The tone curve

Learning goal

You will be able to read the tone curve, with which you can manage all the tones of a photo through a single line, and give it the shape you want. You will explain that the curve's horizontal axis shows the original tones in the photo and its vertical axis the new values these tones will take, and that the straight diagonal line changes nothing. By bending the curve with control points, you will brighten and darken a particular tonal range, show that the curve's slope decides the contrast in that range, and work out what an S-shaped tone curve loses at the ends while gaining contrast in the midtones. You will get faded blacks by raising the black end, give color to the shadows and highlights or correct a color shift with channel curves, and check how the curve affects detail in the tonal ranges where dark and light skin sit.

The question

When the portraits you shot in the drummer's small room open on the computer, they all look a little dull. The white walls filled the shadows, the blacks slipped slightly toward gray, the highlights don't quite shine. When you raise the contrast slider in the basic panel the photo comes to life, but on the dark skin of the drummer's sibling the shadow cheek drops to pitch black, and their forehead almost blows out. When you pull the slider back, the photo goes dull again.

In a corner of the editing program, inside a box, a straight line stretches from corner to corner. When you click the middle of the line and pull it up a little, the whole photo brightens; when you pull a point near the bottom end down and a point near the top end up, the line turns into an S and the photo suddenly deepens. But this time the face's detail is kept. How does a single curve create contrast? And how does it manage what the contrast slider can't?

Context

In the basic panel you learned to adjust the white and black points, the exposure and the contrast each with a slider (Lesson 17.4). Sliders are fast and understandable, but each one changes a wide part of the photo together; the contrast slider, for example, always pulls the shadows and highlights apart in the same way. You can't decide which tone will change by how much.

The tone curve leaves this decision to you. It shows all the tones on a single line and lets you change only the tonal range you want by bending any part of that line. The advanced editing and retouching module opens with this tool, because most of the following lessons, masks, adjustment layers, color and black-and-white, are built on the same logic: deciding knowingly which tones, in which area, to change and by how much.

Simple explanation

How is the tone curve read?

The tone curve is the line that shows which tone each tone in a photo will turn into after editing. The box's horizontal axis lines up the photo's original tones from left to right, from darkest to lightest; its vertical axis shows the new value these tones will take from bottom to top, from darkest to lightest. When nothing changes, the line is a straight diagonal stretching from the bottom-left corner to the top-right corner: every tone is mapped to the same value as itself.

Pulling a point on the line up maps the tone at that point to a lighter value; pulling it down to a darker value. In most programs the photo's histogram also shows behind the box (Lesson 9.4); this way you bend the curve while seeing which tones are plentiful in the photo and which are scarce.

A control point is a marker placed on the curve that bends the curve when dragged. When you move a point, the curve bends softly not only at that point but around it too; tonal transitions don't break. To keep other tones while changing one tonal range, you can put a separate point there and leave it in place; this point fixes the curve in that area. Many programs also let you click a place on the photo and drag the mouse up and down to move the point belonging to that place's tone directly; this is the fastest way to find the tone of a particular area like a face.

Why does the curve create contrast?

The curve's most important secret is in its slope. Slope is how steep or flat the curve is in a tonal range. When the curve is steeper than the diagonal in an area, tones close to each other in that area move apart; contrast increases. When the curve is flatter than the diagonal in an area, the tones in that area come closer to each other; contrast decreases. Everything you do by bending the curve is actually deciding which tonal range to give more room and which less.

An S-shaped tone curve is a curve that forms when a point near the shadow area is pulled down and a point near the highlight area pulled up, and whose middle gets steeper. Because its middle is steep, contrast increases in the midtones, that is, in what is most often the photo's most important area, and the photo comes to life. But the curve's total height doesn't change: a curve that steepens in the middle has to flatten at its two ends. That is why the S-curve reduces contrast in the darkest and lightest tones; the detail in these areas comes closer together, and an excessive S pushes them completely to black or white. Conversely, an inverse S with a flat middle lowers contrast in the midtones, makes room at the ends and softens a harsh photo.

The contrast slider also most often applies an S-curve, but it applies it from the middle and symmetrically. In the tone curve, on the other hand, you choose where the S will steepen: you can place the steep area in the tonal range where the face sits and push the loss in the shadows and highlights to areas where the face isn't. This was what the slider couldn't manage in the portrait of the drummer's sibling.

What are faded blacks and channel curves for?

Faded blacks is preventing the darkest tones in the photo from going down to full black by raising the curve's bottom-left end. Because the curve's start is no longer at the corner but a little higher, even the darkest shadow becomes a dark gray. This gives the photo a matte, hazy air reminiscent of an old print. It isn't a mistake but a deliberate stylistic choice; but when it is used, you have decided what the photo's deepest tone will be. By the same logic, lowering the top-right end prevents the highlights from reaching full white.

A channel curve is a curve that changes the tones of only one of a photo's red, green and blue channels. Because light is made of three channels, bending one channel's curve changes that color: in the blue channel, pulling the shadow area up gives the shadows blue, pulling the highlight area down gives the highlights a yellowish tone. This tool does two jobs: correcting a color shift only in a particular tonal range, for example removing a green that has seeped into the shadows; or building a mood by giving the shadows and highlights different colors. This second use will form the basis of color editing in the module's later lessons.

How does the curve affect skin?

Skin tones sit in different places on the tone curve, and an S-curve affects them differently. Dark skin most often sits in the lower-middle area of the curve. When the shadow point is pulled down too far, the curve flattens in this area; the fine tonal differences of the cheek, the area around the eyes and the hairline come closer to each other, and the skin drops to a uniform darkness. To protect dark skin, you need to place the S's shadow point below the skin, in the photo's truly darkest areas, and keep the curve steep in the skin range. Light skin, on the other hand, most often sits in the upper-middle area; when the highlight point is pulled up too far, the forehead, the tip of the nose and the cheeks get squeezed toward white. In this case moving the S's highlight point above the skin helps.

Channel curves also change the skin's color directly. Even a small change in the red channel flushes or fades the skin; that is why, when working with channel curves, you need to enlarge the face and check whether the skin stays close to the person's real color. In every case, the goal is to give the rest of the photo the contrast you want while keeping the skin's own detail and color.

Visual evidence

You open the portrait of the drummer's sibling and look at the tone curve's box. The histogram has gathered in the middle; there is neither full black nor full white. On the photo you first click the dark skin of their sibling's cheek; that tone's place on the curve is marked with a point in the lower-middle area. Leaving this point in place, you put a point much lower down, at the tone of the darkest fold of the blanket behind, and pull it down a little. Then you put a point a little above the brightest area on their forehead and raise it slightly.

The curve turns into a soft S whose steep area falls exactly on the range where their sibling's skin sits. The blanket deepens, and the areas where the light from the window shines come to life; on the shadow side of their sibling's cheek, fine tonal transitions and the texture under the eye can still be made out. Their forehead shines but doesn't blow out. Finally, in the blue channel you raise the shadow end very slightly, cleaning the yellowish color seeping in from the walls out of the shadows. The photo is no longer dull, but their sibling's face is still their own.

Common mistakes

Summary

The tone curve is the line that shows each tone's new value in the photo; the straight diagonal changes nothing, pulling a point up lightens that tone, pulling it down darkens it, and control points bend the curve softly. The curve's slope decides the contrast: in a steep area the tones separate, in a flat area they come closer; an S-shaped curve gives contrast in the midtones while squeezing detail in the shadows and highlights, and unlike the slider its steep area can be placed in the tonal range you want. Raising the black end gives faded blacks, channel curves give color to the shadows and highlights or correct shifts; skin detail is protected by keeping the curve steep in the ranges where dark and light skin sit, and every change is checked by enlarging the face.

Mini assignment

In this assignment you will edit the same photo only with the tone curve; don't touch the contrast slider in the basic panel.

1. Open a portrait that looks dull and, without doing anything first, look at the histogram behind the tone curve; note where the tones gather.
2. Apply a light S-curve once and a strong one once to the same photo; compare the detail in the shadows and highlights in the two frames.
3. Click on the face on the photo and find that tone's place on the curve; prepare one more frame by placing the S's steep area around this tone.
4. In one copy get faded blacks by raising the black end, in another copy give the shadows a cool tone with the blue channel curve; check the skin by enlarging it each time.

Check yourself

• Looking at a tone curve, can you say in which tonal range the contrast increases and in which it decreases?
• Did you see in your own frames what the S-curve loses at the ends?
• While bending the curve, did you check by enlarging the face whether the skin's detail and color were kept?

Glossary

Mini quiz

1. What happens to the photo when the tone curve is a straight diagonal?

• A) The photo darkens
• B) No tone changes
• C) Contrast increases
• D) The colors fade

2. What does the curve being steeper than the diagonal in a tonal range lead to in that range?

• A) The tones come closer to each other
• B) The colors shift
• C) The tones darken
• D) The tones move apart from each other and contrast increases

3. While an S-shaped tone curve raises contrast in the midtones, what happens in the darkest and lightest tones?

• A) Because the curve flattens at the ends, these tones come closer together and detail gets squeezed
• B) Contrast increases at the ends too
• C) The ends aren't affected at all
• D) The colors get saturated

4. After a strong S-curve is applied to a dark-skinned person's portrait, the shadow side of the cheek drops to a uniform darkness. Which is the most fitting correction?

• A) Raise the red channel
• B) Raise the contrast slider too
• C) Move the S's shadow point below the skin and keep the curve steep in the skin range
• D) Pull the black end down

Answers

1. B — The straight diagonal maps every tone to its own value.
2. D — A steep slope moves close tones apart from each other; this is contrast itself.
3. A — The curve's total height doesn't change; when its middle steepens, its ends flatten.
4. C — A curve that stays steep in the skin range keeps the fine tonal differences of the cheek.

Summary card

• Horizontal axis the original tone, vertical axis the new tone; the straight diagonal changes nothing.
• Slope is contrast: separates in a steep area, comes closer in a flat area.
• The S-curve brings the midtones to life, squeezes the ends; place the steep area on the face.
• Raise the black end for faded blacks; give color to shadow and highlight with a channel curve.
• On dark and light skin keep the skin range steep; enlarge the face with every change.

Bridge to the next lesson

After their sibling's portrait, the drummer opened a landscape photo they had shot on the balcony: a deep blue sky, a lush green hill, orange-tiled roofs. They wanted to darken the sky's blue a little, but every time they tried with the tone curve's blue channel, the color of the tiles and of the green hill shifted too. The curve worked by tones, not by colors. How do you change only a blue sky? The next lesson tells how to manage each color's hue, saturation and luminance separately.