Apple Color 1.5 - Manual (Page 239)

Moving the same control point down lowers the distribution of midtones, darkening the
image while leaving the white and black points pinned in place.
While these three control points can mimic the functionality of the Shadow, Midtone,
and Highlight contrast controls, the true power of curves comes from the ability to add
several control points to make targeted adjustments to the lightness of specific tonal
regions in the image.
The Luma Curve Limits the Range of the Primary Contrast Sliders
One important aspect of the curves controls is that they can limit the range of subsequent
adjustments with the primary contrast sliders in the same room. This can be clearly seen
when you make an adjustment to lower the white point of the image using the Luma
curve. Afterward, you'll find yourself unable to use the Highlight contrast slider to raise
the image brightness above the level that's set by the Luma curve. You can still make
additional contrast adjustments in other rooms.
An Example of the Luma Curve in Use
The following example illustrates how to make very specific changes to the contrast of
an image using the Luma curve. In this shot, the sky is significantly brighter than the rest
of the image. In order to bring viewer attention more immediately to the subject sitting
at the desk, you need to darken the sky outside the window, without affecting the
brightness of the rest of the image.
239 Chapter 9 The Primary In Room