Digital Prints

42
Mastering Digital Printing
However, there’s a downside to more pixels. The higher the ppi and/or the greater the bit
depth, the more space the files take up, the slower they are to edit and work with, and the
harder they are to print since extra pixels are simply discarded by the printer or can cause
it to choke, stall, or even crash. The goal is to have a file that’s just big enough for the job,
but not so big that it causes extra headaches.
So what is the best file or image resolution for digital printing? There is no standard ruleof-thumb
for all digital devices as there is with commercial offset lithography. There, it’s
well accepted that the ppi-to-lpi ratio (lpi is the “screen frequency”), which is also called
the “halftone factor,” should be somewhere between 1.5 and 2.0. In other words, if you
have an image that will be printed as a poster by a commercial print shop, the normal
screen frequency would be 150 lpi. Multiply that by 1.5, and you get 225 ppi. Substitute
2.0, and you get 300 ppi. So your best image resolution in this example of commercial
offset printing is usually between 225–300 ppi at final print size.
However, with most high-quality digital processes, there is no “lpi” in the same sense as
with offset. In the early days of inkjets, some people used the 1/3 Rule: Take the highest
resolution of the printer and divide by 3. For example, an older Epson inkjet printer with
a 720 maximum resolution would require a 240 ppi file for optimal results (the “Magic
Resolution Number”). But then Epson printhead-based printers started coming out with
1440, then 2880, and now 5760 resolutions. One-third of 5760 is 1920 ppi, an absurdly
high and unnecessary image resolution. Some photographers and artists still swear by the
240-ppi formula for even the latest models of desktop printers, claiming, correctly, that,
for desktop Epsons, the “native driver resolution” is still 720, so the 1/3 Rule remains in
effect. (According to Epson data, the “input resolution”—the resolution that data is rasterized
at—is 720 “dpi” for desktops and 360 “dpi” for wide formats.) However, Epson
now recommends 300–360 ppi at the size you intend to print as their current Magic
Number; if you get below 240 you may start to see a difference in image quality, and conversely,
you won’t see any improvement with bitmapped images by going over 360 ppi.
(Note: unlike bitmaps, vector art is “resolution-independent,” which means that you can
blow it up or down without any loss of definition or clarity.)
Hewlett-Packard (HP) has an “internal render resolution” of either 600 dpi or 1200 dpi,
depending on the quality setting, and they recommend 150–200 ppi (or even up to 300
ppi) at final size for their inkjet printers. (HP likes to call it “pixels per printed inch” or
PPPI.) They claim that scientists doing satellite photo reproduction for the government
on their printers typically find that 125 ppi is adequate. In my own experience, 200 ppi
is a good image resolution target for most HP inkjet printers.
Canon, also with a native printhead resolution of 600 dpi on many of its inkjets, says that
an image must be greater than 180 ppi “to avoid pixelation that shows as staggering in
contrast points.” They go on to recommend 200 ppi (see Table 2.1) as the target with 300
ppi as the maximum needed for their inkjets. (To see what printheads look like, go to the
“Inkjet” section near the end of this chapter.)
For continuous-tone printers that don’t use halftoning or dithering (explained below), try
to have your image resolution match the printer resolution. Most dye sublimation printers
are around 300 dpi, so make your final image also 300 ppi. Same for LightJets and