Why can a RAW image shot at ISO 100 and brightened in post look noisier than the same exposure at ISO 3200?
Asked 2/6/2019
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I expected a RAW photo taken with the same shutter speed and aperture to show about the same noise whether I raised brightness using ISO in-camera or by adding exposure in post. But in my tests, an image shot at ISO 100 and pushed about +5 EV looks noisier than the same scene shot at ISO 3200. Why does this happen if ISO is "just amplification"? Does lower ISO introduce more read noise, or is something else going on in the camera's analog/digital pipeline?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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First you need to capture RAW to make any sort of comparison in that manner: JPEG is lossy compression based on the premise that it will not waste storage for differences that are comparatively imperceptible (applying a metric of image quality) by human vision. If you are doing image procession afterwards, this premise is violated. You say that you were using RAW but I wanted to point this out anyway. A similar consideration would be the noise reduction algorithms cameras employ at higher ISO: sometimes they are even active in RAW mode to some degree.
Second, your theory only holds for photon noise and in the digital domain. ISO values tend to be implemented in the analog domain, however. Sometimes there is some differentiation when you have intermediate ISO values: the intermediate values are sometimes implemented juggling thresholds and scales in the digital domain while the "proper" powers of 2 are done in the analog circuitry. This means that significant underexposure may lead to significant quantification noise (part of it being caused by an analog noise floor, part simply because of the D/A converter step size).
Originally by user81797. Source · Licensed CC BY-SA 4.0
user81797
7y ago
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Because ISO usually is not only a post-capture brightness change. In many cameras, much of the ISO gain happens in the analog stage before the signal is digitized. That matters because the sensor output contains both photon noise and read/electronic noise.
If you shoot at very low ISO and then push +5 EV later, you also amplify the read noise that was added before or during digitization. At a higher ISO, the camera boosts the sensor signal earlier, so the signal reaches the ADC at a stronger level relative to downstream read noise. The result can be a cleaner file, even with the same shutter speed and aperture.
A fair comparison should be done with RAW, since JPEG processing and in-camera noise reduction can change the result. Some cameras may also apply processing that affects even RAW to a degree.
So your expectation is only partly right: if noise were only photon noise, ISO vs post gain would be nearly equivalent. In real cameras, analog gain, ADC behavior, and read noise mean a high-ISO RAW shot can look less noisy than a low-ISO shot pushed heavily in post.
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