Apple Color 1.5 - Manual (Page 211)

What Is the Contrast Ratio of a Shot?
One of the most important adjustments you can make to an image is to change its contrast
ratio. The contrast ratio of an image is the difference between the darkest pixel in the
shadows (the black point) and the lightest pixel in the highlights (the white point). The
contrast ratio of an image is easy to quantify by looking at the Waveform Monitor or
Histogram set to Luma. High-contrast images have a wide distribution of values from the
black point to the white point.
Low-contrast images, on the other hand, have a narrower distribution of values from the
black point to the white point.
The Shadows, Midtones, and Highlights contrast sliders let you make individual
adjustments to each of the three defining characteristics of contrast.
Note: Contrast adjustments made with the primary contrast sliders can affect the saturation
of the image. Raising luma by a significant amount can reduce saturation, while reducing
luma can raise image saturation. This behavior is different from that of the Color Corrector
3-way filter in Final Cut Pro, in which changes to contrast have no effect on image
saturation.
211 Chapter 9 The Primary In Room