Histograms are the key to understanding digital images.
A digital image is basically a mosaic of square tiles or "pixels" of uniform color which are so tiny that it appears uniform and smooth. Instead of sorting them by color, we could sort these pixels into 256 levels of brightness from black (value 0) to white (value 255) with 254 gray levels in between. Just as we did manually for the mosaic, an imaging software automatically sorted the pixels of the image below into 256 groups (levels) of "brightness" and stacked them up accordingly. The height of each "stack" or vertical "bar" tells you how many pixels there are for that particular brightness. "0" and "255" are the darkest and brightest values, corresponding to black and white respectively.
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On this histogram each "stack" or "bar" is one pixel wide. Unlike the mosaic histograms, the 256 bars are stacked side by side without any space between them, which is why for educational purposes, the vertical bars are shown in alternating shades of gray, allowing you to distinguish the individual bars. There are no blank spaces between bars to avoid confusion with blank spaces caused by missing tones in the image. Normally all bars will be black as indicated in the second histogram.
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| Correctly exposed image |
This is an example of a correctly exposed image with a "good" histogram.
The smooth curve downwards ending in 255 shows that the subtle highlight
detail in the clouds and waves is preserved. Likewise, the shadow area
starts at 0 and builds up gradually.
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| Underexposed image |
The histogram indicates there are a lot of pixels with value 0 or close to
0, which is an indication of "clipped shadows".
Some shadow detail is lost forever as explained in the
dynamic range
topic. Unless there is a lot of pure black in the image, there should not
be that many pure black pixels. There are also very few pixels in the
highlight area.
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| Overexposed image |
The histogram indicates there are a lot of pixels with value 255 or close
to 255, which is an indication of "clipped highlights".
Subtle highlight detail in the clouds and waves is lost. There are also
very few pixels in the shadow area.
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| Image with too much contrast |
This image has both clipped shadows and highlights. The
dynamic range
of the scene is larger than the dynamic range of the camera.
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| Image with too little contrast |
This image only contains midtones and lacks contrast, resulting in a hazy
image.
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| Image with modified contrast |
When "stretching" the above histogram via a Levels or Curves adjustment,
the contrast of the image improves, but since the tones are redistributed
over a wider tonal range, some tones are missing, as indicated in this
"combed" histogram. Too much combing can lead to
posterization.
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| Example of camera histogram review with overexposure warning |
Most prosumer cameras and all professional cameras allow you to view the histogram on the camera's LCD so you can adjust the exposure and take the shot again if necessary. Some cameras come with an overexposure warning, whereby the overexposed areas blink, as indicated in this animation. In certain cameras the blinking areas are not necessarily overexposed, but an indication of potential overexposure.
It is essential to keep an eye on the histogram when taking pictures and when editing them to ensure proper exposure and avoid losing shadow and highlight detail.