In the Curves tool, you adjust points throughout an image’s tonal range. Initially, the image’s tonality is represented as a straight diagonal line on a graph. The upper-right area of the graph represents the highlights and the lower-left area represents the shadows. The horizontal axis of the graph represents the input levels (original image values) and the vertical axis represents the output levels (new adjusted values). As you add control points to the line and move them, the shape of the curve changes, reflecting your image adjustments. The steeper sections of the curve represent areas of higher contrast while flatter sections represent areas of lower contrast.
Moving a point in the top portion of the curve adjusts the highlights. Moving a point in the center of the curve adjusts the midtones, and moving a point in the bottom section of the curve adjusts the shadows. To darken highlights, move a point near the top of the curve downward. Moving a point either down or to the right maps the input value to a lower output value, and the image darkens. To lighten the shadows, move a point near the bottom of the curve upward. Moving a point either up or to the left maps a lower input value to a higher output value, and the image lightens.
If you have any questions about this or other tools, or have any comments, suggestions, error messages, please feel free to contact us.