by Paul Gallagher Published 01/06/2007
The reign of terror started when I casually remarked to the visiting McNamee that while my mono prints (made using the Epson Advanced Black and White setting) matched my screen perfectly, my colour prints were always too dark. A steely look came over his face and by the time he had turned the electrodes up to 300 volts I was singing like a canary:
"No, I didn't have a calibrated monitor, I'd lost the profile in a disk failure."
"No, I didn't know what Dot Gain I was using."v"No, I didn't have the Color Management policies turned on."
"Yes, I was using the setting that Photoshop came on with, when it was installed."
Eventually he calmed down, detached me from the lie detector, unclipped the electrodes and started muttering about in the Photoshop colour settings thingy. The problem had assumed some importance as I had, by now, confessed to writing a book and, as things were, the outcome of the printing looked a little shaky. Here in a nutshell is the problem that we face - and it is based partly around the fact that I was using mixtures of dot gain values.
Dot Gain is an effect in the printing process in which a blob of ink hits the paper and then expands a little, so that the dot on the page is bigger than the blob of ink. The increase in the size of the mark on the page is termed the Dot Gain. Just to keep everyone on their toes, dot gain is nothing in the pure whites (no ink to spread or gain) and nothing in the pure blacks (no uncovered space for the ink to go into). The dot gain values are thus largest at the 50% grey point (the 18% grey mid-tone out in the real world of the camera).
Now clever little Photoshop knows all about this and bulges out the tone curve to lighten the ink coverage by the expected amount that it will darken due to the dot gain. Hence if you set up for bigger dot gains than you actually get, you end up light and vice versa. This only happens if you tell Photoshop about it, by turning the colour management on and obeying the rules.
Monochrome dot gain settings are just like colour space selection. Incorrectly handling an sRGB file by moving it, in an uncontrolled way, to ProPhoto RGB will create a change in saturation; doing the same thing with a monochrome by moving it, in an uncontrolled way, from one dot gain to another, will produce a change in the tone values.
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