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Digital camera sensors and the "film analogy"
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TimFox:
The interesting thing about "kT/C" noise is that it is independent of the ON resistance of the switch, although the resistance changes the time required to fully reset the capacitance.
magic:

--- Quote from: CatalinaWOW on July 15, 2021, 12:11:51 am ---Some architectures may have very short integration times and sum the results from a series of samples.  The result is analogous to a single longer integration time.   Noise is reduce by the square root of the number of samples and signal increases linearly unless the implementation is incredibly inept.  So shutter matters. 

This isn't just theory.  I have observed it in low light conditions with my consumer digital cameras.  Short exposure times result in noisy images.  Noise is reduced on longer exposure times

--- End quote ---
Do you mean longer exposure at lower ISO in the same light or longer exposure at the same ISO in lower light?
Noise reduction in the former case seems explainable by more photons being captured and lees gain applied. My experience agrees.
In the latter seems it seems extremely dubious that any improvement could be had. My experience is that noise increases.
Kleinstein:
The modern smart phones and consumer cameras are quite good in combining multiple pictures taken with a moderate exposure time (e.g. 20 ms) instead of a long exposure. The magic here is in also taking the movement of the camera into account to do a digital steady shot without a tripod even with an effective long exposure. So there can be some picture averaging for longer exposures, but usually only at the long end (e.g. > 100 ms).

The gain / iso setting is often also allowed higher than actually justified noise wise. So the highest ISO setting may be of limitd use. So a lower iso setting and longer exposure can really help, even without picture averaging.

For the noise, there is also shot noise than can become relevant: The number of photons hitting the pixel has an uncertainty the square root of the number. The analog sensro can't get better than photon counting, where the statistical error becomes obvious. AFAIK the modern sensors can in the low light range resolve some 50-100 electrons - so the shot noise can become relevant with more than 6-7 bit resolution for the brighter pixels. So it needs a certain amount of light for a really good picture. Amplification can't help here very much and the sensors are already quite good with the quantum efficiency.

AFAIK some CMOS sensors aready used coherent double sampling, so there is a way around the kTC reset noise. A good read-out circuit can be lower noise than kTC, though it is not easy. So the starting charge is also read and subtracted instead of assuming a fixed starting point.
Siwastaja:
You have incorrect understanding of the shutter.

CCD and CMOS sensors do have very real shutters, either electronic, mechanical, or combination thereof.

They do continuously collect photons, like film. There are no gaps in time, nothing digital, no DSP magic involved. (Of course, you can also digitally merge multiple shots but that doesn't usually make sense because doing a longer exposure on the "analog" level to begin with is simpler, and superior.)

Now I don't know if some modern camera chip primarily optimized for shooting video does use "virtual" longer exposure by combining multiple frames digitally, I don't know, but it doesn't need to be this way, fundamentally the sensor chips easily support arbitrary length exposures like film and this is how it classically is done.

A simplified model is; a photon is converted into electron; electrons (i.e., charge) are collected into buckets. A pixel is a bucket. The bucket can be emptied by the electronic "reset shutter"; another electronic shutter mechanism can prevent more charge accumulating in the bucket because readout of the buckets take time and you don't want to continue the exposure during that time. The latter electronic mechanism can be replaced with mechanical shutter which was common just a decade ago, because it doesn't waste the fill rate on the sensor. A very common failure mode on the digital pocket cameras of the 2000's is the failure of the mechanical shutter, which can be seen as striping as some lines of the sensor are read out later than others, and receive more light during readout.

Leakage currents may limit the maximum exposure time, but decent sensor chips can do minutes at room temperatures. Astrophotographers who need hours of exposure and prefer low noise levels benefit from sensor cooling.

Similarly to the leakage currents, film had reciprocity law failure, but modern film stocks of 2000's had all but got rid of that. This means that basically, if the time difference between photons hitting the same grain is too long, the chances of "missing" one of them gets higher. Thanks to modern (late 1990's to early 2000's) features like 2-electron sensitization, modern films can detect much smaller photon counts making them more sensitive / less grainy, and also making reciprocity law failure almost disappear.

I would hazard a guess that all good-brand sensor chips (or conversion ICs, on CCDs that may be a separate chip) have implemented correlated double sampling to get rid of reset noise at least for 15 years, my guess it's a concern of discussion, not a feature present in final products exactly because it's being taken care of. I'm saying this because I developed a CCD scanner module in 2010 and used a very low-cost commodity scanner ADC IC which did have this feature built-in.
magic:

--- Quote from: Siwastaja on July 15, 2021, 09:36:52 am ---They do continuously collect photons, like film. There are no gaps in time, nothing digital, no DSP magic involved. (Of course, you can also digitally merge multiple shots but that doesn't usually make sense because doing a longer exposure on the "analog" level to begin with is simpler, and superior.)

--- End quote ---
As explained above, cellphones do it to eliminate shake blur in software. If you move your camera during a single 100ms exposure there is no recovery from that.
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