Adobe Photoshop CS6 User Manual page 268

Note: Choosing Image > Adjustments > Curves applies the adjustment directly to the image layer and discards image information.
2. (Optional) To adjust the color balance, in the Adjustments panel (CS5) or the Properties panel (CS6), choose the channel you want to adjust
from the menu to the left of the Auto button.
3. In the Adjustments panel (CS5) or Properties panel (CS6), do any of the following:
Click directly on the curve line and then drag the control point to adjust a tonal area.
Select the On-image adjustment tool and then drag in the area of the image you want to adjust.
Select the On-image adjustment tool and click the tonal areas in the image that you want to adjust. This places control points along the
curve line.
(CS6) Choose a preset from the Preset menu.
Dragging a control point up or down lightens or darkens the tonal area you're adjusting. Dragging a control point left or right increases or
decreases the contrast. You can add up to 14 control points to the curve. To remove a control point, drag it off the graph. As you adjust the
tonality, the graph continues displaying the original diagonal baseline and image histogram as references. These options can be turned off,
see Set Curves Display Options.
4. (Optional) Do any of the following to modify the adjustment:
Add more points directly to the curve to adjust different tonal areas.
Click the On-image adjustment tool in other areas of the image, and drag up or down.
Move the Set Black and White Point sliders or use the Eyedropper tools to specify the darkest and lightest values in the image.
Click a point on the curve, and enter values in the Input and Output text boxes.
Select the pencil icon and draw a new curve over the existing one. When you have finished, click the Smooth the Curve Values icon or
to smooth the curve you drew. Clicking more than once continues to smooth the curve further.
Points on the curve remain anchored until you move them. You can make an adjustment in one tonal area without affecting other areas.
Clicking the On-image adjustment tool in the image adds control points to the curve. Moving the control points adjusts the tonality of the image.
Removing control points from a curve
To remove a control point, do any of the following:
Drag the control point off the graph.
Select the control point and press Delete.
Ctrl-click (Windows) or Command-click (Mac OS) the control point.
Set Curve display options
You can control the curve grid display using the Curve display options.
1. Apply a Curves adjustment.
2. In the Adjustments panel (CS5) or the Properties panel (CS6), choose Curves Display Options from the panel menu.
Note: If you chose Image > Adjustments > Curves, expand the Curve Display Options in the Curves dialog box.
3. In the Curves Display Options dialog box, select any of the following:
Light (0-255) Displays the intensity values for RGB images in a range from 0 to 255, with black (0) at the lower-left corner.
Pigment/Ink % Displays the percentages for CMYK images are displayed in a range from 0 to 100, with highlights (0%) at the lower-left
corner.
Simple Grid Displays gridlines in 25% increments.