Introduction
Histogram is a very useful tool in photography, found on all the digital cameras (compact cameras and DSLR cameras), as well as in all the photography editing programs.
With the histogram you can analyse the tonal value of the image or in other words: it shows how bright or dark the image is (number of tones captured at each brightness level). The color histogram additionally shows the amount of specific colour in the image.
What does the histogram look like?
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 gray 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).
Some new cameras already offer a live histogram preview of the image BEFORE it is taken! How cool is that!
What does 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.
So how do you use it?
When you take the image (or before it, if the camera has this feature), check the histogram on the camera to determine wheather the image is underexposed or overexposed.
By learning the "common" shapes of the histogram you will quickly know whether the image you have taken is good or do you need to change the settings and take the image again.
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!
Additionally, 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 above; while if taking images in JPEG, when contrast is applied by the camera automatically, it is usually going to look a bit “better”.
Arkady Pogostkin
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Good intro, but only a starting point.
I suggest the following links for more detailed info:
http://www.cambridge
Robert Gingher
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A lucid introduction to the histogram