Understanding Light Ratios: Additive and Source Methods

Lighting ratios, how they are measured and how they are captured in the portrait, have been confusing to many. This is because often there is a discussion of the appropriateness of one of two specific methods of how to measure and create desired ratios and the results in the portrait. These methods are based on either the measurement of relative intensity used for key and fill lights or the measurement of the light within the key-lit or filled areas or the subject.

Using lighting ratios is a means of measuring the different levels of illumination in a scene. In a portrait we are dealing with the light from the highlight area compared to the light in the shadow areas. Lighting ratios are numbers that represent the relative difference in intensity between these specific areas. A typical numerical representation might be a 3:1 lighting ratio. That means that the highlights (in this case represented by the “3”) receive three times more light than the shadows (represented by the “1”). A 5:1 ratio means that the highlights are illuminated by five times more light that the shadows.

Source Vs. Additive Method
Confusion enters the discussion first with how the light ratio is measured. There are two methods used to determine the light ratio. These are the Source Method and the Additive Method.

The Source Method measures both key light and fill light independently. In this method the light from each source is measured without the other light illuminating the subject. Thus the fill light is turned on and the light value is measured and then it is turned off. Then the key light (main light) is measured without the fill being on. This method gives the light intensity values of each light unit.

The Additive Method is based on the assumption that there will be interaction between the key and fill lights. Here the fill light is measured first independently with the key light off. The measurement is done on what will be the shadow side of the subject. Then the key light is turned on without turning off the fill light. The measurement is then taken of the highlight side of the subject. This measures the illumination of the key and fill lights that can combine on the highlight side of the subject.
At first glance the distinction doesn’t seem that significant. It is! Fundamentally, one measures the light emitted from each source separately at the subject and the other measures the amount of light illuminating the subject that can be captured in the photographic process. Understanding this concept eliminates the confusion.

In describing the measuring techniques, let’s start first by redefining the key and fill lights as they will be used in this discussion. In this discussion we are using a light placed at about a 45-degree angle to the right of the camera subject axis, about 45 degrees above the camera’s axis. This is a very typical lighting setup in portraiture and is often called the 45/45, Rembrandt or a closed-loop lighting pattern.

For this example, the fill light is in the traditional position directly above and behind the camera. Its sole function is to add light to the shadow areas. Placing it directly behind the camera eliminates any additional shadows that might be confusing in the portrait.

With this setup, using the Source Method, we measure a 2:1 relative intensity difference between the key and fill lights. When we metered the fill light independently, it read an intensity of f/5.6. (The shutter speed is irrelevant since we are using strobe lights.) Metering the key light independently, we get a meter reading of one stop brighter at f/8. In the Source Method it is important to make sure that each light does not affect the meter reading of the other. Also the incident dome of the meter will point at the light from the subject.

The Source Method assumes that the key light will cast a shadow and that the fill light will only affect the shadow. Further, it assumes that none of the fill light adds intensity into the area lit by the key light. With these assumptions the Source Method has one-stop difference between the intensity of the key and fill lights. While the intensity ratio as measured with the Source Method is 2:1, the actual lighting ratio is 3:1 because the key light is strengthened in intensity by the overlay of the fill light as it spills into the key lit area. The only situation where this could be correct is with pure split lighting, when the key light illuminates only half of the subject and the fill light illuminates only the other half of the subject.

If we use the Additive Method to determine the ratio on the same setup, we measure the light differently and arrive at a different ratio. With the Additive Method we measure, with the meter pointed at the camera from the subject, the fill light the same and arrive at the same f-stop, f/5.6. However, when we read the highlight side with both the key light, that independently reads f/8, and the effect of the fill light, f/5.6 or one half stop additional light, we find that the measurement reads f/8.5 (f/10). This means the light ratio between the fill in the shadows and the total light in the highlights is 3:1 with the Additive Method.

Therein lies the confusion. Both measuring systems used the same lighting setup and can be said to result in the same functional ratio. However, they use different methods of description. The Source Method deals with the intensity of lights and the Additive Method deals with the light on the subject. Source methodology uses an intensity ratio measured in the intensity of individual lights. Additive methodology states that the ratio is about light that is available to be reflected back to the camera from the highlighted and shadow areas of the subject; therefore the total light in the shadows compared to the total light in the highlights is output based. It is an issue of words not light… the light stayed the same.

Both systems are valid, but is one better to use than the other? Not a simple answer. Both are correct and whether you are a commercial or portrait photographer or where/how you learned ratios will probably determine which system you use.

We have found, after years of using both methods, that standardizing on the Additive Method provides better results in all situations, particularly with setups outside the studio environment that require metering the key and fill combined. In the outside environment, fill light is usually ambient light and is therefore impossible to turn off to get a true reading of just the key light alone. That same system has translated well for the studio situation and requires less calculation and simplifies the process. However, if working exclusively in the studio, the Source Method works well and can be standardized.

Ultimately, the capture and output devices you use will have just as much effect on how your images reproduce as the original ratio. Learning to quickly reproduce the look you want is crucial. Most professional photographers choose one system and produce ratios by knowing what the difference is between the intensity of the key and fill lights and using a metering technique with which they are most comfortable.

To provide a way of both seeing the relationship of the two methods and as a guide to using ratios, the following table is presented. We have based this on a setup that has the fill light set to read f/5.6. The key light is listed as both the f-stop compared to the fill light and the stop differential based on the fill stop. Exposures for these recommendations are the f-stop listed as the additive method reading regardless of the method used. If you use the source method and use an f-stop other than f/5.6, the Key Stop Differential provides the additional stops needed to achieve the desired light ratio.

One final thought about lighting ratios. What camera setting should you use when exposing the previously described ratios? The answer is whatever your meter indicates. You should be taking a meter reading with both the key and the fill lights on and use that reading.

Glenn Rand and Tim Meyer both teach at Brooks Institute and this article is excerpted from their upcoming book The Portrait: Understanding Portrait Photography to be released later this year by Rocky Nook.