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Essential characteristics of noise

Sunday May 10 2009

Sensor Insight
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