Printing to a Press - Color Management for Photographers - page 258

Different GCR settings can be built into groups of profiles and used
based on image content. The amounts of GCR specified usually are
labeled light, medium, heavy, maximum, or none, as seen in Photoshop's
CMYK dialog in Fig. 7-4. Based upon what setting is selected, the GCR
begins at different percentages within the tonal range of the image. For
example, light GCR compared to medium and dark GCR; the curves are
more pronounced as the tonal range of the image gets darker as illus-
trated in Fig. 7-4. Low-key images with lots of shadow detail can benefit
from light or perhaps no GCR, whereas a medium-to-light GCR setting
is better for mid-key images.
Higher GCR settings can assist with images that contain lots of neu-
trals such as a photograph of silverware. Neutrals made up with less CMY
inks tend to produce better gray balance. Therefore, once you measure
a target to build a CMYK output profile, you may wish to create a number
of different GCR settings to use based upon image content. Note that the
differences in UCR and GCR separations are not visible on screen or even
to a proofing system since these options affect the actual inks on the press.
You can see the differences in individual color channels. A tutorial
in Chapter 9 goes through the steps to produce a number of separa-
tions with different UCR/GCR settings, allowing you to get a better idea
of how this affects conversions to CMYK. (See Chapter 9, Tutorial #14:
“UCR/GCR Settings.”)
Users that need a fine degree of control over the separation should
consider building several output profiles from the same measured data
using different GCR settings. Otherwise, a light-to-medium GCR setting
for high quality offset printing should produce acceptable results. Photo-
shop and some products also specify a setting called UCA (Under Color
Addition). The idea is to add back density to shadow areas as a compen-
sation for GCR. In Fig. 7-5 you can see I added 50 percent UCA, which
caused the CMY inks in the darker tonal areas to increase. This huge addi-
tion is used for illustration only. Such a setting would never be used. If
you find that GCR produces muddy blacks, adding a small amount of
UCA can reduce the problem. UCA is usually an image-specific correc-
tion. That is, if a group of images with a great deal of darker shadows are
looking flat, a small amount of UCA can help.
Black Start and Black Limit
Some products allow users to decide at what point black ink begins to be
used in the separation; this is known as Black Start . Since this is tied to
the Total Ink Limits, as black start increases from zero, CMY inks are also
affected. Suppose I set the black start at 10 percent. Once CMY inks have
reached this 10 percent mark, black ink will begin to be used and mixed
with these inks for the rest of the separation. The opposite is Black Limit ,
sometimes called Max Black , which is an option in some products, notably
Photoshop. This is an option for ensuring that too much black ink isn't