The following blog posts explain the use of Levels & Curves which is the single most powerful digital image processing tool for adjusting/controlling image contrast.
The first five posts listed below provide background related to contrast, especially as related to images. Starting with “…Part 6…”, we get into Levels & Curves proper.
These were written four years ago for photographers, but apply equally to anyone working with digital images regardless of the source.
Contrast part 1: The Beginning …. Light
Contrast part 2: Tonal & Color Contrast
Contrast Part 3: Measuring Tones & Color
Contrast part 4: Luminosity and Lightness
Contrast Part 5: Contrast Adjustment Basics
Contrast Part 6: Levels and Curves, 101
Contrast Part 7: Levels & Curves – LEVELS
Contrast Part 8: Levels & Curves – Refining Adjustments
Levels & Curves – Color Shifts & Changes
Levels & Curves – Input & Output Tonal Levels
Levels & Curves – Curves are Levels on Steroids
Curves Effects – An Illustrated Wrap-up
Levels and Curves at the Extreme