The Histogram is accessible from Tools tab or with Alt+H shortcut.
The Histogram Aid is an important and powerful feature that can be used to see if image is properly exposed or to make Screen Stretch or Levels over image to see fainter details. This is "on screen" operation and the saved image is not altered!
The histogram is a graphical representation of the tonal distribution in the image. It shows the number of pixels for each tonal value. The horizontal axis of the graph represents the tonal variations, while the vertical axis represents the number of pixels in that have this particular tone. The left side of the horizontal axis represents the black and dark areas, the middle represents medium bright areas and the right hand side represents bright and pure white areas.
You can set one black and one white point. The black point defines the left part of the histogram that you want to exclude (dark areas). The white point defines the right part of the histogram that you want to exclude (bright areas). The range between the black and the white points is the one that is showed on the screen. When this range is smaller than the original one stored in the image the remaining tones are intensified. This action is known as Screen Stretching or Levels filter. When the back point is in the very left and the white point is in the very right, the image is un-stretched and all tones are showed as they are stored in the image.
In order to provide better control and more clean user interface, the Histogram Aid has different look in DSLR and CCD/CMOS modes.