Photoshop Tutorials - Brightness/Contrast
Brightness/Contrast is the easiest option for tonal correction of the whole image. But the simplicity of this tool does not let you change the brightness and contrast at selected color ranges and separate color channels, as Curves and levels options do.
In the Brightness/Contrast dialog window, two sliders that let you change the Brightness and Contrast values from -100 to +100. Formally, the brightness and contrast are not dependent on each other, but you should know that changing one parameter leads to changing the other one. Increasing or decreasing the brightness leads to decreasing the contrast; increasing the contrast increases the brightness while decreasing the contrast decreases the brightness.
Increasing the brightness of the balanced image you move the color range towards the Highlights, therefore the black color and dark tones get cut off and the black pixels get replaced with dark grey pixels.
And if the full tone range the maximum contrast varies between the black (0) and white (255) colors (255-0=255), then if the image has the increased brightness then the maximum contrast is the difference between dark grey (for example, 55) and white (255-55=200). Therefore in the second case the available range of contrast values is smaller. When the brightness is decreased the white color and bright tones get cut off from the tonal range and the white pixels get replaced with light grey pixels. In this case, the contrast range equals the difference between light grey (for example, 200) and black (0) and has the value range of (200-0=200).
When the contrast is increased, the tonal range is stretched to the edges (towards black and white color), therefore the amount of dark and bright tones increases and the image becomes brighter.
When the contrast is decreased the tonal range contracts toward the center and the amount of light and dark tones decreases, therefore the image becomes darker.