A histogram is a graphical display that represents the distribution of tonal values across an image or video frame, showing how many pixels fall at each brightness level from pure black on the left to pure white on the right. Rather than evaluating exposure and tonal range by eye: which is unreliable because the appearance of an image on a monitor changes with ambient light, monitor calibration, and viewing angle: a histogram provides an objective, quantitative picture of what tonal information the image actually contains.
The horizontal axis of a histogram represents luminance, or brightness, with the leftmost point corresponding to the darkest possible value (pure black, or 0 in an 8-bit image) and the rightmost point corresponding to the brightest possible value (pure white, or 255 in an 8-bit image). The vertical axis represents the number of pixels at each brightness level, with tall peaks indicating many pixels at that tone and low or absent sections indicating few pixels of that brightness. The overall shape of the histogram describes the tonal character of the image: a histogram clustered toward the left indicates a dark, underexposed or low-key image; one clustered toward the right indicates a bright, high-key or overexposed image; and one distributed across the full range indicates an image with a wide tonal spread.
In photography and cinematography, a histogram clipped at the left edge indicates crushed blacks ( areas of complete shadow with no detail ) while clipping at the right edge indicates blown highlights: areas of complete overexposure where all detail is lost. The histogram is therefore one of the most reliable tools for judging whether an image has retained detail in the extremes, independent of how the image looks on a specific screen. Camera manufacturers display live histograms during shooting, and colour-grading software such as DaVinci Resolve shows histograms in real time as adjustments are applied.
In colour work, the histogram is commonly displayed as three overlapping channels ( red, green, and blue ) allowing colourists to identify colour casts and imbalances between channels as well as overall exposure. In post-production, using the histogram alongside the waveform monitor and vectorscope gives the colourist a complete objective view of the image's technical state.
