Specifications
The Digital Fine Print Course
Editing a Fine Print
Image Posterization
Practical experimentation alerts us to the fact that a minimum number of levels of tone
are required to create the illusion of a smooth tonal scale from black to white on the
screen and in the digital print. With less than 8-bits or 256 levels of tone per channel the
individual levels or steps may become increasingly noticeable as distinct jumps or bands
of tone. This effect is known as posterization, because the normal smooth tonal transi-
tions of reality and the photographic image begin to break up into noticeable bands or lay-
ers of tone resembling a poster or screen print.
Adjusting the image in a pixel editing program (eg. Adobe Photoshop) results in a redis-
tribution and/or elimination of levels of tone. Even a simple contrast adjustment can
result in a significant loss of levels of tone in an 8-bit per channel image. For example:
Copyright Les Walkling 2012
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Normal Contrast Greyscale
Increased Contrast (N+2) Greyscale
The upper histogram shows the pixel distri-
bution in the original image. Applying a mod-
erate ‘two stop’ increase in contrast results in
the right hand histogram which reveals the
actual levels of tone that are now missing
from the image. As the gaps increase the
image becomes increasingly posterized. This
effect may eventually become visible in the
print as obvious bands of tone especially in
smooth tone areas like sky and skin. These
effects are often accumulated and multiplied
with repeated image adjustments.
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