The Apply Custom Curve Output AOV node can be used to adjust the tonal range, color balance, and contrast using a curve (figure 1). The Curve Input/Output parameter determines the value(s) to use as curve input for each pixel, and how to use the curve output to modify the color of the pixel. The Maximum HDR Value parameter only affects the curve display and the smooth maximum limit for the Inflated modes. For an SDR image, this should be 1.0. 


apply custom curve


Figure 1: The Apply Custom Curve Output AOV node used to adjust the mid-tones of a composite node tree