How an Image Becomes Data
Anything in the universe can be measured in analog or digital terms.
Analogsimply means that the expression of the measurement is analo-
gous to whatever it’s measuring. An old-fashioned thermometer with red-
dyed alcohol in a tube gives an analog representation of how hot it is
because the alcohol rises, literally, as the temperature rises. A digital
thermometer expresses the temperature in numbers on a small LCD screen.
The numbers themselves don’t grow larger as the temperature rises.Film is an analog method of
recording a photo. Where
light strikes the silver-halide
crystals embedded in film, the
crystals clump together.
Where the light is stronger,
more crystals clump. Where
the light is dimmer, fewer
crystals clump.(^2) The photodiodes that replace film in a digital camera don’t
look any different after a picture is snapped than they did
before. They don’t shift about on the surface of the image
sensor to clump where the light is stronger. But there is an
unseen analog process afoot when a digital photo is taken.
Each of the photodiodes collects photons of light as long as
the shutter is open. The brighter a part of a photograph is,
the more photons hit the pixels that are analogous to that
part of the scene. When the shutter closes, all the pixels
have electrical charges that are proportional to the amount
of light they received. If you picture the photons piling up
like little heaps of glowing pebbles, you’ve got the idea.
3
1
The street you’re driving down doesn’t suddenly end here and start again over there, with no way to bridge
the gap. Time doesn’t stop for 5 minutes and then pick up again where it left off. Of course, if it did, how
would we know? The point is that we’re used to thinking of things as analog—smooth, continuous objects
without any quantum gaps between here and there. But in the computer world—and your digital camera is a
computer—nothing is smooth and continuous. It’s digital. There are gaps between this point and that
one, between this moment and the next. Before we can do all the wonderful things available to
us now that a computer is packed into our camera, we and our cameras have to communicate—
we with our words; the cameras with a mathematical alphabet of only 0s and 1s.
(^108) PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES