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Posterization

Introduction:
Posterization in a photographic image refers to a region of continuous gradual tone being replaced by a regions of fewer tones resulting in visible steps from one tone to another. This effect can be quite subtle, or very obvious.
 
Posterization:
Any tool in an image editing program that adjusts the histogram (such as Levels and Curves in Photoshop) has the potential to create posterization.

Below is the histogram of an 8-bit image.
 
8 bit histogram 
 
Now watch what happens when we make an extreme change using Levels in Photoshop.
 
8-bit Histogram post Levels 
 
This is what is called posterization. The comb effect. The missing vertical bars are caused because the original color intensities of 0 to 255 are being forced to spread over a larger range than existed in the original image. This causes the missing gaps to appear because there is no data available to fill those columns.

Below is a 16-bit histogram of the same image.
 
16 bit histogram 
 
Now watch what happens when we change a 16-bit image using the same extreme adjustment in Levels as before. 
 
16 bit Histogram post Levels 
 
Bingo, no missing data. This is because now there is a huge amount of data available to display the tonal range of the image and making extreme adjustments still leaves plenty of room for smooth tones. 16-bit images have up to 256 times more color levels as 8 bit images.
 
This is what an image would look like before posterization and after. 
 
No Posterization  Posterization 
 
Posterization can easily be seen in smooth graduations and it will ruin a image.

Always edit in 16-bit mode to prevent this from happening. 
 
Cheers 

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