Use the Adjust Color by Zone
window when you want to make adjustments that affect different parts of
an image differently, say when an image has some areas that are too light
or dark or washed out, but other areas that are not. To access this window,
select Color/Adjust by Zone from the image
menu.
Running across the top of the window are controls for selecting the attribute you want to adjust (brightness, contrast, or gamma); the channel to adjust (RGB=all colors, or red, green, or blue separately); and the number of zones you want the image divided into.
The lower portion of the window is taken up by the area where you make the adjustments, with the zones running from darkest parts of the image on the left (aka shadows) to the lightest parts of the image on the right (the highlights). The horizontal line running across the middle indicates the neutral state, while the space above it represents increased values (e.g. more brightness) and the space below it is decreased values (e.g. less brightness).
To make adjustments, set the controls at the top as desired, then increase or decrease the values by either clicking in each zone above or below the line, or by holding down the left mouse button and dragging the mouse across the adjustment area. Each time you make an adjustment, Easel shows the value by drawing a red bar, as shown. When the mouse stops moving, Easel updates the image to show the effect of the adjustments.
Hold down the Ctrl key and hit the R key to reset the current attribute to neutral in all zones.
Hold down the Ctrl key and hit A to toggle between absolute and relative zoning. Absolute zones divide the picture into mathematically equal zones, while relative zones divide it by frequency. Easel defaults to absolute, which is generally more useful.
Resize the window to get more or less adjusting precision.
Note: It is often best to make gradual changes between zones, as shown. Abrupt changes can have visibly weird effects. It often takes a little experimentation to get the correction you want, including picking the right number of zones.