Specifications
The Digital Fine Print Course
Editing a Fine Print
Minimising Image Posterization
Posterization can be minimised (and therefore the illusion of smooth photographic tone
preserved) by working with files that contain more than 8-bits (2
8
= 256 levels) of data per
channel. As many printer drivers and RIPs only accept 8-bits of data per channel it is also
logical to only convert the image down to 8-bits of data per channel just before printing.
This happens naturally in a program like Live Picture, but prior to version 8, Photoshop
only supported limited editing in more than 8-bits, therefore it was more difficult in
Photoshop to reduce the posterization that resulted from excessive image manipulation.
Whenever it is possible (or practical) digitize the image in greater than 8-bits, and com-
plete all global colour corrections in the scanning/raw convertor software. Save the image
in 16-bits per channel, open in Photoshop and complete all image adjustments and pro-
file conversions in 16-bits. Once the image is converted to 8-bits endeavour to achieve the
desired result with as few adjustments as possible.
Use an RGB working space with a Gamma of 2.2 (rather than 1.8) because it more equal-
ly distributes bit-depth between an image's shadows and highlights. At a Gamma of 1.8
much less of the image's bit depth is devoted to the shadows (approximately 102 levels)
compared with the highlights (153 levels), therefore posterization will appear more readi-
ly in the shadows than in the highlights when they are manipulated to the same extent.
When archiving JPEG camera files first optimize image quality in the capture software
(white balance, tone and contrast, and sharpening), with teh aim of limiting subsequent
image editing to minimse the risk of posterization in the 8-bit JPEG format. Also try to
avoid resaving an edited JPEG file back into the JPEG format because it will further com-
press the image data and therefore reduce image quality.
Copyright Les Walkling 2012
19/50
Gamma 2.2
Gamma 1.8
Zones
Zones
Levels
(Photoshop)
0 26 51 77 102 128 153 179 204 229 255
Levels
(Photoshop)
0 52 84 112 136 156 180 201 220 237 255
Screen Density 2.49 1.97 1.35 1.00 0.75 0.56 0.40 0.28 0.18 0.08 0.00
Screen Density 2.49 2.39 1.79 1.35 0.99 0.75 0.53 0.38 0.23 0.11 0.00