Canon MP-C635 User Manual page 122

Consider the following example: You may wish to enhance an image
which has lost detail in the shadow areas due to excessive darkness.
By raising the lower portion of the Master curve, you affect the shadow
portion of the image without touching the mid-range or highlights.
It's a good idea to experiment with these curves to gain the necessary
experience to use them effectively.
Changing Gamma Curve Values
Another method of adjusting the tone curves is to use the Gamma Bar.
Click and drag within the Gamma Bar, located above the numerical
values.
6-16 Scanning With the MultiPASS C635
MID-TONES
1/4 TONES
SHADOWS
HIGHLIGHTS
3/4 TONES
Chapter 6