Essential characteristics of noise  Sunday May 10 2009  Camera
Article Introduction | Noise in shadows | Noise in mid-tones |
Noise in highlights | Summary | Annex  One of the major reasons
for noise in an image is that the signal itself is noisy.
Although this seems very far afield from photography, this is
actually a direct consequence of quantum mechanics! The light
falling onto the sensor is both an electromagnetic wave but also
composed of particles—photons.  A finite number of photons hit a
pixel within a given time interval. This is comparable to cars
driving on a highway. Assume that the traffic flows at a
constant pace and you count the cars passing under a gate in one
minute. If you perform this experiment many times (under similar
traffic conditions), then this number fluctuates around a mean
value. This fluctuation is noise. Another point which is
intuitive is that the SNR is lower for low observation time,
because the flow of cars (or photons) is quantized. For very
small pixel pitch (a few µm) a