variance is signal-dependent and comes from three main independent sources:
- Dark (electronic) noise,
which is a constant, depending on the quality of the electronics, and which
influences the quality of images in shadows.
- Photonic noise, which is
a quantum phenomenon whose variance is proportional to the signal.
- Pixel response
non-uniformity, which is due to electronic inhomogeneity of the sensor, and
whose variance is proportional to the square of the signal.
an equation, the variance corresponding
to an output signal can
be approximated by
consequence, the SNR also has three regimes:
- Shadows (): the SNR increases 6dB for every EV
and loses 6dB for each doubling of the ISO setting.
- Midtones (): the SNR increases 3dB for every EV
and decreases by 3dB for each doubling of the ISO setting.
- Highlights (): the SNR is constant and does not
depend on the ISO.
The three regimes of noise
Further readings for the Essential characteristics of noise
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