The Histogram Tool
The Histogram tool lets you make global changes to the brightness values for a selected object or group.
You can split the Histogram into sections, then change only the values that lie in one of the split areas.
The Histogram displays 255 possible brightness values for each object or group, with each row in its display representing one brightness level. The scale, shown at the left side of the Histogram, should be centered around the median area of the Histogram.
You can set the following options for the Histogram tool:
Update on Drag: When this box is checked, the tool is interactive. To improve performance, clear the check box. You can see the results of your changes each time you release the mouse button.