Nikon LS-3500 - LS-3510 - Manual (Page 70)

7-6 Scanning for Reproduction
Software Reference for Scanners
overly neutral halftone of medium-percentage black dots., a rainbow effect will
appear where the colored dot midtones break above the gray replacement point.
Also, in dark areas, where a large percentage of common color YMC inks
have been replaced with black, the depth of the ink (the richness) goes flat because
there is low ink surface volume on the page. When the same density is produced
using YMC or YMCK then there is a greater quantity of ink laid down and the
richness or depth is immediately apparent. Flemish painters like Rembrandt
realized this and used the oil medium to build up a shadow area to maximum black
using many layers of colors on top of each other. If they had simply used a black
pigment, there would have been no depthoor penetration of the shadows, no
content. In the above example, the lack of depth in the deep shadows of our
printed image would contrast greatly with the more robust layers of color which
formed the neutral lower-midtones even though they are lighter densities. Also,
this GCR technique, when badly handled, generally leads to a severe tone break
that is not necessarily due to a lack of bits in the digital representation, but due
rather to the jump from four color to one color, being set at a level where their is a
natural break forming in the image shading content, such as strongly defined dark
shadows which merge with the lighter side of a contrasty subject. Portraits that are
low-key and contrasty are the worst candidates for this type of color correction. It
is best to avoid or reduce the use of GCR when printing difficult subjects like
faces (especially on short runs where the cost of ink is not a major component of
the job), since the tone breaks will always appear adjacent to continuous midtones
in a face that requires a delicate gradation balance.
Finally, for gray replacement in the midtones, it is generally true that using
several screens of small dot size fills in the pattern of holes between variable high
contrast dots better than a single large size dot pattern. The surface effect that you
may detect from bad GCR is simply the lack of more diverse layers of ink, where a
single screen of medium black dots does not have the same surface sheen as
several screens laid down at different angles with smaller colored dots. The latter
is easier on the eyes, and in the lower midtones, the dot pattern is harder to detect.
In any multiplexed representation system, it is always better to increase the
frequency of the sampling, i.e.. lots of smaller dots are better than fewer larger
dots. When viewed from a certain distance they both appear to be the same shade.
Of course, there is a natural limit to this in that the finest controllable printing dot
in an ink litho system is difficult to produce at finer than a 1/200th of an inch
screen size.
At that size, the accuracy of the dot size is harder to control because the
capillary action of the paper has already caused it to bleed sideways to perhaps
twice its original size. Normally the dot gain as it is commonly called, is limited to
2-5% on film and perhaps as high as 10-15% with ink and paper. This dot gain
produces a muddy, desaturated and low definition result. The effect cannot be seen
on a film dot proof such as a 3M Match Print® or Dupont Chromalin®. These
proofs use a light sensitive color pigmented emulsion to reproduce the dots from
the original separation films. Because the dots are photographically “hard”, they
don’t bleed by capillary action, their gain is limited to the photographic gain
caused by the irradiation of exposure and processing.