Color Correction - Nuke 101: Professional Compositing and Visual Effects - page 163

The Gamma operation changes the midtones without changing the blacks or
whites. You can tell that the point at the furthest left and at the furthest right are
not moving.
Contrast is next.
12. Replace Gamma2 with a RolloffContrast node in the Color toolbox.
13. Bring RolloffContrast2's contrast value to 1.5.
The contrast operation pushes the two parts of the dynamic range away from one
another ( FIGURE 4.14 ) .
Figure 4.14. A basic contrast curve.