Apple Color 1.5 - Manual (Page 210)

Stage 4: Making More Specific Adjustments
If you still feel that there are specific aspects of the image that need further adjustment
after Stages 1 through 3, you can turn to the curves controls, which let you make targeted
adjustments to the color and contrast of the image within specifically defined zones of
tonality. Past a certain point, however, it may be easier to move on to the Secondaries
room, covered in
The Secondaries Room .
Contrast Adjustment Explained
If you strip away the color in an image (you can do this by setting the Saturation control
to 0), the grayscale image that remains represents the luma component of the image,
which is the portion of the image that controls the lightness of the image. As explained
in
The Y
′
C
B
C
R
Color Model Explained , the luma of an image is derived from a weighted
ratio of the red, green, and blue channels of the image which corresponds to the eye's
sensitivity to each color.
Although luma was originally a video concept, you can manipulate the luma component
of images using the contrast controls in Color no matter what the originating format.
These controls let you adjust the lightness of an image more or less independently of its
color.
Note: Extreme adjustments to image contrast will affect image saturation.
210 Chapter 9 The Primary In Room