I sorry, I don’t understand 
Then what does the ISO setting do? My photos get very noisy when I increase the ISO 
My understanding is that increasing iso increases amplifier gain. If the amplifier circuit is “clean,” then most noise would come from the sensor. That is the noise that is amplified.
Generally, increasing exposure (when possible) is best for reducing noise.
Edit: last sentence should read … increasing light collection …
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Thank you for your answer! It helped me understand ISO better 
That leads me to even more questions!
What is amplifier gain and what is amplifier circuit? Is it part of the sensor and/or how the sensor captures the light?
Does this help?
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Thank you for sharing the article!
It does help
I will do my best studying it since it’s quite informative yet dense 
Ok, good. Understand that this is a simplified explanation; there is much more to know about collecting light, converting that light to a charge, collecting the charge, then amplifying that as a current and digitizing it, etc.
Here is my take at a simplified overview. I’ve worked a bit on making sensors, but I’m not the expert here on cameras. Anyway -
The lens collects light and projects it onto a sensor. The sensor has pixels (little “light buckets”), and these have either red, green, or blue filters. Once light particles (photons) make it past the filter and into the silicon sensor, the photon creates a charge. This light/charge collection continues as long as the shutter is open.
Now with dim lighting or very high shutter speed, there is very little charge (signal) collected. There is some background charge in the sensor (usually quite small) and micro-variations in the quantity of light (called photon noise). This causes variation from pixel to pixel that is called noise.
With a brighter condition or longer shutter time (or wider aperture on the lens), more light is collected, but the same noise is present. So the ratio of signal to noise is improved.
This charge is collected and amplified. The amplification is set using your iso dial - higher iso gives higher amplification (also called gain).
With a high charge (or current) from the sensor, the signal does not need much amplification (gain). But if you turn up the iso too high, the amplifier could saturate which causes clipping.
But with low charge/current from the sensor, it is necessary to boost the signal by amplifying it, and this is where high iso is used. But remember that sensor signal/noise is low, and both the true signal and the noise are amplified. So, it makes more noise visible in the processed image.
So needing to use a high iso is a consequence of collecting a small signal at the sensor. But the photon noise is about the same regardless of the signal, so the signal/noise ratio is low, and both the signal and noise get amplified.
To improve signal/noise, collect more light. Don’t over-amplify (i.e. use the correct iso).
I hope this helps.
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Also, older cameras did do better for noise as ISO was increased compared to compensating in post. Some modern cameras are considered “ISO less” and there’s no noise improvement from increasing ISO on the camera VS adjusting exposure in post. Some are “dual ISO” where there are two special ISO values, base ISO and another, often around ISO 800 which changes noise performance, but other ISO values don’t impact noise significantly.
Talking about ISO and noise on my Canon R7. I find two sources of noise are underexposure (or exposing to the left) and increasing the ISO number. But I shoot bracketed exposures where the ISO changes between the shots. What I have come to realise with my Canon R7 is that increasing the ISO to very high numbers exposes to the right and produces very similar noise to the under exposed image exposed to the left. So while my R7 is not ISO invariant the effect of ISO on noise is counterbalanced by the brighter exposure in most cases. But under exposure seems a greater source of noise for my R7 than ISO. This is not the case for all brands of cameras and I only reflect upon my experiences with the R7.
Hi @Terry
So in my parlance above, you get lower signal/noise when you collect less light by either: under exposure or by increasing iso (gain), which causes the camera to collect less light. To me that makes sense. There are of course some subtleties that I did not cover (and some I do not even know about).
The bracketing situation may be a result of the same light collection with different gain. If the amplifier circuit is clean you should have about the same signal/noise in each exposure. That is an approximation that (these days) seems to hold over a limited iso range.
Have you tried exposure bracketing at fixed iso in either aperture or shutter priority? There you should see variation in s/n as long as exposure bracket is wide enough. It might be easiest to see such a result at elevated iso, but in principle it should be the case even at base iso.
Of course, turn off noise reduction in your raw developer, and turn off any noise reduction in-camera.
I think that part of the confusion is that what we call “exposure” is determined after amplification. So that term confounds light-collection and amplification. That is how I think of it, anyway.
In the film days, of course, there was no such circuit, just film types with different sensitivities. So, once you load a roll of film, the only way to change exposure is to change the amount of light that is collected. Digital is different.
So, to beat this dead horse, collect more light to improve signal/noise.
Yes, there are some subtleties (e.g. with dual-amp circuits), but the general trend follows.
FYI I also think that this explains much of the “dynamic range vs iso” trend chart that was shared earlier. As iso is increased, for the same “exposure,” less light is collected, reducing s/n, and effectively raising the “noise floor.” Curves from most camera sensors are parallel in these plots, with offsets that come from pixel size and sensor/amp technologies (sensor mostly). My thinking, anyway.
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The only time I used P mode was for the first 2yrs I used a DSLR then I switched over to Aperture when I changed over to Raw from Jpeg. I use Manual mode when I do long exposures.
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