Shot Noise
The unavoidable randomness in the arrival of photons, which produces grain even from a perfect sensor.
Formula / rule
SNR = √N
Measured in
e⁻ (electrons)
In depth
Photons arrive as discrete, independent events, so the count in any fixed interval varies statistically. This noise scales with the square root of the signal, meaning brighter areas have more absolute noise but a far better signal-to-noise ratio. It is a property of light itself, so the only remedy is to collect more photons — a larger sensor, a wider aperture, or a longer exposure.
Examples in practice
A photosite receiving 100 photons has about 10 per cent noise; one receiving 10,000 has only 1 per cent.