A histogram is a graph of the brightness in a photo. Dark tones sit on the left, light tones on the right, and the height of the graph at any point shows how many pixels have that brightness. Read it and you can judge your exposure without trusting the glow of a screen.
Practice: read the histogram
Ten rounds of three kinds. In some you pick the photo that made a histogram, in some you pick the histogram that belongs to a photo, and in some you say what a histogram is telling you. Each photo is shown from two stops darker to two stops brighter, a red edge marks clipping, and every answer comes with an explanation.
Did a red edge catch you out? Clipping explains what it means when the graph is cut off at either wall and how to avoid it.
Want to move the exposure yourself and watch the graph follow? The camera simulator has a histogram you can switch on while you change the settings.
How to read the left, middle and right of a histogram
The horizontal axis runs from pure black at the left edge to pure white at the right edge. Roughly, the left third is shadows, the middle third is midtones and the right third is highlights. The vertical axis is a count of pixels, so a tall peak means a lot of the picture has that brightness and a flat stretch means almost none does.
- Piled at the left. Most of the picture is dark. That is correct for a night street or a cave, and a sign of underexposure for a daylight portrait.
- Piled at the right. Most of the picture is bright. That is correct for snow or a white wall, and a sign of overexposure for a moody evening scene.
- A hump in the middle. Most tones are midtones. This is typical of flat light and of an average scene, and a hump with nothing at either end often means low contrast.
- Two humps with a dip between. A bright part and a dark part, such as a sky over a dark skyline or a window in a dim room. Few midtones are present.
The same image can look right or wrong depending on what you meant to photograph, so the shape alone never tells you whether to change anything. What matters is whether the shape matches the scene and whether it is cut off at the edges. A histogram describes the tones in a picture. It is a report, not a target.
A wide spread of tones is called tonal range. A picture can have strong blacks and clean whites and still have most of its pixels in the shadows. That is fine: a wide tonal range gives a picture depth, but you do not need the pixels shared out evenly across it. A portrait with a dark background and a few bright accents on skin and eyes is a good picture with a lopsided histogram.
Clipping and blinkies: when the graph hits the wall
Clipping happens when a tone is brighter or darker than the camera can record. Everything past the limit is stored as the same value: pure white at the right or pure black at the left. On the histogram it shows as a tall bar jammed against the edge, or a graph that is climbing when it is cut off by the wall instead of fading to nothing.
A clipped area has no texture, because every pixel in it has the same value. A white dress that clips turns into a flat patch, and a sky that clips turns into a blank hole. Shadow clipping does the same in the dark, though shadows usually tolerate brightening in editing better than highlights tolerate darkening. A highlight that has clipped in the raw file cannot be rebuilt. For that reason, protect the highlights first when the choice is between the two. The page on blown highlights shows how to spot and avoid them.
Most cameras can also flash the clipped areas on the playback screen. People call these warnings blinkies, and they answer a different question from the histogram. The histogram says how much of the picture is clipped. The blinkies say where. Use both: if the blinking area is a lamp or the sun, you can accept it, but if it is a face, you cannot.
Small amounts are normal. A glint on a chrome edge, a sun reflection on water or the sun itself will clip in almost any exposure, and chasing them would darken everything else. The game marks an edge in red when more than half of one percent of the pixels are clipped there. That is a useful line for a first glance, not a law. A few clipped pixels in a glint are harmless, and the same count across a cheek is a problem.
Luminance and RGB histograms: how a color can clip while the graph looks fine
The default histogram on most cameras is a luminance histogram. It combines the three color channels into one brightness value for each pixel, with green counting for the most and blue for the least. It is a good summary of how bright the picture is.
An RGB histogram shows the red, green and blue channels separately, either as three graphs or as three overlaid curves. This matters because a single channel can clip while the combined brightness stays well inside the range. A saturated red flower, a sunset or a neon sign can push the red channel against the right wall while the luminance graph sits comfortably in the middle. The result is a patch of color with no detail in the petals, where the shading has gone flat and the color looks painted on.
If you search for the red channel on your camera histogram, this is what you are looking at. A red curve that climbs to the right edge and stops means the red channel has clipped. Lower the exposure a little until the red curve pulls away from the wall, or accept the clip if the red area is small and unimportant. Blue skies do the same with the blue channel, and bright yellow can clip red and green together.
Check the RGB view whenever the scene has strong, saturated color. For skin tones, neutral scenes and most landscapes, the luminance histogram is enough.
Where the camera histogram comes from, and why it is cautious for raw
The graph on the back of your camera is not computed from the raw sensor data. It is built from the processed preview image, the same small JPEG that you see on the screen. That preview has had white balance, contrast, saturation and sharpening applied, using the picture style that is currently selected.
That has two effects. First, a vivid or high contrast picture style makes the histogram look more clipped than the underlying data is. Second, a raw file generally holds more headroom than the preview shows, so the histogram can warn you about clipping that is not in the raw file. In practice, the histogram is conservative for raw shooters. If it just touches the right wall, a raw file may still hold detail that the graph has lost. If you want to know how much, shoot a test series, bring the files into your raw editor and see where the real limit is for your camera.
This is one of several differences covered in raw versus JPEG. If you shoot JPEG, the histogram is an accurate description of the file you will get. A neutral picture style makes the graph closer to what the raw file holds.
When you open the same picture in editing software, the histogram you see there is based on the processed image in that program, which is why it rarely matches the one on the camera exactly.
High key and low key pictures are not mistakes
A bright, airy portrait on a white background should have its histogram on the right. A silhouette, a night scene or a moody low key still life should have most of its pixels on the left. Neither is wrong, and forcing either toward the middle makes the picture duller and ruins the mood.
The trouble is that a camera meter aims for an average gray, so it pulls snow and white walls down toward gray and lifts dark scenes up to gray. The histogram is how you catch that. If a snow scene shows a hump at the center and nothing near the right edge, the snow is gray, and you need to add exposure. If a night scene fills the middle of the graph, the night has been turned into dusk and you need to subtract exposure. See exposure compensation for how to apply the correction.
The question to ask is whether the histogram matches what you intended. The page on high key and low key photography goes deeper into both, and the terms underexposure and overexposure only make sense when you compare the result with what you wanted.
Expose to the right: using the histogram for cleaner shadows
A digital sensor records light in a linear way, so half of all its data is spent on the brightest stop of the scene. The shadows get far fewer values to describe them, and that is where noise shows up. Exposing to the right means giving the scene as much light as you can while keeping the histogram off the right wall, then bringing the brightness down when you process the file.
The way to do it is to raise the exposure with a slower shutter speed or a wider aperture, check the histogram, and stop when it is close to the right edge without climbing the wall. Raising the ISO does not help: it makes the image brighter without gathering any more light, and it can push the highlights over the edge. Work with raw files, because a JPEG made this way will look washed out and has little room to bring the exposure back.
This works best when the scene is still and you have time to check, such as a landscape on a tripod or a studio set. It is a poor fit for fast action where you cannot review between frames. Because the camera histogram is conservative for raw, you can often push a little farther than it suggests. The full technique is on the expose to the right page.
Contrast and dynamic range: what the width of the graph tells you
A narrow graph that sits in the middle means low contrast: the brightest tone in the scene is not far from the darkest. This is what you get from fog, an overcast sky or a flat room. A graph that spreads from wall to wall means high contrast, with deep shadows and bright highlights in the same frame. Whether that is good depends on the picture. Fog should look low in contrast, and a backlit scene is full of it.
The width also tells you whether the scene fits inside the range the camera can record. Dynamic range is the span from the darkest tone with usable detail to the brightest one before clipping. If the scene is wider than that range, the histogram will hit a wall at one end or both, and no exposure setting will fix it. You have to choose what to give up, usually the shadows, or take several exposures and combine them. The page on dynamic range covers the limits in more detail, and a pair of frames taken with exposure bracketing is one way to capture a wide scene.
Using the histogram while you shoot
- Take a test shot at your first guess for the exposure.
- Look at the histogram, not the picture. Screens look brighter in a dim room and darker in bright sun, but the graph does not change.
- Check both walls. If the graph is cut off at the right and the clipped area matters, reduce the exposure. If it is cut off at the left and the highlights have room, add some.
- Change the exposure with a shutter speed, aperture or ISO change, or with exposure compensation if your mode sets the exposure for you.
- Shoot again and check again. Repeat when the light changes.
This takes a few seconds when it becomes a habit. It is much more reliable than looking at the screen and guessing, which some people call chimping. A mirrorless camera or a DSLR in live view can show the histogram before you press the shutter, so you can set the exposure and not need the test shot. On a DSLR with the optical viewfinder, you only see it after the shot. The histogram matters even more for long exposures, because a shot of several seconds or minutes is impossible to judge by eye: check the graph after each test exposure and adjust from there.
Different metering also changes the story. If a bright sky fills the frame, the way the camera measures it determines where the graph lands. The page on metering modes explains how each one reads the scene.
Using the histogram when you edit
The histogram in an editing program works like the one on the camera, with the addition that you can use it as a control. Hovering over a region usually shows which slider affects that part of the tones: blacks, shadows, exposure, highlights or whites. Many programs let you drag on the graph itself to move those tones.
Clipping warnings are available too, often as colored overlays, with one color for blown highlights and another for crushed shadows. Turn them on while you work and the lost areas are visible on the picture itself.
The classic levels control has three triangles under the graph. The black triangle sets the darkest tone, the white triangle sets the brightest tone, and the gray triangle in the middle moves the midtones lighter or darker without touching the ends. Dragging the black and white triangles in to meet the edges of the data stretches a flat picture to the full range. The page on the black point explains how the end points work, and the step by step version is in setting your black and white points.
Heavy editing leaves its mark on the graph. A comb of thin gaps and spikes means the tones have been stretched too far, and the gradients in a sky may show visible steps called banding. Making smaller changes, or working from a raw file, reduces it.
Common mistakes with the histogram
- Trying to make every histogram a bell curve. The right shape depends on the scene. A low key image should lean left and a high key image right.
- Trusting the screen over the graph. The screen changes with the room and your eyes. The graph does not.
- Panicking over a few clipped pixels. A glint or a light source is fine to lose. Ask whether the clipped area is something you care about.
- Never looking at the RGB view. A saturated color can clip in one channel while the luminance graph looks fine.
- Looking at it only back home. The time to fix the exposure is on location, and again whenever the light changes.
- Expecting it to tell you about focus. The histogram knows nothing about sharpness or composition. Zoom into the picture to check those.
- Expecting the camera graph to match the editor. The camera graph is based on a processed preview, so some difference is normal.
Try this: a ten minute histogram test
- Put a white sheet of paper and a black cloth in the same light, and fill the frame with each in turn in an automatic mode. Look at both histograms. Both should sit near the middle, because the meter aims for average gray.
- Use exposure compensation to bring the white up by about one stop and the black down by about one stop. Watch the graphs move to the right and to the left, and stop when each one looks like the thing really looks.
- Photograph a bright window with a dark room in the same frame. Find the exposure where the window just stays off the right wall, and note how much of the room is lost on the left.
- Shoot a still scene at normal exposure and again one stop brighter, keeping the highlights off the right wall. Bring the bright one back down in your raw editor and compare the shadow noise at full size.
- Switch to the RGB view and photograph something saturated and red or blue. Raise the exposure until one channel climbs the wall while the luminance graph still looks fine.
Questions about the histogram
What does a good histogram look like?
There is no single good shape. It is one that matches the scene and does not lose important detail to clipping at either edge. A daylight scene often fills most of the width, while snow, night and silhouettes each give a different shape that is just as correct.
Should the histogram touch the right side?
Only if the right edge holds detail you can live without, such as a light source or a glint. Close to the right wall is good for noise, and up against it with a large clipped area is a loss. For raw shooters, a little contact can still hold detail, because the camera graph is conservative.
Why does the histogram in my editor look different from the one on my camera?
The camera graph is made from a processed preview image, and your editor measures its own rendering of the raw file. Different tone curves, color spaces and settings lead to different graphs, so the shape moves a little between them.
Does the histogram show whether my photo is in focus?
No. It counts brightness and nothing else. Zoom in on the picture on the screen to check focus and sharpness.
What do gaps in the histogram mean?
A gap means no pixels have those brightness levels. A few small gaps are normal. A comb of many thin gaps after heavy editing can mean banding in smooth gradients, so make smaller changes or work from a raw file.
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