Nikon D300 User Manual page 669

Figure 9-20. Post-processed cabin picture and its histogram
(in-computer manipulation)
The photographer must be the judge. The greater apparent
detail in this image is the result of compressing the mid-range
of the light values a bit in the computer. If you compress or
make the mid-range light values smaller, that will tend to pull
the dark values toward the light side and the light values
toward the dark side. So, you will have more apparent detail
in your image. It's like cutting a section out of the middle of a
garden hose. If you pull both of the cut ends together, the
other two ends of the hose will move toward the middle, and
the hose will be shorter overall. If you compress or remove
the mid-range of the histogram, both ends of the graph will
move toward the middle. If one end of the graph is beyond
the edge of the histogram window (clipped off), it will be less
so when the mid-range is compressed.
We are simply trying to make the histogram fit into the frame
of its window. If we have to cut out some of the middle to
bring both ends into the window, well, there is usually plenty
in the middle to cut out, so the image rarely suffers.
Remember, this is done outside of the camera in a computer.
You can't really control the in-camera histogram to compress
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