Apple Color 1.5 - Manual (Page 235)

Without any adjustments made, each curve control is a flat diagonal line; in other words,
each source value equals its adjustment value, so no change is made.
Source Value
Adjustmen
t V
alue
If part of a curve is raised by one or more control points, then the tonal area of the image
that corresponds to that part of the curve is adjusted to a higher value. In other words,
that part of the image is lightened.
Effect of raising midtones using the Luma curve
If part of a curve is lowered with one or more control points, then the tonal area of the
image that corresponds to that part of the curve is adjusted to a lower value. In other
words, that part of the image is darkened.
Effect of lowering midtones using the Luma curve
235 Chapter 9 The Primary In Room