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Histogram
A histogram shows how many pixels in a picture are at each of its possible brightness levels. This graph is displayed in the top part of the Side Panel on the right in the Manager, Develop, and Editor modules. Its horizontal axis represents the scale of possible brightness values, usually either for the picture overall or for a single color channel. Its vertical axis represents the pixel frequency in the picture for each of these values.
If the histogram shows too many pixels on the left (that is, in the dark tones) and not enough on the right, then the picture is probably underexposed. If most values are on the right, the picture is probably overexposed. In most cases, a normal photo should make use of the whole range of the histogram.
Histograms are only truly useful for pictures with 24-bit, 32-bit, or 48-bit color, or for grayscale pictures. (However, any picture in a modern photo format will use one of these color types anyway.)
Right-clicking on a histogram will show a context menu where you can switch among several display modes: Brightness, Brightness And Color Channels, Color Channels, Red Channel, Green Channel, Blue Channel. The histogram can also be shown at Half Size/Full Size/Double Size.
If the selected photo contains High Definition Range data (HDR) and your computer and monitor support HDR display, the histogram displays HDR data as well.
More Information
[Infographic] Histograms for Beginners
HDR Display in the Histogram
When HDR mode is active, the histogram is visually split into two halves by a vertical line. The left half represents the picture’s standard dynamic range (6 EV), and the right half represents the picture’s high dynamic range in the lights.
Because current HDR monitors are able to add up to 4 additional EV, the right half of the histogram is split using vertical lines into four parts, each indicating one EV.
The bottom part of the histogram contains a horizontal line, whose left portion shows a gradient from black to white. The right potion has a white line instead of a gradient; it indicates how many additional EV the given monitor is able to provide.
This monitor can show +1.2 EV.
This monitor can show +2.9 EV.
The red line represents the HDR data in the picture that the monitor cannot support. If you have turned on HDR mode on an SDR monitor, there will be a red line for the whole right part of the histogram, indicating that the given monitor is not able to display HDR content. The red exclamation point icon at the bottom right corner of the histogram alerts you to this fact as well.
Blowout Display
Show Blowout button. It can temporarily show overexposed areas (blowout) and underexposed areas in the current picture. To turn it on, use the control in the histogram or the View | Blowout top-menu item, or use the [Shift+O] keyboard shortcut.
In regions with blowout, one or all color elements have reached their maximum values. No room to go higher means no room for variation, so regions with blowout lack detail. Overexposure is actually not the only cause of blowout; it also will appear in regions with strong light sources, like the sun or a fire.
Zoner Studio can highlight eight different types of blowout: blowout in the R, G, and B channels, combined RG, RB, and GB, RGB (places where all three elements have blowout), and summary blowout, where even though there is no blowout in the individual channels, their sum exceeds a certain threshold. You can choose to have areas without blowout be displayed in grayscale or in color. Areas with blowout are shown in saturated colors representing the affected channels: red for R, green for G, blue for B, yellow for RG, purple for RB, cyan for GB, and bright yellow for RGB. Summary blowout is displayed in medium yellow.
Zoner Studio can also call attention to underexposed areas – overly dark places lacking detail. These places are displayed in bright sky blue.
You can change how blowout is highlighted using Settings | Preferences | Other | Display of blowout and underexposure. For more on this, see the topic about the “Other” section of the program preferences.