While Levels adjustment is often the first step photographers use for adjusting parameters like brightness, contrast and colour balance, when you want those adjustments to be subtle, the Curves control should be the tool of choice. Like Levels, the Curves adjustments are based upon a histogram, which can be globally adjusted through the default RGB channel, or each colour channel can be adjusted individually.
But unlike Levels, which only provides adjustments for black, white and midpoints, Curves can be controlled with any number of points. This makes the Curves function a very powerful tool.
In the dialog box for Curves adjustments, shown on this page, the horizontal axis plots the input (original image values), while the vertical axis plots the output (adjusted values). The diagonal line (curve) maps inputs to…