Canon EOS REBEL T8i Advanced User's Manual page 492

[Brightness] display
This histogram is a graph showing the distribution of the image's brightness level. The
horizontal axis indicates the brightness level (darker on the left and brighter on the right)
while the vertical axis indicates how many pixels exist for each brightness level. The
more pixels there are toward the left, the darker the image. The more pixels there are
toward the right, the brighter the image. If there are too many pixels on the left, the
shadow detail will be lost. If there are too many pixels on the right, the highlight detail
will be lost. The gradation in-between will be reproduced. By checking the image and its
brightness histogram, you can see the exposure level inclination and the overall
gradation.
Sample histograms
Dark image
Normal brightness
Bright image
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