80 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
How Speed Creates
Noisy Pictures
We’ve all driven through the night, trying to tune in any radio station to keep us company and keep us
awake as we motored through stretches of countryside barely on the fringes of distant radio signals. What
faint broadcasts we could receive sounded as if they were being pan-fried as the incoherent sizzle of static
would swell to drown out music and voices. Radio static comes from the random dance of electrons and
stray electromagnetic waves that have been throwing a party throughout the universe since the Big Bang.
The same type of electromagnetism, though not with so grand an ancestry, peppers digital photographs with
visible static, noiseas it’s called in photography despite its silence. Here’s where it comes from, what you
can do to avoid it, and how, sometimes, you need it when it isn’t there.
Dark noise comes from the imaging sensor
and its accompanying circuitry. The heat these
electronics produce boils off electrons that
bounce around in the form of dark current
until they end up in one of the photosites,
where they’re counted along with the legitimate
photons captured through the lens. Depending
on the type of image sensor used, an increase
of 6°–10° C can double dark noise. Some pho-
tosensors, calledhot pixels, generate more
dark current than their neighbors and so pro-
duce more noise, but at known locations.Photographic noise is digital photography’s equiva-
lent of the grain in photos derived from film whose sensi-
tivity to light has been pushed to the limit. In film, grain
shows up as dark clumps that rob the photographs of
finer resolution. In digital photographs that have also
stretched normal exposures to eke out some detail in the
shadows, noise appears as white or faint pastel specks
that can add texture or simply be distractions. Noise com-
monly comes from several sources.