Noise
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Noise, in digital photographic terms, is the occurrence of random values of colour or brightness, due to imperfections in the sensor or variations in its environment.
For a particular sensor, noise is generally caused by heat, and it becomes apparent when the output of the sensor is boosted.
The sensor is boosted when:
- The ISO is set above the base ISO for that sensor
- Shadow values are boosted in postprocessing, perhaps to deal with underexposure
Noise is also apparent in long exposures, where the random action of environmental heat has a proportionately greater effect.
Noise can be reduced by
- Correct exposure
- Use of noise reduction algorithms, either in-camera, or in postprocessing through NoiseNinja or NeatImage or other plugins.
- For long exposures, through switching on in-camera long exposure noise reduction, which executes a second exposure with the shutter closed after the first one, thereby only recording the values caused by heat fluctuations or sensor imperfections, and then subtracts these from the original exposure. Long exposure noise reduction doubles the time it takes to capture an image.
Noise is either (or both) chrominance noise or luminance noise.
- Chrominance noise is sporadic variations in colour, from pixel to pixel
- Luminance noise is sporadic variations in brightness. from pixel to pixel
Generally speaking, luminance noise is closer in character to film grain, and is therefore less undesirable.
