How the Nature of Light affects the performance of digital cameras — (Part 2) Noise

ImageJ-G9
G9, pixel plot across sample

In the second of two articles I look at another natural phenomenon, photon noise (also known as Shot noise). As with diffraction blur, the problem becomes more serious as the physical size of the sensor is reduced.
Whilst this is not the only source of noise, it is now the dominant one in the darker areas of an image where only a relatively small number of photons are incident on the sensor. It is the counting of photons, which is subject to Poisson statistics, which produces the noise.

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Click here to read the first article: Diffraction blur

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How the Nature of Light affects the performance of digital cameras — A new Technical Note

G9-f8-2.38
Image of a point source: Blue = Intensity, Red = Sensor output. Computer model of G9 at f/8

Reducing the physical size of a camera, even if the total number of pixels is maintained, inevitably reduces the quality of the images because of two fundamental properties of light itself. This technical article looks at diffraction, usually explained by considering light as waves. A future article will look at photon noise, explained by considering light as particles.
A simple rule-of-thumb is established for determining the “diffraction limited f-number” by relating this to pixel pitch on the sensor.
The earlier, pre-digital, criterion for diffraction limited aperture (based on required print sharpness) is revisited and considered to be still valid – perhaps with a little sharpening.

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