graphic images by a camera employing an electronic
sensor and digital data storage rather than film—and
indirect ‘‘digital conversion’’ through the scanning of
film-based photographic images or existing prints.
Once created—by either method—digital image
files (computer data that represent a photographic
image) may be easily modified, duplicated, trans-
mitted, displayed on screen, or ‘‘output’’ to printed
form by a wide variety of means. Some of the print-
ing technologies used within digital photography are
fully digital themselves (computer-driven printers
such as dye-sublimation or inkjet, or even mass-pro-
duction offset) and some are hybrids (using compu-
ter-driven lasers, for example, to expose standard
photographic printing paper that must then be chem-
ically developed).
Traditional photographic technologies are now
oftenreferredtoas‘‘analog’’todistinguishthem
from those that are ‘‘digital.’’ Analog recordings
retain a physical relationship to the phenomena
they record; the densities of film are proportional to
the intensities of light that produced them in the same
way that the grooves of a record emulate sound
waves.Digitalrecordings,ontheotherhand—
whether of light or of sound—maintain no analogous
relationship to what they represent. An image is
translated by the digital camera or scanner into a
series of numbers (ones and zeros). To again become
intelligible to human senses, digital data must be
converted back into analog form; this happens as it
is being displayed on a monitor or being printed.
In traditional photography, millions of micro-
scopic grains of silver halide molecules dispersed
through a film’s emulsion respond proportionally
to the intensity of light striking them. In digital
photography, millions of microscopic sensors in a
grid respond proportionally to the intensity of light
striking them. The sensitivity of either a film or a
digital capture system may be expressed as an ISO
(International Standards Organization) rating; this is
a fixed value in the case of film but can be user-
adjustable in digital cameras. Generally, the higher
the ISO rating of a film, the higher its visible ‘‘graini-
ness.’’ As the ISO rating of a digital sensor is
increased, its ‘‘noisiness’’ generally increases. After
film has been exposed, no visible image exists; a
pattern of changed energy states in the silver halides,
known as a ‘‘latent image,’’ must be chemically
developed for the image to be revealed. Neither
does a visible image exist in a digital sensor after it
has been exposed; it is created through sets of calcu-
lations—or algorithms—performed within the cir-
cuitry of the camera or in a separate computer.
After an image has been created using film, the
image continues to reside in the film itself as a unique
deposit of silver and/or dyes. After an image has
been created by a digital imaging system, it is ‘‘writ-
ten’’ as a digital image file to some form of digital
storage and may be identically reproduced at will.
A digital image is defined by the number of ‘‘pix-
els’’ (picture elements) that make it up and the
numerical value for the brightness of each pixel (or
three numerical values in the case of a typical
‘‘RGB’’ color image—one for red, one for green,
and one for blue). A five ‘‘megapixel’’ digital cam-
era (five million total sensing elements), for exam-
ple, will typically produce an image file 2560 pixels
by 1920 pixels. In the case of a standard mono-
chrome—or ‘‘grayscale’’—image, each pixel will
be assigned one of 256 possible levels of brightness
(represented by numbers from 0, which signifies
absolute black, to 255, which signifies absolute
white) and the image file will use approximately 5
Mb (megabytes) of computer memory. In the case
of a standard RGB color image, there are three
brightness values per pixel, each expressed as a
number between 0 and 255 (yielding 16.7 million
possible color variations at each pixel) and the
image file will use approximately 15 Mb of memory.
Many digital imaging devices can record an even
higher number of tonal variations at each pixel,
which may be helpful when photographing scenes
of great contrast, but, currently, only 256 variations
per pixel are used for screen display. It is worth
noting that a higher density of pixels will generally
produce an image of higher resolution, but there are
other factors affecting image quality, just as in a-
nalog photography. Two different five-megapixel
cameras may yield very different results, depending
upon lens quality and the amount of noise and
digital artifacts caused by the sensor and related
electronics. To put digital resolution in a traditional
photographic context: the largest truly ‘‘photo-rea-
listic’’ color print that could be produced from an
RGB image file created by a five-megapixel camera
would be about 810 inches.
One additional technical consideration is that of
file formats. A digital image file, to be readable by
more than just one computer, must be written to
storage using widely agreed-upon protocols, or
‘‘formats.’’ The most universal file formats for digi-
tal images are PICT (from ‘‘picture’’) and TIFF
(‘‘tagged image file format’’). Also very popular—
virtually ubiquitous—is the JPEG format (named
for the ‘‘Joint Picture Experts Group’’ that devel-
oped it). JPEG files differ from most PICT or TIFF
files in that they are significantly ‘‘compressed.’’
They take up less space in storage media and trans-
mit more quickly across networks. Compression is
achieved by using special algorithms to find pat-DIGITAL PHOTOGRAPHY