With the histogram you can analyse the tonal value of the image. It shows how bright or dark the image is (number of tones captured at each brightness level) and also with a colour histogram it shows the amount of specific colour in the image.

You can find the histogram in the histogram pallet (windows / histogram) or in adjustment layers like Levels and Curves.

It is made of individual columns (each column representing how many pixels there are in specific tonal value) which range from darkest (value 0) to brightest (value 255). Between the two are the grey values.

Most of the cameras show you a histogram of the image already on the LCD display, so you can see the condition of the image already at the location (and correct it if needed – take a new image). Some of the cameras also show you if the image is overexposed (by blinking those areas on the LCD screen).

Please note that there is no correct or wrong shape of the histogram! It is there only to give you more (very valuable!) information of the image. Sometimes you want, e.g., the shadows to be clipped for the effect!

Also, if you are taking images in RAW format (when no contrast is applied by the camera), the histogram probably won’t look “ideal” as shown below; while if taking images in JPEG, when contrast is applied by the camera automatically, it is usually going to look a bit “better”.

Here are some examples of what shapes of the histogram tell you:

Correctly exposed image. This is an example of a “ideal” histogram. The tonal values are distributed from shadows (0) to highlights (255) with no gaps.

Underexposed image. The detail in the shadows is lost (shadows are clipped). Image is dark.

Overexposed image. Image is burned. The detail in the highlights is lost (highlights are clipped). Image is very bright.

Strong contrast. The detail is lost both in shadows and highlights.

Little contrast. Lots of midtones, no deeper shadows or highlights. No rich tones.

Combed histogram. It usually appears when the image was manipulated (applied contrast, using adjustment layers like Levels, Curves) or with a poor scan. Some of the tonal values are missing (seen as white spaces of columns inside the histogram).

When editing an image with Levels or Curves, the histogram in those tools provides a visual feedback where the highlights and shadows on the image are going to be after the edit. Note also that whenever you have been manipulating the image in any way, the histogram will likely to look combed – which doesn’t necessarily mean that the image’s tonal information is too low.

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