62 Advanced image processing
A short guide to tone curve corrections
Image processing is a highly specialized and difficult field that takes years of practice to master.
This basic guide to using tone curves covers a few simple procedures to improve your pictures.
For more about digital-image processing, consult your local book dealer about self-help guides
on this subject.
About the tone curve
The tone curve is a graphic representation of the brightness
and color levels of the image. The bottom axis is the 256 lev-
els of the original image (input data) from black to white. The
vertical axis is the corrected image (output data) with the
same scale from top to bottom.
The bottom left portion of the graph represents the dark col-
ors and shadow areas of the image. The middle section repre-
sents the mid-tones: skin, grass, blue sky. The top right sec-
tion is the highlights: clouds, lights. Changing the tone curve
can affect the brightness, contrast, and color of the image.
Input
Output
Highlights
Mid-tones
Shadows
Bring out detail in the shadows
This is a simple technique to make a subject hidden in
the shadows brighter. Unlike the brightness level control
(p. 40), this method of correction will not lose details in
the highlight areas of the image.
With the RGB channel selected, place the smooth-curve
cursor on the center of the curve. Click and drag the
curve up. Look at the displayed image to judge the
result. The adjustment can be very small and still have a
significant impact on the image. Moving the tone curve
down will make the image darker.
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