Photoshop CS6 Tutorial
How to Use Curves in Photoshop
Curves can be applied as a one time process or non-destructively as an Adjustment Layer. The purpose of curves is to re-map the values of the original image and assign them either lighter or darker values.
Curves can be applied to the entire image or adjusted one channel at a time (red, green or blue). To use curves on a channel, you would select the channel and choose Image > Adjustments > Curves.
Open an image in Photoshop (File > Open...).
With the image layer selected, click the Create New Adjustment Layer icon in the Layers palette. Choose Curves... from the menu.
The Vertical Scale represents the Output image luminosity values. The darker values are at the bottom and the lighter are at the top.
The Horizontal Scale represents the original image luminosity values. The darker values are towards the left and the lighter are on the right.
By default, the line is straight and the input and output values are identical. If you click in the center of the line and raise the curve slightly, the output image becomes lighter.
If you take the same point you added in step five and lower it, the final image darkens.
A steeper curve adds more contrast to the image.
Using Adjustment Layers is considered a non-destructive workflow because you can always return to your original curve settings. You can also apply Curves to an image by choosing Image > Adjustments > Curves. Using this method doesn't give you the ability to return to the curves dialog box and edit the settings in the future.
Removing Curve Edit Points
If you need to remove any curve edit points, simply click-drag the points from the line to beyond the graph in the edit panel.