Histogram & Curves
The Curves panel is an effective tool for correcting Exposure as well as Color of pictures. The Curves let you adjust the whole tone range of an image. You can adjust any point along a 0-255 scale while keeping other values constants.
The horizontal axis of the graph represents the original intensities of the pixels or input values. The vertical axis represents the new values or output values for the same pixels. On the diagonal line all pixels have identical input and output.
Click any point on the curve that you want to remain fixed. For example if you want to adjust the midtones, click on a point in the center of the diagonal: this will create a new fixed point. Move, by drag & drop, this point horizontally to the right to decrease midtones or to the left to increase midtones. You can add more "control points" the same way.
To remove a control point just right click on it. You cannot delete the endpoints of the curve, but you can move them. To move a point, pass the mouse over it and when the cursor becomes a hand symbol, click and drag the point to the desired position.
You can edit all 3 RGB channels at same time (luminance) or just a channel per time.
In the Levels Tab it's possible to regulate the Exposure (Contrast) and Color for each RGB channel separately, by changing the amount of , and . The tab is divided into and . You can modify the original levels in the picture (input), and then further modify the resulting levels (output). Basically, this dialog is a more advanced version of the Curves panel described above.
The combination of these 2 operations on Shadows and Hilights is called the Contrast Stretch and is provided to increase the level of Contrast in the picture. It is recommended that you don't move the Shadows & Hilights points beyond the histogram left and right limits, which you can easily visualize on the histogram display.
Oppositely to Input Levels where you can only increase contrast, in Output Levels you can decrease contrast by moving the Shadows and Hilights sliders. It's not very common to use this correction, since it looses part of the color information of the picture, trying to achieve a washed-out effect. To reduce contrast and hilights more easily, you can also use the Exposure correction dialog..
Example of Levels adjustment:
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