V1.02
Thom Hogan’s Complete Guide to the Nikon D300
Page 74
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Shot noise.
Photons, like almost everything in life, have a
bit of randomness to them. In absolutely even light, there
still is a statistical chance that any given photodiode does
or doesn’t see a photon. This statistical photon noise,
called shot noise, is equal to the square root of the
number of signal photons collected, and this relationship
does not vary from sensor to sensor. What does vary from
sensor to sensor is the number of photons collected.
Remember that the D300’s collection area is smaller than
that of the D3, but bigger than that of the Coolpix
cameras. Thus, it should be obvious that the visibility of a
D300’s shot noise is lower than that of a Coolpix, higher
than that of a D3. The generally accepted value is that it
takes a signal-to-noise ratio of 2.7 before you can get
meaningful data beyond the shot noise. Put another way,
the minimally theoretical light fluctuation that can be
accurately discriminated by a sensor is 8 photons. That’s
usually okay for us, as our brightest values in our shots are
often collecting tens of thousands of photons, but do
watch out for underexposure of your shadow areas—there
might not be a lot of photons represented there, and shot
noise will rear its head when there aren’t.
•
Read noise.
Likewise, we have to store the photons, then
move the electrons from the photosite to the ADC and
count correctly. Sometimes that all goes wrong, too. We
might lose or gain a few electrons in the process, and our
Analog-to-Digital converter might not count accurately.
Sensors tend to produce more noise when:
•
left exposed to light for long periods of time
. Electrons get
more chance to migrate from where they should be to
places they shouldn’t.
•
exposed to low levels of red or blue wavelengths of light.
The demosaic routine uses all adjacent photosites in
calculating pixel values, so when one channel (color of
photosite) is very low in value and thus has a low signal-
to-noise ratio, it can contribute that noise to all the
channels in the final adjacent pixel values.
