter to measure the key to fill ratio in your shot, simply measure the key side and the fill side of your subject. It’s worth noting that in using this method, measurements have to be done at the moment. There’s no way to measure the light ratio of a scene after it’s been shot, once it’s on a tv screen. This means you have to experiment and make adjustments in real-time, another time-consuming task. 

A false-color monitor can provide a much quicker way to measure light ratios. A monitor showing false-color images converts light levels into colors based on brightness, similar to how a thermal camera works. This way, you can clearly and instantly see which areas are most and least lit based on color. Not only is a false-color monitor less time consuming, but it can also be more accurate. Light meters do provide accurate readings, however, a false-color monitor can measure light bouncing off other objects and provide a more accurate measure of relative brightness. Unfortunately, false color monitors are many times more expensive than light meters, easily costing thousands of dollars. 

Where Light Ratios Fall Short

Although measuring light in ratios is an excellent technique for cinematographers to master, there are a few areas where this method isn’t adequate. You can still use light ratios to illuminate your scene, but these additional elements need to be considered outside of this method. Light ratios are incredibly useful for setting the mood, but other lighting factors still need to be taken into account: 

  • Type of lighting: Cinematographers make great use of soft and hard light, and lighting ratios aren’t much help here. You’ll still need to make decisions regarding the type of lighting you’ll use on set, as well as choosing a light ratio. 

  • Lighting direction: You’ll still need to make decisions about the angle and direction of your studio lights, as ratios are powerless in this regard. 

  • Lighting color: The color and hue of your chosen lighting have a massive impact on the moods and emotions a scene will convey. In this area, lighting ratios are determined separately. 


Measuring light in ratios is an advanced technical skill that every cinematographer should master. This basis of set lighting in film is a powerful tool that can be used to really connect with your audience. Using a low light ratio (for example fill to key) helps scenes to maintain a light-spirited and happy appearance, where there is less shadow on the subjects and most details are clearly visible. A much higher light ratio will cast your subject in bright light on one side, leaving the other in much darker shadow.

This technique can help convey more mystery and intrigue, and present a more dramatic image. You can also use light ratios in film to help draw the audience’s attention to specific things, like when using the key to background ratio. This way, you can direct the eyes of your viewer more effectively, and ensure they’re focusing on the right part of the scene. 

Remember, light ratios aren’t the be-all and end-all of scene lighting. They’re a great place to start, but it’s important to keep in mind the other elements of lighting which aren’t as easily meters or controlled. The type, color, and direction of your lighting all work completely independently of light ratios and have an equally important impact on the end result.

However, we recommend that every cinematographer learn the theory behind light ratios and put it into practice, to gain a better-rounded knowledge of the craft. 

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