ISO is the electronic gain applied to the signal given by the sensor.
- The light hits a pixel sensor.
- The sensor emits a signal proportional to the intensity of the light.
- This signal is amplified, then recorded as an image.
Noise
If the light is very weak, the Signal to noise ratio is low, so the excessive amplification necessary to have a usable image makes the noise visible.
Solutions are to:
- reduce the ISO and compensate with exposure time
- reduce the ISO and compensate with a bigger aperture
- Reduce the noise in post-processing
ISO sensitivity
ISO sensitivity is a property of the camera sensor