Like Levels, curves serve for correcting tone range and color balance.
They will help you to perfect pictures' tone range and a exposure, especially if they were taken under extreme conditions.
You can call up this function using Edit | Adjust | Curves [Shift+C] in the Browser menu or Adjust | Curves in the Editor menu.
Work with curves is more demanding than work with levels. On the other hand, it lets you very precisely set the individual color channels in the picture. While the Levels filter only lets you affect the range of input and output levels and gamma correction, in the Curves filter, you can set the shape of the conversion function (shown as a curve for the sake of visualization) almost at will. The horizontal axis shows input values. The vertical axis shows output values.
The Channel menu lets you choose which color channel to edit. RGB means the brightness channel. You can configure all these settings separately for each channel. To get a better idea of how the individual RGB channels are responding, it can be good to turn on Show color elements. The Linear option determines whether an interpolation curve should link the nodes you add to the curve, or a series of straight lines should link them instead.
Click Select pixel in image if you would like to use a pixel from the picture itself to set the color for the current node. If you hold down [Shift] at the same time, a node with this color will be added to the curve.
You can easily edit the curve using the mouse. Clicking anywhere on the curve between the red endpoints will add a new node to the curve. You can click any node on the curve to then move it to another position, using either drag-and-drop, the arrow keys, or direct editing via the Input and Output fields. If you right-click on a node, you can delete that node. You can also select a node by left-clicking it and then pressing [Del]. You can keep on deleting nodes until the only two nodes left are the red endpoints. Click Erase Curve to restore the original "curve" for the selected Channel. That curve is formed simply by a straight line leading from the top left to the bottom right. If the curve is erased for all color channels, then the picture will stay unchanged. Clicking Automatic Contrast or Automatic Levels will erase the current curves within the R, G, and B channels and set up new ones corresponding to each button's purpose. To control the sensitivity of these automatic settings, use the Cropping controls (separate for Lights and Shadows).
- Shifting the endpoints will change the input values for the white point and black point, just as if you were using the Levels function.
- Moving the nodes horizontally to the positions where the histogram for the RGB channel "begins" and "ends" will cause an effect similar to using Automatic Contrast within the Levels filter: the brightness of the whole photo will be "stretched" to the maximum values.
- Moving the nodes horizontally to the positions where the histogram for the R, G, or B channel "starts" or "ends" causes an effect similar to when you are using Automatic Levels: the individual color channels throughout the photo will be "stretched" to their maximum values. If each channel is edited using a different curve, then the picture's tinting will change.
- If you wish to influence shadows, create and move nodes on the left sides. If you wish to influence lights, work on the right side.
- Moving a node you have created down on the curve or to the right will darken a part of the tones in the picture. Moving a node up or to the left lightens part of the tones in the picture.
- The curve you create should have an ascending path; if it is descending in some section, that section will be tinted.
- But to get the most contrast for a photo, create an S-curve. This will slightly darken part of the midtones, while "stretching out" the remaining midtones and light tones.
In practice, using the Curves tool well simply requires experience. There are no particular settings that can be recommended universally, because each photo has different tonal characteristics and requires specific edits. To gain the experience mentioned above, you mainly need to keep using the Curves window and pay a close eye to the preview pane and to changes in the histogram.