Adjusting Curves for Hard-to-Correct Photos

The Curves command is one of the most advanced Photoshop correction tools available, offering sophisticated control over the brightness, contrast, and gamma levels in an image; I’m talking about control that is far beyond that offered by the Levels and Brightness/Contrast dialog boxes. This section introduces you to the functions of the Curves command, but you’ll
want to practice using it a great deal to gain the kind of experience you need to work with it effectively.

Whereas the Brightness/Contrast dialog box lets you change an image globally, and the Levels command allows you to change the shadows, highlights, and midtones separately, Curves goes far beyond either of those settings. It lets you change pixel values at any point along the brightness level, giving you 256 locations at which you can make corrections. You can work with the combined Red, Green, and Blue color channels (or CMYK channels) or apply your changes to the individual colors. Here are some tips for using Curves:
- Access the Curves dialog box by choosing Image-->Adjustment-->Curves or by pressing Ctrl+M (Ô+M on the Mac).
- The horizontal axis maps the brightness values as they are before image correction.
- The vertical axis maps the brightness values after correction.
Each axis represents a continuum of 256 levels, divided into four parts by finely dotted lines. In the default mode, the lower-left-hand corner represents 0,0 (pure black) and the upper-right-hand corner is 255,255 (pure white). By default, the dialog box shows a 4 x 4 grid; Alt+click (Option+click on the Mac) inside the grid to toggle it to a 10 x 10 grid, as shown in the figure.
- Whenever you open the Curves dialog box, the graph begins as a straight line. Unless you make changes, the input is exactly the same as the output, a direct 1:1 correlation.
- When you use the Eyedropper tool from the Tools palette to click in the image, a circle appears on the graph to show you the value of the pixel being sampled. At the bottom of the Curves dialog box, you can read the pixel’s input and output values.
- When you click the Auto button, the darkest pixels in the image (the deep shadows) are reset to black and the lightest areas are set to white. As with the Levels dialog box, this option is the easiest way to make a correction.
- The Curves dialog box has black, white, and gray eyedropper tools you can use to set the black, white, and midtone points, just as you can with Levels.

Adjusting curves
If you click at any point on the curve other than the endpoints, Photoshop adds a control point that shows your position. You can remove a control point by dragging it downward until it is completely off the graph. Experiment with the curves to see how they affect the image. For example:
- Flattening a curve lowers contrast.
- An S-shaped curve increases contrast, especially in the highlight and shadow areas. Using a curve like this also helps to define the midtones.
- For ultimate control, Photoshop lets you draw a curve with the precise shape you’d like, creating an arbitrary curve or map. Click the Pencil tool in the dialog box, and then draw peaks and valleys in the Curves dialog box. Watch the changes in your original image.