Use the Gamma Correction command to adjust the brightness and contrast of an image in unison. The gamma of an image is a standard measure of its contrast and brightness.
The gamma curve has three components: the red, green, and blue of the image. In addition to adjusting the brightness and contrast of the image, with the Gamma Correction command you can adjust the color balance of an image by unlinking the three components and changing them separately. For example, if the image has a red cast to it, you can reduce its red value.
Important: To apply this command to a selection, the image must be a 16 million (24-bit) color image (otherwise the command is applied to the entire image). To increase the color depth of an image, see Increasing the Color Depth of an Image.
To use the Gamma Correction command:
To limit the adjustment to a specific area, make a selection in the image. See Using the Selection Tools.
Choose Adjust
> Brightness and Contrast > Gamma Correction.
In the Gamma Correction dialog, the graph shows the gamma curves for
the three colors. If the three colors have the same gamma value, there
is one white line representing the three colors. If the gammas are different
values, there is a line for each color.
The left side of the graph represents the shadows of the image; the
right side represents the highlights.
Do one of the following:
To adjust the brightness and contrast in unison, mark the Link check box.
To change the color balance of the image rather than the brightness and contrast, clear the Link check box.
For the Red, Green, and Blue values, drag the sliders or type new numbers in the edit boxes. A value of 1 indicates the original gamma. If colors are linked, increasing the value lightens the image and decreasing the value darkens the image. If colors are not linked, increasing or decreasing a individual color value increases or decreases the amount of that color in the image.
Click OK.
Note: The Gamma Correction command can result in an image looking somewhat flat; you may want to increase the contrast after correcting the gamma. See Improving Contrast.
Adjusting Individual Brightness Values
Adjusting the Brightness of the Highlights, Midtones, and Shadows
Adjusting the Lightness Levels
Correcting Images by Adjusting the Histogram
Manually Adjusting Brightness and Contrast