Primary Color Correction: Tonal Correction Tools - The Art and Technique of Digital Color Correction - page 52

the graph—128—down to 120, that will mean that the pixels that were
originally at a brightness value of 128 are now mapped lower, to 120.
That also means that all of the other values that were between 128 and
0 are slightly compressed into a smaller tonal range between 120 and 0
and all of the values between 128 and 255 have been lowered but into an
expanded range between 120 and 255. So each time you place and move
a point on the curve, you are not only remapping the tonal values of that
point but also compressing and expanding the tonal ranges on either side
of the point on the curve.
A valuable thing to understand about using curves is that the steeper
the angle in a curve, the greater the contrast of the image in that range.
Although few professional colorists use curves, they utilize the concept of
expanding tonal range where it is needed and collapsing it where the eye
does not need the information as much.
This concept is the basis for a great little tip about how to quickly make
images look better quickly using curves.
S Curve Tip
In the master curve, place a point—by clicking on the diagonal line of the curve—about
a quarter to a third the way up from the bottom of the curve and another about a quarter
or a third the way down from the top. Now drag the top point slightly upwards and the
bottom point slightly downwards. This creates a shallow “S” curve. The curve makes
the blacks rich and the whites brighter (possibly clipping detail in each of these areas,
depending on how much the points are moved) and then spreading the tonal values out
over a wider range across the middle of the picture.
Fig. 2.23 This is what the S curve looks like in Color's Luma curve in the Primary In room.
A valuable thing to understand about using curves is that the steeper
the angle in a curve, the greater the contrast of the image in that range.