The histogram

Learning goal

You will be able to read the histogram as a counting graph that shows how the tones in a photo are spread from dark to bright. You will explain that the left of the graph shows darkness, the right shows brightness, and its height shows how many points there are in that tone; you will work out the photo's overall brightness and contrast from the position and shape of the mountain; you will notice that the graph pressing against an edge means a loss of detail; you will not mistake the histograms of white or dark scenes, which naturally pile up to one side, for an error; and you will know that the histogram counts the whole frame, so a small face needs to be zoomed in on and checked separately.

The question

On the screen of a camera or phone, a small graph sometimes appears in the corner of the photo: a drawing that looks like a mountain range, sometimes leaning to the left, sometimes piled up to the right, and sometimes standing in the middle as a single peak. Most people see it as decoration, or switch it off in the settings.

What is that mountain-shaped graph telling you? In fact it is the photo's most honest witness. When you look at a small screen in the sun, your eye can fool you; the screen's brightness can show the photo brighter or darker than it really is. The histogram, on the other hand, is independent of the eye: it counts the tones inside the photo one by one. In this lesson we will talk about reading this count, and when to trust it and when not to.

Context

In the editing module we talked about the photo's tonal steps, the values ranked from dark to light, and left the graph that shows these steps for later in the exposure topic. At the start of this module you saw that the meter reduces where it looks to an average tone, and in the previous lesson you saw how to tell the camera the scene's tone with compensation. The histogram is the measure that shows the result of these decisions: after you set the exposure, it tells you what you got.

On many cameras you can see the histogram while reviewing the photo after the shot, and on some, live before the shot. On mirrorless cameras and in the pro mode of many phones, the histogram can also be shown on the screen while shooting; that way you can judge the exposure before pressing the shutter button. Most editing apps also have a histogram next to the light settings. In all of them it speaks the same language: left is dark, right is bright.

Simple explanation

What does the histogram show?

The histogram's horizontal axis shows tones: pure black at the far left, pure white at the far right, and every point in between is a shade of gray. This can be called the tonal axis. The vertical axis shows how many points of that tone there are in the photo. The graph rising somewhere means there are many points of that tone in the photo; staying low means there are few of that tone.

The histogram is a count, not a map. It doesn't show where the points in the photo are, only how many there are in each tone. A peak on the right says there are many bright points in the photo; but it doesn't say whether these points are in the sky, on a white shirt or in a window. That is why you always read the histogram together with the photo itself: the graph tells you how much of what there is, and the photo tells you where.

What do the mountain's position and shape tell you?

Where the mountain's weight sits tells you the photo's overall brightness. If the graph leans to the left, dark tones are in the majority in the photo; if it leans to the right, bright tones are. If it is gathered in the middle, the photo is mostly made of mid tones. The mountain's width tells you the contrast: a histogram spreading wide from left to right shows a contrasty photo with both dark and light tones, while a narrow peak in the middle shows a low-contrast, soft photo, like a misty morning.

The edges carry special importance. The graph pressing against the right edge and rising there like a wall shows that some points have been recorded as completely white, that is, that detail has been lost; this is called clipping. Pressing against the left edge shows that some points have been recorded as completely black. The graph touching an edge isn't always a mistake; the sun itself or the bulb of a lamp should be pure white. But if an important area, a face or a sky, has been clipped, that detail doesn't come back. When clipping is acceptable is the subject of the next lesson.

Is there a right histogram?

There is no single right histogram. The histogram of a snowy hill or a white room naturally piles up to the right, because bright tones are in the majority in the scene. The histogram of a night street or of a portrait in front of a black background naturally piles up to the left. Photos made mostly of bright tones are called high key, and those made mostly of dark tones low key. Both low key and high key are conscious choices, and you shouldn't try to pull their histograms to the middle.

The real question is this: does the histogram tell the scene truthfully? Remember the meter's middle gray error: when the camera pulls snow toward gray, the histogram gathers in the middle, and this is a sign that the exposure is wrong. The histogram of a snowy scene should stretch toward the right, but shouldn't press against the right edge and form a wall. Many cameras can also show separate histograms for the red, green and blue color channels, called an RGB histogram. A red dress or an orange sunset can be clipped in a single color channel even if the overall histogram looks good; in a colorful scene, looking at these separate graphs is useful.

Does the histogram show faces?

The histogram counts the whole frame, and a face that covers a small part of the frame can get lost in this count. The face of a small figure inside a wide landscape leaves a trace of only a few points in the histogram; however evenly spread the graph looks, it doesn't tell you whether that face has stayed in the dark. That is why, when photographing people, the histogram isn't enough on its own; you need to zoom in on the face on the screen and look.

Different skins also fall in different places on the histogram. A face darker than a mid tone leaves a trace in the left half of the graph, and a face lighter than a mid tone in the right half; this is so even when the photo is correctly exposed. Always trying to pull the histogram to the middle can lighten dark skin toward gray, or darken light skin toward gray. In portraits, the histogram's job is narrower: to confirm that the face isn't clipped, that is, that on dark skin the shadows don't stick to the left edge, and on light skin the bright points don't stick to the right edge. The skin's right tone, on the other hand, is decided by looking at the face itself.

Visual evidence

Imagine photographing a woman hanging out laundry on a rooftop on a sunny noon. White sheets hanging on the line fill most of the frame. In the frame you shoot in auto mode, the sheets have turned gray; the histogram is a single peak gathered in the middle. You set compensation to plus one and a half. The histogram shifts right, comes close to the right edge but doesn't touch it; the sheets are white and their folds can be made out. The woman's dark skin has left a small trace in the left half of the graph; when you zoom in on her face, the warm color of her skin and the lines around her eyes can be made out.

In the evening you photograph the same woman in a dim café with dark walls. The histogram has piled up to the left; on the right there is only a thin trace left by the lamps. Your first instinct is to pull the graph to the middle; when you set compensation toward plus, the walls turn gray and the woman's face grows pale. You take the compensation back. The histogram again leans to the left, but it doesn't stick to the left edge; when you zoom in on her face, her skin is in its own depth, and her eyes and smile can be made out. Two scenes, two different histograms; both are right.

Common mistakes

Summary

The histogram is a graph that counts the tones in a photo: its left shows darkness, its right brightness, and its height how many points there are in that tone; the mountain's position tells you brightness, its width contrast, and pressing against an edge means detail has been clipped. There is no single right histogram; snowy scenes can pile up to the right and night scenes to the left; in colorful scenes a single channel can be clipped on its own. Because the histogram counts the whole frame, a small face gets lost in it, and different skins fall in different places on the graph; that is why in portraits you need to check clipping with the histogram and the skin's tone by looking at the face itself.

Mini assignment

In this assignment you will make reading the histogram a habit.

1. Turn on the histogram on your camera or phone; if possible, make it appear live while shooting.
2. Photograph three surfaces such as a white sheet, a mid-toned wall and a dark fabric, filling the frame, and note where the three histograms gather.
3. Adjust the white surface with compensation until the histogram stretches to the right without pressing against the edge, and compare the result with your eye.
4. Photograph a friend in a wide frame, and judge the histogram and the zoomed-in face separately.

Check yourself

• Can you explain why the histograms of the three surfaces gather in different places?
• When the histogram pressed against an edge, did you find which detail was lost?
• In the wide frame, was your friend's face recorded in its own tone, even if the histogram looked good?

Glossary

Mini quiz

1. What does the left side of the histogram show?

• A) The left half of the photo
• B) Dark tones
• C) Red colors
• D) The photo's sharpness

2. If the histogram presses against the right edge and rises there like a wall, what has happened?

• A) The photo is too dark
• B) The photo is blurry
• C) Some areas have been recorded as completely white and detail has been lost
• D) The white balance is wrong

3. Why can the histogram of a snowy scene pile up to the right, and is this a mistake?

• A) Because bright tones are in the majority in the scene; it isn't a mistake as long as it doesn't press against the edge
• B) Because the camera is broken; it is a mistake
• C) Because snow is too contrasty; it is a mistake
• D) The histogram always piles up to the right

4. What do you do to judge the exposure of a small face in a wide frame?

• A) Look only at the histogram
• B) Pull the histogram to the middle
• C) Check clipping with the histogram and look at the face's tone by zooming in on the screen
• D) Turn off the color channel histograms

Answers

1. B — The histogram's horizontal axis shows tones; the left end is pure black, and the tones get brighter as you move right.
2. C — The graph pressing against the right edge means some points have been recorded as pure white and detail has been lost there.
3. A — In a snowy scene, bright tones are in the majority; the graph can stretch to the right, as long as important areas don't stick to the edge and get clipped.
4. C — Because the histogram counts the whole frame, a small face gets lost in it; you check clipping from the graph and the tone by zooming in on the face.

Summary card

• The histogram counts tones: left is dark, right is bright.
• The mountain's position tells you brightness, its width contrast.
• A graph pressing against the edge: detail has been clipped.
• There is no single right histogram; snow can pile up to the right, night to the left.
• The histogram can't see a small face: check clipping from the graph, and the skin by zooming in on the face.

Bridge to the next lesson

You have seen that the histogram pressing against an edge means a loss of detail. But can an area clipped to pure white be rescued later, or is that detail gone forever? And how can the camera warn you about this loss at the moment of shooting? The next lesson describes the limits of overexposure and underexposure, the blinking highlight warning and the zebra pattern.