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The art of halftone photography
came into being because of the need to reproduce
photographs using printing plates. In the graphic
arts industry, photographs, watercolor paintings
or other images that contain a broad range of
tones are referred to as continuous-tone images.
The films used for creative black-and-white photography,
for example, contain silver, which appears black
in the photographic emulsion. After the film
is developed the amount of residual silver remaining
in any given area of the image will determine
how dark that area of the image will be.
In letterpress and offset lithography, however,
it is not possible to create varying shades of
color by applying varying amounts of ink to the
paper. If the ink fountain of a printing press
is filled with black ink, the press is capable
of printing black and only black.
A NEED FOR HALFTONES
As photography moved from the scientific laboratory
to the consumer marketplace, advertisers desired
to use photographs of their products in their
promotional material, rather than the engraved
illustrations that had gone before. A method
was needed to simulate, on a printing press,
many different shades from only one color of
ink.
Halftone photography converts continuous-tone
images into a pattern of small, precisely-defined
dots of varying sizes.
The halftone principle is actually an optical
illusion: the human eye cannot distinguish between
the small, individual dots of ink and instead
sees them as varying shades of whatever ink color
is used for reproduction. The nine halftone examples
on this page, although appearing to contain many
shades of gray, are composed only from tiny dots
of black ink.
EARLY TECHNIQUES
Traditional halftone photography is accomplished
by using a graphic arts camera to photograph
a continuous-tone original using extremely high-contrast
film, which is usually held in place by a vacuum
pump. A halftone screen is placed in the bellows
of the camera, between the camera¹s lens
and the film.
The first halftone screens were made of two
sheets of glass, each ruled with a grid of fine,
parallel lines. The sheets of glass were turned
at a 90-degree angle to each other, creating
a grid of fine squares. As light entering the
lens passed through the halftone screen, the
high contrast film (which can reproduce only
black or white, not gray) was "forced" into
creating a pattern of small dots. The size of
each dot varied according to the intensity of
the light passing through that particular area
of the halftone screen. This type of halftone
photography required precision equipment and
highly- skilled technicians.
CONTACT SCREENS
The use of contact screens took a lot of the
guesswork out of halftone photography and this
method is still used today. A contact screen
is a piece of continuous-tone film containing
of a pattern of vignetted, "blurry" dots.
The contact screen is placed into direct contact
with the high-contrast film and is held in place
by the same vacuum that holds the film to the
camera back. For all other intents and purposes,
they create halftone images in the same way as
the old glass screens.
Screens used to create halftones come in a
variety of resolutions, which are expressed in
terms of lines per inch (lpi). This is because
different types of paper and printing processes
have different tolerances. Photographs reproduced
in newspapers, for example, typically use screen
rulings of 60100 lpi. Newspaper printing
requires more space between the individual halftone
dots, due to the tendency of the ink to spread
out as it hits the absorbent paper. Commer-cial
printing uses screen rulings of 133175 lpi,
resulting in smoother-looking, more highly-detailed
reproduction. High-quality process color printing
frequently uses screen rulings of up to 250 lpi
or greater.
In addition to the standard, round-dot contact
screens used to produce basic halftones, there
are also contact screens that produce special
effects. Some, for instance, contain a random
pattern of speckles and can be used to create
what is known as a mezzo-tint effect. Even a
traditional, orderly dot pattern can be used
to create a striking special effect, simply by
altering the angle and lpi of the screen ruling.
DIGITAL HALFTONES
For graphic designers, one of the most exciting
aspects of today's electronic prepress technology
is the ability to scan images and create halftones
at the desktop level. Today¹s image-manipulation
software has the ability to analyze the individual
pixels that comprise a scanned photograph and
recombine them into halftone dots that simulate
the traditional angles and screen rulings formally
achieved with contact screens. These images can
be output to film negatives using high-resolution
imagesetters. Moreover, experienced graphics
professionals now have complete control over
adjusting the highlight, midtone and shadow areas
of any photograph. The traditional (and iffy) "dodge
and burn" method employed by expert technicians
to adjust specific portions of an image can now
be performed with great precision - all it takes
is knowledge and a few mouse clicks!
Today's computer programs are capable of instantly
generating traditional darkroom effects such
as solarization and posterization, two creative
techniques that formally had to be accomplished
at the continuous-tone level. As if this isn't
enough, third-party manufacturers sell a wide
array of special-purpose electronic "filters" that
work in conjunction with image-manipulation software.
Many of these filters are capable of producing
multi-dimensional and "painterly" effects
that can only be achieved by electronic means.
MODERATION IS THE KEY
Of course, just because a photograph
can be digitally manipulated doesn¹t mean
it should be. Use restraint. Applying effects
with a heavy hand can destroy the meaning of any
image. On the other hand, an ordinary "snapshot",
cleverly manipulated by a professional, can gleam
from the cover of your company¹s annual report
like a work of art.
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