this post was submitted on 17 Nov 2023
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Photography

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Where does noise really come from? (alien.top)
submitted 11 months ago by VexTheMaster@alien.top to c/photography@viewfinder.pro
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I know, I know, it can be caused by various factors, like high ISO, poor lighting conditions, long exposure time, heat. (not that I know why the last two cause noise)

But where does noise REALLY come from? What is noise? What does it have to do with photons and photosites?

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[–] oldlurker114@alien.top 1 points 11 months ago

If you're talking film the answer is film has literal grain because it's physical

Grain is only one part of the noise function. The other is the random nature of light itself.

If you're talking digital the answer is sensors use gain to increase and decrease sensitivity

Nope. We can of course define "sensitivity" is multiple ways, but typically in this context the image sensors have one sensitivity (though different for each wavelength of light).

When you increase the gain you start having cross-talk as you overload individual pixels and the detection kinda bleed across them.

No, this is plan wrong.

Here's what happens typically:

  1. Lens draws a noisy image (because of photon shot noise - light is noisy, and this noise dominates outside of very light starved places)
  2. Pixels capture this image - at this stage pixels add such a tiny amount of noise to the signal that is usually irrelevant.
  3. Signal moves to PGA (programmable gain amplifier) - here the analogue signal is amplified according to selected settings (typically the ISO setting controls this)
  4. Signal moves to ADC (analogue to digital converter) where it is digitized - this process is slightly noisy, thus some noise is added to the signal
  5. A raw file is written or the camera processes a JPG and writes it

The reason why there is step 3 is in step 4 noise. If you increase the signal before ADC, the noise that the ADC adds becomes less relevant in comparison. The drawback for amplification is that the signal may "burn", or the ADC operational range is exceeded.

A fun experiment is that you take two shots with identical exposure settings: one at ISO 100, the other at ISO 6400, both in raw format, and then process them in Lightroom or other such raw processor to the same lightness. For practically all today's cameras the ISO 6400 will appear cleaner.