CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 81
Signal noise is the noise that’s inherent to all imaging and electromagnetic transmissions, which are the signal. The photons raining
onto the photosensors do so in a statistical pattern calledPoisson statistics. They decree that the rate of photons arriving at each
photodiode, or photosite, fluctuates over time so that one pixel may record twice as many hits as the one next to it, producing
inevitable noise. Put simply, noise is the everyday background clatter you can’t get away from because there is no such thing as a pure
signal. It’s the electronic equivalent of traffic sounds, birds, and kids playing outside. Some signal noise is necessary to give objects tex-
ture. Photoshop backgrounds and computer-generated art without noise look flat and unrealistic. There are, in fact, programs that let
you add noise to photos that are too quiet. The ideal is a noise-to-signal ratiothat is low but still has some presence.Hushing Your Photo
With so many sources of noise, like the visible chatter in the
shadows of the photography on the left, you’d think there
must be many ways to silence it. You’re right. No one
method works best in all situ-
ations, and unless you do a
lot of night photography—
pushing the exposure with
long shutter times and high
ISOs—you usually don’t have
to worry about noise. If you
do shoot in noisy situations,
you’re still covered. Some
camera makers include cir-
cuitry to baffle noise.
Because the locations of
some noise are predictable,
the camera’s processor sub-
tracts from noisy pixels a value that brings them in line with
less raucous photodiodes. As shown in the picture on the
right, retouched with Noise Ninja, if the camera doesn’t get
rid of noise, most photo editing software can.Amplified noise is akin to the grain you get using a high ISO film, especially when you
pushthe film by exposing it as if the film had a higher ISO than it does. When you use a
high ISO setting on a digital camera, the camera’s sensor doesn’t become more sensitive.
Instead, the camera increases its amplification of the signals created by the light photons. As
you amplify the signal, you also amplify the background electrical noise that is present in
any electrical system. Even if no extraordinary amplification is used, reading the signals so
they can be converted to digital values still contaminates the signal with noise originating in
the amplifiers. This is also calledread noise orbias noise.Accumulative noise is akin to
amplified noise. When you use
extremely long shutter speeds in
low light situations, you also give
noise from its various sources a
longer time to accumulate until, like
amplified noise, it becomes visible.