June 26, 2011, 4:15 a.m.
posted by lordnikon
After you adjust the overall tonal range of a photograph by using the Auto commands and the Levels dialog box, you can use curves to adjust particular portions of the tonal range without affecting other parts of the range.
As the section title states, curves are levels on steroids. Instead of adjusting just three tonal areas — shadows, midtones, and highlights — as you do with levels, you can adjust up to 14 areas along the entire tonal range.
You can apply curves to an entire image, layer, selection, or as an adjustment layer. Open an image and try using curves to adjust the tones in your image. While the Curves dialog box looks a bit scary at first, as you become familiar with it, you may find yourself using it frequently for precise adjustments.
To adjust the tones in your images with curves:
Choose Image>Adjustments>Curves or Press Ctrl/z +M to directly correct a layer or selection.
Or if you want to apply curves as an adjustment layer, click the Create New Fill or Adjustment Layer button at the bottom of the Layers dialog box and choose Curves from the menu.
The Curves dialog box opens, as shown in Figure.
Select a channel using the Channel drop-down list, if you want.
The default selection is the composite channel that represents a blending of all the channels in the image.
Position the mouse over the graph area and click the plotted line to create a point.
You can create up to 14 points on the plotted line. By the way, the plotted line is also called the curve, hence the name for this type of adjustment.
You can change the graph from a 4 x 4 grid to a 10 x 10 grid by Alt/Option+clicking the graph.
Drag the point to adjust the tone and color.
You can delete a point by Ctrl/z +clicking it.
Continue adding points and adjusting the curve until you get the results you need.
Keep an eye on the Histogram palette to see how your corrections are going. To find out how to read the Histogram palette, take a look at the sidebar, “How do you read a histogram?” earlier in this technique.
Click OK to close the Curves dialog box and apply the adjustments to your image.
If you want to start over while working in the Curves dialog box, press Alt/Option and click Reset.