Lighting ratios and the light meter

Learning goal

Instead of estimating by eye the difference between the lit side and the shadow side in a portrait, you will be able to measure it with a light meter and write it as a ratio. You will define the lighting ratio as the quotient of the lit side's reading and the shadow side's reading, turn this ratio into a stop difference, and calculate how many stops each of ratios like 2:1, 4:1 and 8:1 is. You will know that two different ratio definitions circulate in the industry, and state which one you use. You will read the incoming light by putting a handheld light meter's incident dome at the subject's face and turning it toward the camera or toward the lights one by one, measure each light separately in flash metering mode, and record the setup you built as a lighting recipe. You will check that incident light metering is independent of the skin's color, but that the same ratio places the shadow side at different points on the tonal scale on different skin tones.

The question

The band wants a series of portraits that resemble each other, one for each of its four members; they will stand side by side on four pages of the album. You shot the bassist, the guitarist and the keyboardist on different days. Now it is the drummer's turn, but there is a problem: looking at the screen, you notice that the earlier three portraits don't quite match each other. In one the shadow side of the face is soft and open, in another almost black, in the third somewhere between the two. Each time you had set the fill by eye, said "looks good" and shot.

In the studio's drawer you find a small handheld device with a white half-sphere; it has a screen and a few buttons. How will you set up the lights to make the drummer's portrait the same as the earlier ones? How do you measure the balance between lights?

Context

In the butterfly and clamshell patterns we talked about how many stops weaker the fill is kept than the key light, and defined this difference in stops. But we always built that difference by eye, looking at the image on the screen. The eye responds differently to the same scene on different days; the screen's brightness, the room's light and tiredness change this decision. To rebuild a setup or keep a series consistent, this difference needs to be turned into a number.

This number's name is the lighting ratio, and the most accurate tool for measuring it is the handheld light meter. The camera's own light meter measures the light reflected from the scene; the light meter measures the light falling on the subject. This lesson tells how to define the lighting ratio, measure it, and record a setup with numbers.

Simple explanation

What is the lighting ratio?

The lighting ratio is the light falling on the lit side of a face divided by the light falling on the shadow side. The lit side receives both the key light and the fill; the shadow side receives only the fill. In this lesson's definition, the ratio is the direct quotient of the two readings the light meter takes from the two sides. A second definition also circulates in the industry: some sources calculate the ratio by dividing only the key light's power by the fill's power. The two definitions give different numbers for the same light, which is why when writing or reading a ratio you need to state which definition is used. Because the light meter measures the two sides directly, this lesson's definition is the one that matches the meter most easily.

Turning the ratio into stops follows the doubling logic of exposure. One stop is twice the light; that is why 2:1 is a difference of one stop, 4:1 two stops, 8:1 three stops, 16:1 four stops. In general, the stop difference is the base-two logarithm of the ratio. When the meter gives readings from the two sides in f-numbers, this calculation gets easier: if the lit side reads f/8 and the shadow side f/4, the difference between them is two stops and the ratio is 4:1. If the lit side is f/8 and the shadow side f/5.6, the difference is one stop and the ratio 2:1.

Ratios change the face's look and are associated with certain moods. A low ratio, for example 2:1, keeps the shadow side open and soft; the face looks bright and open. A medium ratio, for example 4:1, gives the face a distinct shape. A high ratio, for example 8:1 and above, deepens the shadow side and builds a strong contrast. These associations are common interpretations, not rules; the same ratio can also say something else under the effect of the background, the clothing and the expression.

How is a light meter used?

A light meter is a handheld device that measures the light falling on the subject and shows the aperture needed for a given ISO and shutter speed. You know incident light metering from earlier lessons (Lesson 9.1): the meter reads the light falling on the subject, independently of its reflective properties. The incident dome is the white, translucent half-sphere on top of the meter; it gathers light coming from different angles and imitates the light falling on the face's three-dimensional shape.

Basic use is simple. You put the meter right in front of the subject's face, where the face is, and turn the dome toward the camera. The aperture read is the value needed for the face to be exposed correctly in that light. To measure the ratio, on the other hand, two readings are needed: you turn the dome first toward the lit side, toward the key light, then toward the shadow side, toward the fill, or turn on each light on its own and measure separately. Just as you built the multi-light pattern in layers, doing the metering in layers shows each light's contribution clearly. To measure the back light, you also turn the dome toward that light, behind the subject.

Flash metering mode is the mode of operation in which the light meter can catch and measure a very short flash burst. In this mode the meter goes into waiting; when you fire the flash with a trigger or through the meter's own connection, it shows the aperture that burst requires. With continuous lights, on the other hand, the meter reads the light directly.

How is a setup recorded?

A lighting recipe is a written record of a lighting setup that includes the positions of the lights, their power, the measured readings and the ratios. Next to a lighting diagram you write each light's reading, the readings of the lit and shadow sides, the ratio, the ISO, the shutter speed and the aperture. Weeks later, when you want to build the same setup, you place the lights according to the diagram and bring each light to the value in the recipe with the meter. This is what keeps a portrait series consistent.

Incident light metering is independent of the skin's color. Because the camera's light meter measures the light reflected from the face, it tends to brighten dark skin and darken light skin; the light meter, on the other hand, reads the light falling on the face, so it gives the same correct exposure on every skin. This is a big advantage in a series made up of different skin tones. But the same ratio places the shadow side at different points on the tonal scale on different skin tones. On dark skin a 4:1 ratio can bring the shadow side down to the lower end of the tones, to an area close to black, and detail can be lost; in this case you need to lower the ratio a little or check the shadow side on the histogram. On light skin, on the other hand, at a high ratio the lit side can approach the upper end of the tones and blow out. The ratio is a starting point; you make the final decision by enlarging the face and looking at the detail of the shadow and lit sides.

Visual evidence

In the studio you put the key softbox to the drummer's left, forty-five degrees to the side and a little above, and the fill umbrella beside the camera. You set the light meter to ISO 100 and a hundred twenty-fifth of a second, and hold it in front of the drummer's chin. First you turn on only the key light and turn the dome toward it; then you turn on only the fill and turn the dome toward the fill. With both on together, the lit side reads f/8 and the shadow side f/4: a two-stop difference, a 4:1 ratio. Looking at the diagrams of the earlier three portraits, you guess which one was closest to this.

On the drummer's dark skin, the shadow side looks almost black at this ratio; on the histogram the shadow side's tones are pressed against the left wall. You open the fill by a third of a stop and bring the shadow side to f/4.5; the ratio drops a little below 4:1, and the curve of their cheek and the line of their eyelid can be made out again. You write all the readings, the positions of the lights and the ratio on a sheet of paper. When you reshoot the band's other three members with this recipe too, the four portraits, with four different faces, stand side by side in the same light.

Common mistakes

Summary

The lighting ratio is the lit side's reading divided by the shadow side's reading; 2:1 is a difference of one, 4:1 of two, 8:1 of three stops, and because of the second definition circulating in the industry, which definition is used should be stated. A handheld light meter reads the light falling on the subject with its incident dome; with the dome turned to the camera it gives the exposure, turned to the lights one by one it gives each light's contribution, and in flash metering mode it catches short bursts too. When a setup is written in a lighting recipe it can be rebuilt weeks later; incident light metering is independent of skin, but because the same ratio places the shadow and lit sides at different points on every skin, the final decision is made by looking at the face itself.

Mini assignment

If you don't have a light meter, you can make the same measurement approximately using the camera's light meter with a gray card; hold the card in front of the face and turn it first toward the lit side, then toward the shadow side.

1. Light a person with a key light and a fill; take the readings of the lit and shadow sides and calculate the ratio.
2. By changing the fill, build the 2:1, 4:1 and 8:1 ratios and shoot each with the same person.
3. Put the three frames side by side, write which one suits the portrait's purpose with your reasoning, and compare with a friend's choice.
4. Record your favorite setup as a lighting recipe: the lights' positions, each reading, the ratio, the ISO, the shutter speed and the aperture. A week later rebuild the setup by looking only at the recipe and compare the two frames.

Check yourself

• Can you turn the difference between two readings into stops and into a ratio?
• Do you know when to turn the light meter's dome toward the camera and when toward a single light?
• Did you check with the histogram and by enlarging the face where the same ratio places the shadow side on different skin tones?

Glossary

Mini quiz

1. If the lit side reads f/8 and the shadow side f/4, what is the lighting ratio according to this lesson's definition?

• A) 4:1
• B) 2:1
• C) 8:1
• D) 16:1

2. What is the basic difference between the light meter and the camera's light meter?

• A) The light meter measures only flash
• B) The light meter is less sensitive
• C) The light meter measures the light falling on the subject, the camera's meter the reflected light
• D) The camera's meter measures only continuous light

3. A ratio of 8:1 corresponds to a difference of how many stops?

• A) 2 stops
• B) 3 stops
• C) 4 stops
• D) 6 stops

4. In a 4:1 portrait of a dark-skinned person, the shadow side is almost black and pressed against the left wall on the histogram. Which is the most fitting correction?

• A) Strengthen the key light
• B) Cut the exposure by two stops
• C) Turn the light meter toward the camera and measure again
• D) Open the fill a little to lower the ratio, and check the detail of the shadow side

Answers

1. A — Between f/4 and f/8 is two stops; two stops means four times the light.
2. C — Incident light metering is independent of the subject's reflective properties; the camera's meter decides by the reflected light.
3. B — Each stop doubles the light; two to the power of three is eight.
4. D — Opening the fill moves the shadow side up the tonal scale and brings the detail back.

Summary card

• Lighting ratio: the lit side's reading divided by the shadow side's reading.
• 2:1 is one, 4:1 two, 8:1 three stops; always state the definition.
• The light meter reads incoming light; turn the dome toward the camera or a single light.
• Measure each light separately; write the setup in a lighting recipe.
• The metering is independent of skin, the look isn't; check the shadow and lit sides with the histogram and by enlarging the face.

Bridge to the next lesson

When you finished the drummer's series, the band got stuck between two separate ideas for the cover: one, a dark frame where the members stand against a pitch-black background, at a high ratio, with half of their faces in shadow; the other, an airy frame against a bright white background, at a very low ratio, almost shadowless. Both are the same four people, but two completely different worlds. Dark and dramatic, or bright and clean? The next lesson tells about these two extreme atmospheres the background and the lighting ratio build together.