On the camera screen, in Lightroom, or in Photoshop, a histogram shows the distribution of pixels in an image. How can we interpret this graph, which is both enigmatic and indispensable?
The shot is in Raw format. However, the histogram displayed on the camera’s rear screen is that of a JPEG. At the top is the brightness histogram. Below are the histograms of the image’s RGB channels.
Histogram
This word first appeared in its English form, histogram, in 1895. British mathematician Karl Pearson coined it. It is constructed from two Greek roots, histos (mast) and the affix gram (line). This is because the graph, composed of bars, resembles a series of masts. Used in digital imaging, it represents the quantity and distribution of recorded tones. A high bar indicates a strong presence of a tone, while a small bar indicates its low frequency. The absence of a bar shows the absence of a tone. The histogram of an image is essential, used both in a camera’s display menu and in post-production software.
The histogram is an essential tool in digital photography. It represents the quantity and distribution of recorded tones.
4 histograms for one image
The histogram can be displayed in two ways. The first is brightness or lightness. A synthesis of the RGB channels, with its bars being white. This is the most important for evaluating clipping. Next are the histograms for the R, G, and B layers, which are also helpful to analyze. Clipping can occur in a channel even if it does not appear on the brightness histogram. In a portrait, if the red channel (skin) is clipped on the right, there is a risk of losing skin tones. This often occurs when the white balance is inappropriate. Adjusting it will solve the problem. Otherwise, reducing exposure is necessary to avoid information loss.
The brightness histogram, displayed in white on a black background, is a synthesis of the primary color channels red, green, and blue.
Histogram, JPEG, and RAW
In a camera body, the histogram is generated from a JPEG version of the photo, even if you shoot only Raw. In post-production software, the Raw histogram may vary depending on the file development settings. Also, in the case of a Raw file, clipping in shadows or highlights that may be indicated on the camera screen may disappear in post-production software. With experience, photographers can assess, for each of their cameras, whether the clipping indicated on the camera can be recovered in post-production.
Depending on the development profile (Adobe Color, Adobe Neutral, Adobe Vivid), the histogram of a Raw file in software such as Lightroom may change. No other development settings are modified here.
Tone distribution
For an 8-bit encoded image, the histogram consists of 256 vertical bars, one for each tone. The left end represents black, and the right end represents white. These extremes are fundamental reference points. Unless you want pure black or pure white in the image, no bar should touch either of these two sides. Between these extremes, the histogram shape varies from one subject to another, depending on the brightness range specific to each subject. If a clear sky occupies a large part of the image and a small area is dark, a prominent peak appears on the right. If the opposite is true, the prominent peak is on the left. If midtones dominate, a mountain shape appears in the center of the image. The histogram, therefore, reflects specific situations each time. There is no such thing as an “ideal” histogram, except for the recommendation to avoid it hitting either the left or right side. It provides information about the distribution of tones in the subject.
Histogram display varies depending on the device (here, an Olympus camera). The histogram stops on the left, leaving the right half empty, which is a sign of obvious underexposure. In post-production, this defect can be corrected on a Raw file, with a risk of increased noise and less accurate colors.
If the histogram spreads across the entire range without touching the extreme edges, all tones in the image are correctly recorded. The peak in the highlights indicates a high presence of light pixels. These correspond to the sky, which represents a quarter of the image area.
The histogram is clipped on the right. The left half is empty, indicating severe overexposure. The histogram shown is for a JPEG. In post-production, overexposure can be corrected on a Raw file, even if the bars are compressed on the right. Based on experience, we can estimate the correction tolerance of an image whose histogram is clipped on the right.
Histogram and contrast
If the subject has little contrast, the histogram will only occupy a portion of its range, half or even a quarter of its range. There is room to expose the image satisfactorily. We recommend exposing as far to the right as possible, without letting the tones hit the right side of the histogram. The image may appear too bright. It will be darkened in post-production. The specific characteristics of the Raw format mean that much more information is recorded in the mid to light tones (the right half of the histogram) than in the dark to mid tones (the left half).
When the subject is highly contrasted, to the point of clipping both shadows and highlights, the tones hit the left and right sides of the histogram. If the post-production software cannot recover any detail in the dark or light tones, there is only one solution: bracket the shot and perform HDR merging in post-production (see article on high-speed handheld bracketing).
The histogram of a high-contrast image is U-shaped, with peaks on the left for very dark tones and on the right for very light tones. In between, there are almost no midtones. Here, the contrast of the scene and the studio lighting was deliberately high, creating a silhouette effect.
In Rajasthan, India, on a foggy day, the dome of a palace is bathed in a diaphanous light. The contrast is significantly reduced. The pixels occupy only a quarter of the dynamic range. In this case, it is better to adjust the exposure so the histogram shifts toward the right, improving tone depth.
An 18% gray card is placed in front of the lens. Opened to f/4, the lens vignettes slightly. On the histogram of the camera screen (Nikon D850 with a 60mm macro lens), the gray reproduction is not perfectly uniform. As a result, despite the uniform gray of the photographed object, the histogram shows a peak with a slightly spread base. A single-value gray image, created in Photoshop (RGB value: 118), produces a histogram with a narrow bar.
Discover the photography courses at Spéos
Spéos offers various training courses ranging from simple one-week photography workshops (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.
Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.
Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos