Essential characteristics of noise
| Introduction | Noise in shadows | Noise in mid-tones | Noise in highlights | Summary | Annex |
Sensor noise 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.
Expressed as
an equation, the variance 
corresponding
to an output signal 
can
be approximated by
As a 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