Unsampling Photos: Don't Do It
When you scan a photo for placement
in a file that will be offset printed, the general rule
of thumb is to choose a resolution 1.5 to 2 times the line
screen of the final, printed piece. For example, if you
plan to print a photo at 100 percent size using a 150 lpi
halftone screen, you would scan the photo at between 225
and 300 dpi. If you intend to shrink the photo to 50 percent
of size before placing it in your design file, you can cut
the initial scanning resolution in half to (rounding) 113
to 150 dpi. I'm sure most of you have read this rule in
many books, magazine articles, and printing seminar handouts.
But what if you want to increase the
size of the photo? Simple--you don't. Here's why. When you
scan a photo, the computer captures the image in a grid
of squares of a certain size (dots or pixels per inch).
These squares (together called a raster grid) can be made
larger or smaller by enlarging or reducing the photo, but
you have the same number of squares after you have enlarged
or reduced the photo. If they are reduced, they become less
evident to the naked eye, but if they are enlarged, they
become visible and soft or fuzzy, or even jagged. The photo
loses detail. This effect is called pixellation, and unless
it is what you intend, don't scan a photo and then enlarge
it. Simple as that.
Of course, all rules are meant to be
broken, and if you want a futuristic, computerized effect
that draws attention to itself as a product of the computer,
fine. If you are looking for a non-realistic, painterly
effect, you can also "upsample" (type a larger
number in the resolution box, under Image > Image Size
in Photoshop), but when you do this, the computer just increases
the number of pixels of color in the area (called interpolating)
without adding detail.
Photoshop TIFF files are based on rasters,
while Illustrator or Freehand EPS files are based on vectors
(and are scalable without loss of detail). For the most
realistic, least pixellated effect, remember that rasters
are like a checkerboard, while vectors are based on mathematically
defined arcs and curves. Reduce or enlarge vectors as you
will, but avoid enlarging or upsampling rasters.
Real vs. Fake Duotones
Duotones come in two varieties: "real"
and "fake." A real duotone uses two separate negatives
and two inks to add dimension to a photo. Each halftone
focuses on a limited range from the lightest light to the
darkest dark it can reproduce. This equals roughly four
f-stops on a camera. Therefore, with one halftone you can
emphasize highlights and midtones, or midtones and shadows,
but usually not both. Using two negatives printed over one
another in two different inks and with their screen rulings
angled slightly to avoid moire patterns, one can double
the "dynamic range" of the photo. Whether you
use two distinct colors, like a black enhanced by a green
or a red to give the photo character, or you use two blacks
(or a black and a grey) to produce a more naturalistic photo,
you can achieve a superior range from highlights to midtones
to shadows.
As a side note, a four-color photo
is actually a quadtone (as opposed to a duotone). Technically,
it is created in much the same way as a duotone: four halftones
are laid over one another and set at angles to each other
to avoid moire. The only difference in four-color process
work is the choice of inks: CMYK inks to approximate full
color vs. blacks or browns or variants of gray used in most
duotones.
On the other hand, a fake duotone is
one halftone of a photo printed in one color over a flat
screen of another color. For instance, you can print a black
halftone over a twenty-percent screen of blue and get an
interesting effect. But it isn't a real duotone.
The best way to tell which is which
is to look for the white of the paper (or whatever color
the base paper is). A real duotone will have areas with
no halftone dots, revealing the paper itself. A fake duotone
will show the base screen color (the twenty-percent screen
of the second color) through areas of the overprinting halftone
that otherwise would be white (or paper colored).
Neither approach is right or wrong;
both can be useful options depending on your aesthetic goals
for your printed piece. Do keep in mind, however, that process
inks and many PMS inks are transparent. Therefore, if you
print a dark blue over a screen of gold or yellow, your
blue will become green.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]