What is a histogram?

A histogram is a graph that measures the brightness of an image by representing the frequency of each tone as a value on a bar chart. The horizontal axis moves from pure black on the left side of the histogram, through shadows, mid tones, and highlights, all the way to the brightest white on the right side. The vertical axis represents the frequency, or intensity, of each tone, with peaks for high frequency and valleys for low. Most digital cameras have both a luminosity histogram (measuring total brightness) and color histogram (measuring the intensity of red, green and blue tones).

Left side run-off - If you see high-frequency tones or peaks running off the left side or your histogram, that means your blacks are being clipped, and your camera is not picking up all the shadow detail that it might. This type of “low-key” image might be what you’re going for, but if it isn’t, you can let in more light by lowering the shutter speed, widening the aperture, or raising ISO (light sensitivity) of your camera. Each pf these fixes can reduce the image quality, but you can experiment with slight adjustments to all three to get the combination of brightness and sharpness that you want.

Right side run-off - If you image is “high-key,” you might expect peaks on the right side of the histogram. But if those peaks are cut off the right edge, the image may be overexposed, meaning the highlight details washed out. In this case, take a shorter exposure, narrow your aperture, or lower the ISO to reduce the light your camera is capturing.