Posterize
Creates a poster effect on the bitmap by quantizing the bitmaps colors to a specified number of color levels per plane. i.e. the image is made out of discrete blobs of color with obvious boundaries between them.
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Posterize
To achieve the impression of continuous tone we need to reproduce 256 differentiation's of tone from white to black. If we break down the tonal range into fewer differentiation's you begin to see the breaks between the individual tones.
But 256 colors (8-bit color) appear posterized and fake when viewing a color image. The gradation of colors is jumpy. If we give 256 steps (8 bits) to each of the RGB colors that create the color palette, these colors can be mixed in 16.
The images below were produced from our test shots, they have been deliberately 'posterized' to indicate different levels of falloff. Each band represents a 10% drop in luminance.
It now looks much better, but for fun let's see how it looks posterized. Pretty cool, but the contrast is still too high for this effect. So I go back, reduce contrast some more, and then come back to the "posterize" icon.
A set of such solarized masks, corresponding to a different color filter for each density range, is then used to make a posterized color print. The print is posterized in terms of colors, not densities.
The term "posterization" comes from the word poster, as images that have been posterized resemble the mass-produced look of poster images.
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In Photoshop CS2, there are twelve adjustment layers: Levels, Curves, Color Balance, Brightness/Contrast, Hue/Saturation, Selective Color, Channel Mixer, Gradient Map, Photo Filter, Invert, Threshold, and Posterize (it is beyond the scope of this ...
Move the slider slowly to either side and see how the adjustment affects the image. It is important not to move the mid-tone slider too far as this will break up the histogram meaning you will lose tonal information and create a posterized effect.
Favor longer gradients with smoother transitions. They can always be tightened and repositioned by adjusting contrast. Reducing contrast won't produce the opposite effect; it will posterize the gradient.
 See also: Image, Light, Color, Digital, Photograph
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