Printer Resolution (dpi)
While digital cameras use pixels to measure image resolution, printer resolution
is based on the number of dots per inch (dpi) the printer lays down on paper.
The higher the dpi, the smaller the dot, and the harder it is to discern one
dot from another with normal viewing. Very high-quality, photorealistic ink-jet
printers produce dots so small that you can only see them with a magnifying
loupe, but there are many levels of acceptable printer quality between high-end
and low-end models.
Printer resolutions vary from 300 to 1,400 dpi and higher, depending on the
technology used to create the dot (see "Printer Technologies" below)
and the size of the nozzles or heating elements in the print head. A printer
that delivers 600 dpi resolution is generally considered photo-quality, but
there are other factors that influence how a print looks to the naked eye. The
kind of paper you use to print the image has a dramatic effect on print quality,
as does the number of colors the printer uses, and the way the ink is applied.
Read on to find out more about these factors.
How Many Colors?
Most digital printers use a combination of three, four, or six colors to print
full-color images. Ink-jet printers dispense each color individually, either
from a single chamber in a multi-chambered ink cartridge, or from a single ink
cartridge that can be swapped out when one color gets low. Dye-sub printers
use heat-transfer ribbons, each dispensing a different colored dye. All printers
use cyan, magenta, and yellow -- the three primary colors used in printing --
as their base colors, with a few variations as noted below:
- Three color printers: Cyan, magenta, and yellow are known as "subtractive"
colors. If you combine equal amounts of these three colors, you get black
-- the absence of all color. Based on this theory, a CMY printer should be
able to produce black without any problem. In the real world, however, CMY
blacks usually come out looking muddy or gray, so the printing industry has
traditionally added a "composite" black ink to the mix to help clean
up the shadows and dark areas of an image. If you want high-quality photos,
we recommend that you avoid three-color printers.
- Four color printers: Like the professional printing presses mentioned above,
most high-quality digital printers use a combination of cyan, magenta, yellow,
and black (CMYK) inks to recreate -- as closely as possible -- the full spectrum
of tones and colors that you see when you take a picture. When shopping for
a digital printer, look for one that uses at least four colors.
- Six color printers: Many printer manufacturers have expanded the traditional
CMYK ink set to include two additional colors -- light cyan and light magenta.
These two color variations make it easier for the printer to reproduce light-colored
image tones, without having to leave excess white space between the ink dots.
The result is an image with a more continuous-tone quality.
Ultraviolet Coating
Concerns with print permanency and the adverse effects of UV light have prompted
a great deal of research into methods of protecting digital prints. In addition
to improvements in ink quality, some printer manufacturers have added a UV coating
to the print production process. Originally introduced in high-end, dye-sub
printers, the a UV layer may add decades to a print's life expectancy.
Choosing the Right Paper
Paper is a key component to the quality of a digital print. You can't expect
to obtain good results from an inexpensive, porous paper that is not designed
for printing photo-quality images. Your best bet is to buy papers (and inks)
recommended by the printer manufacturer, which in most cases, will be made or
marketed by the manufacturer itself. Its papers are optimized for use with its
printers, and will probably give you the best results.
Once you become familiar with your new printer, then you can start experimenting
with different brands and textures of papers. There are dozens of creative possibilities
out there, especially in the realm of ink-jet printing.
Try Before You Buy!
There is no better way to shop for a printer than to try it hands-on. Have
a salesperson run a test print, preferably one with text on it. If possible,
run the same print on other models you're considering.
Observe the printer in operation. Is it noisy or quiet? Does it take a long
time to print? Check the printer's specification sheet, it usually publishes
the print speed in prints-per-minute (this time will vary depending on ink coverage
and quality setting).
Look at the printed image. On an 8x10-inch print, there should be no visible
dots or dithering from a distance of 8 inches. Check to see that the edges of
the text are smooth, and that there are no signs of aliasing or stair-stepping
along hard edges or lines in the image (straight or diagonal).
Printer Technologies
There are many types of digital color printers on the market. Each approaches
the task of depositing ink or dye on paper in a different fashion. The following
are the three most common types of printers used for digital color printing. Among
them you'll find models priced for the amateur, advanced-amateur, and professional
photography markets.
Ink-Jet
Ink-jet printers operate exactly as their name implies: Ink is sprayed onto
the printing substrate through tiny nozzles (about the diameter of a human hair),
depositing small droplets of color as they move over the image area. These nozzles
are part of a cartridge assembly that makes up the "print head," which
passes back and forth across the paper horizontally, squirting ink as it goes
along. When one strip of paper is covered with enough ink to form that portion
of the image, the printer's "stepper motor" advances the paper to
the next strip, so the print head can continue to deposit ink until it has covered
the entire sheet of paper.
There are several methods by which ink is transferred from the nozzle to the
paper in ink-jet printing. Thermal ink-jet technology, originally developed
by Canon U.S.A. as "bubble-jet" printing, uses heat to force the ink
through the nozzle openings. Canon has refined the thermal process so that its
current bubble-jet printers are capable of producing variable-size dots instead
of the uniform size dots that are usually associated with this technology. By
varying the dot size, the printers are better able to manipulate ink density.
Resolutions for thermal ink-jet printers usually start at 300 dpi.
Micro Piezo technology, a development of Epson America, employs an electrical
charge to deliver the ink to the substrate. This method allows more precise
control over the size and shape of the ink droplets, which are generally smaller
than dots created by the thermal ink-jet process. Smaller dots mean that you
can fit more of them per inch, and therefore achieve higher image resolution
(typically starting at 720 dpi). Another benefit of the Piezo method is that
the ink does not have to stand up to the high temperatures associated with thermal
ink-jet technology, so there is more latitude for developing new and improved
ink sets.
Hewlett-Packard's PhotoREt III is a color layering technology that produces
extremely small droplets, resulting in more colors per pixel than other ink-jet
printers (as many as 3,500 printable colors per dot). This allows HP to improve
print quality through increased color range, rather than increased dpi. Regardless
of which technology they use, ink-jet printers are rapidly becoming the most
common color printing devices used in homes, offices, and even graphics businesses
-- thanks to constant improvements in performance and significant decreases
in price.
Dye-Sublimation
Dye-sublimation was long considered the "only" technology capable
of producing real photo-quality digital prints. Based on a heat transfer process,
thermal dye-sublimation uses thousands of tiny heating elements that come in
contact with a "donor ribbon," releasing a gaseous dye that is transferred
to the paper one color at a time. Each heating element is controlled individually
by electronic impulses from the printer's internal processor. The gaseous nature
of the dyes allows them to blend seamlessly on the printing substrate, producing
continuous-tone prints that are nearly indiscernible from conventional photographic
prints.
Early dye-sub printers were large, bulky, and cost tens of thousands of dollars.
Because each color ribbon is processed individually (one pass each for cyan,
magenta, yellow, and -- in some printers -- black), the dye-sublimation process
can also be somewhat time-consuming. However, the results usually speak for
themselves, and some of the newer, less expensive consumer-level dye-subs, like
those made by Olympus for use with its digital camera line, are beginning to
gain popularity.
Thermo Autochrome
Thermo autochrome is a relatively new printer technology that uses heat-sensitive
pigment layers incorporated directly into the paper. The three color layers
-- cyan, magenta, and yellow -- are each sensitive to a different temperature.
The printer selectively heats areas of the paper, one color at a time, to activate
and then fix the pigments with ultraviolet light. Fuji Photo Film U.S.A. uses
thermo autochrome technology in its NC and NX printers specifically designed
for use as companion printers with its popular line of digital cameras.
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A Guide
to Digital Printers
