Tonal and color contrast

Learning goal

You will be able to tell whether the difference that separates the subject from the background in a photo comes from brightness or from color. You will read tonal contrast in stops; you will see that hue, saturation and the warm-cool difference are each a contrast too; you will explain why two colors of the same brightness fall to the same gray when converted to black and white; you will see a scene's tonal structure before the shot by squinting or with a monochrome preview; and by checking with which difference each skin separates from the background, you will keep the face from blending into the background.

The question

In a garden, you are photographing a woman in a red headscarf standing in front of dense green leaves. In the color photo, the red headscarf burns among the green leaves almost like a flame. When you convert the same frame to black and white, though, the headscarf disappears into the leaves; both have turned the same middle gray.

Does contrast only mean black and white? When we say contrast, most of us first think of bright whites and deep blacks. But in the color frame, what separated the headscarf wasn't brightness. In this lesson we will talk about telling apart the two big families of contrast, tonal contrast and color contrast, and seeing which one a photo relies on.

Context

Contrast is the difference between two things; every difference that lets something be seen in a photo, that separates it from somewhere, is a contrast. In the exposure and histogram lessons we always worked with one kind of difference: the difference between bright areas and dark areas. With the tangerines in the blue crate, though, we came across another difference: the difference in color between orange and blue, independent of brightness.

These two differences can work independently of each other. In one scene the brightness difference can be big and the color difference small; in another, the reverse. Knowing which difference a photo's strength comes from is useful when you decide how to light it, how to edit it, and whether to convert it to black and white.

Simple explanation

How do you read tonal contrast?

Tonal contrast is the distance between a frame's bright areas and its shaded areas. This difference can be read in stops: a one-stop difference between two areas means the bright area reflects twice as much light as the dark area. Lighting experts call the difference between the bright side and the shadow side of a face the lighting ratio. A 1:2 ratio is a one-stop difference, 1:4 two stops, 1:8 three stops. As the ratio grows, the shadow deepens and the face reads harder and more dramatic; as the ratio shrinks, the face softens.

The eye mostly reads form and detail in a scene from these brightness differences. The lines of a face, the corner of a building, the fold of a fabric show through tonal difference. To see tonal contrast, it helps to take color out of the way: when you squint, the colors fade, and big bright and dark patches are left. Many cameras and phones can also show a monochrome preview on the screen; if you shoot RAW, the file still stays in color, only your view changes.

What does color contrast consist of?

Color contrast is the difference between colors independent of brightness, and it shows up in several forms. Hue contrast is the difference between colors that stand far apart on the wheel; the strongest is between complementary colors. Saturation contrast is a vivid color standing among faded colors: a single red umbrella on a gray, rainy street. Warm-cool contrast is a warm subject standing in cool surroundings, or a cool subject in warm surroundings.

These differences can bring a subject forward even when there is no tonal difference. That is why the headscarf was burning among the leaves: red and green stand far apart on the wheel. But the eye can perceive a border built by color difference alone, with no brightness difference, as an edge that shimmers a little and doesn't quite settle in place. When strong color contrast and strong tonal contrast meet in the same place, separation takes its clearest form.

Why does color contrast disappear in black and white?

A black-and-white photo records only brightness. The red headscarf and the green leaves were different colors, but at about the same brightness; when converted to black and white, both fell to the same gray. This situation, where colors are different but brightness is the same, is called isoluminance. If a color photo's strength comes only from color contrast, its black-and-white version looks flat and scattered.

This is also a good way to see a scene's real tonal structure. If you convert a frame to black and white for a moment before shooting it, or while editing, you see whether the subject separates only by color, or by brightness too. In the black-and-white conversion of editing programs there are sliders that let you set separately where each color will fall on the gray scale; by brightening the red and darkening the green, you can separate again two colors of the same brightness.

By which difference do skin and background separate?

In a portrait, the face's separation from the background relies on one of these two differences, or on both. When a dark face comes in front of a shaded bush, an olive face in front of sand-colored plaster, a light face in front of a white curtain, the tonal difference shrinks. If the background is in a color close to the skin's complement, the color difference can still separate the face; but this separation disappears when converted to black and white, and weakens in the eye of someone looking at a screen with faded colors.

If you want the face to read under every condition, build the separation with tonal difference too. You can move dark skin in front of a brighter background, let a little light fall on the edge of the hair and shoulders from behind, or leave the background darker than the face; on light skin the same logic works the other way round. Tonal contrast is measured with the same stop calculation on every skin; what changes is where the skin and the background stand on this scale.

Visual evidence

You go back to the woman in the garden. The woman is dark-skinned, in a red headscarf; the leaves behind her are in shade and dark green. In the color frame the headscarf separates from the leaves, but when you convert it to black and white, both the headscarf and the shadow side of her face blend into the dark gray of the leaves; the edge of her face has almost disappeared.

You ask the woman to move two steps to the side, to a place with light green leaves in the sun behind her. Now the background is about two stops brighter than her face. In the color frame the red headscarf glows again in front of the green; in the black-and-white frame, the dark gray of her face and headscarf separates with a clear line in front of the light gray leaves. The difference between the light and shadow on her cheek and the curve of her eyebrow can be made out too. The same woman, the same headscarf; this time the separation was built in both color and tone.

Common mistakes

Summary

Contrast is the difference between two things, and it comes from two big families: tonal contrast between light and dark, and color contrast independent of brightness. Tonal contrast is read in stops and carries form; color contrast is built with hue, saturation and the warm-cool difference, and when converted to black and white, colors of the same brightness fall to the same gray. For a face to read under every condition in a portrait, you need to build the separation between skin and background, on every skin, with tone as well as color.

Mini assignment

In this assignment you will find tonal and color contrast separately, and build both together in a portrait.

1. Find a scene separated only by color difference, for example a red object in front of green, and photograph it; then convert the frame to black and white and look at whether the separation stays.
2. Photograph a scene separated only by tonal difference, for example a bright face in a dark doorway.
3. In the black-and-white conversion, try to separate two colors of the same brightness with the color sliders.
4. Photograph a person first in front of a background at the same brightness as their skin, then in front of a background a few stops brighter or darker than the skin, and compare the two frames in black and white.

Check yourself

• Did the scene separated only by color flatten in black and white?
• Were you able to separate the two grays with the color sliders?
• When you changed the background, did the edge of the face, on light or dark skin, become readable in black and white too?

Glossary

Mini quiz

1. How many stops of difference does a 1:4 lighting ratio on a face correspond to?

• A) One stop
• B) Three stops
• C) Four stops
• D) Two stops

2. A single red umbrella on a gray, rainy street is an example of which contrast?

• A) Saturation contrast
• B) Lighting ratio
• C) Isoluminance
• D) None of them

3. Why does a red headscarf in front of green leaves disappear in black and white?

• A) The black-and-white conversion erases red
• B) Because red and green are at about the same brightness, they fall to the same gray
• C) The headscarf is out of focus
• D) The headscarf's fabric absorbs light

4. If a dark-skinned person's face disappears against a dark background in black and white, what do you do?

• A) Making the background's color complementary is enough
• B) Raise the saturation
• C) Choose a background a few stops brighter than the face, or let light fall on the edge of the face
• D) Raise the ISO

Answers

1. D — 1:2 is a one-stop, 1:4 a two-stop, 1:8 a three-stop difference; each stop doubles the brightness.
2. A — The vivid red stands forward through a difference in saturation, because it stands among faded grays.
3. B — Black and white records only brightness; two colors of the same brightness fall to the same gray.
4. C — Color difference disappears in black and white; tonal difference is needed to separate the face under every condition.

Summary card

• Contrast = difference: tonal contrast and color contrast are two separate families.
• Tonal contrast is read in stops: 1:2 one, 1:4 two, 1:8 three stops.
• Color contrast: hue, saturation, warm-cool.
• Colors of the same brightness fall to the same gray in black and white.
• In a portrait, on every skin, build the separation with tone too: brighten the background, darken it, or let light fall on the edge.

Bridge to the next lesson

We always thought of tonal contrast between big areas: a face and a background, a headscarf and leaves. But light and dark also stand side by side at a very small scale: every bump of a stone wall has a bright face and a small shadow. These small differences make a surface feel touchable. The next lesson talks about texture and form, and how the angle the light comes from flattens a surface or makes it stand out.