What causes chroma noise, and why does it increase so much at high ISO?
Asked 5/21/2015
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I’m trying to understand the source of chroma noise in digital image sensors and why it becomes so visible at very high ISO settings. My understanding is that on a CMOS sensor, raising ISO increases analog gain before the ADC, so both signal and some forms of noise are amplified.
What noise sources are mainly responsible for the colored blotchy noise seen at high ISO? Is it mostly read noise from the sensor/electronics being amplified, or can dark current / thermal effects also contribute? I’m also curious why the pattern can appear low-frequency enough that it takes a fairly large noise-reduction filter to suppress it.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Shot noise goes with the square root of the number of photons, so every time you have 1/4 the photons you have 2x the noise (with respect to the signal). (Example - 400 photons has 20 average noise, 100 photons has 10, so go from noise is 1/20 to 1/10.)
Note that pixel electron capacity goes down as ISO goes up, as there is a limit as to how far you can multiply the signal before you run out of range. For example a Nikon D800 has 81,000 electron capacity per pixel at ISO 100 but only 175 at ISO 51,200. White is still a full pixel, so you can see how much more noise you will get by applying the first para.
Also the read noise (from sensor/electronics) gets amplified by the ISO amplifier along with the signal. If you have 10,000 electrons of signal 2 electrons of read noise is very little (especially compared to the average 100 electrons of shot noise). If you have 16 electrons then the 4 electrons of shot noise and 2 of read noise being to look very large, especially after the ISO amplifier multiplies the whole lot by 32x, or whatever.
Originally by user40667. Source · Licensed CC BY-SA 4.0
user40667
11y ago
0
Generated from our catalog & community — verify before relying on it.
High-ISO chroma noise is mainly the result of low signal plus amplified sensor/electronic noise.
Two key effects are at work:
- Shot noise becomes more visible when fewer photons are captured. Shot noise scales with the square root of the photon count, so when light levels are low, noise is a larger fraction of the signal.
- Raising ISO amplifies both the signal and the sensor’s read noise. ISO doesn’t create more light; it increases gain before/around readout, so any read noise from the sensor/electronics is boosted too.
At high ISO, the effective electron capacity per pixel is reduced, so the camera reaches full-scale output with far fewer collected electrons. That means the image has less headroom and a worse signal-to-noise ratio, making noise much more obvious.
The “chroma” appearance comes from differences in noise between color channels after demosaicing and color processing. In practice, the colored blotches you see at very high ISO are usually not just photon shot noise alone, but a combination of weak signal and amplified read/electronic noise. Thermal/dark-current effects can contribute, especially in long exposures, but high-ISO color blotching is commonly dominated by the low-signal + amplified-read-noise side of the problem.
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