What happens in the camera when you increase the ISO?

Question

Cameras

Hello!
When we take pictures with ISO 200, 800, etc., what happens in the camera at these settings?
Does this process take place in the processor during image processing or do these settings affect the matrix?

Answer 1

The camera matrix is ​​a device consisting of many light-sensitive cells, which are, in practice, photon counters. The number of electrons in each cell after exposure is proportional to the number of photons that hit it. The coefficient that determines this proportion is called the quantum efficiency. And this characteristic is implemented in the matrix. That is, the number of electrons (almost, with some error) does not change when the same number of photons hits the matrix pixel.
After exposure, the charge in the matrix pixels is converted into digital values. This is done by an analog-to-digital converter. When you change the ISO value in the camera settings, it affects two things: the sensor ADC input gain and the ADC input range. Thus, taking a picture in the same conditions, but changing the sensitivity, you can get a brighter picture. But this, in a rough approximation, is nothing more than multiplying the same number of electrons in a pixel by a different number (larger with a higher sensitivity value in the settings). That is, increasing the sensitivity practically only makes the final picture brighter, but does not make the camera more capable of detecting light. This is the answer to your original question.
You may ask, why then do different sensitivity values ​​​​if it only affects processing. The answer is simple. Firstly, in many cases, you need to get a picture already in the camera, without any processing, with an exposure of no more than a certain time. Secondly, as with any analog-to-digital conversion, it is important to have the settings for that same conversion (gain, range shift, input range limitation, in the most general case) so that the input data maps as well to the output range as possible. In the case of photography - so that objects in the shadows and in the light are visible to the desired degree. That's why they allow the user to change the sensitivity setting.
Since the ADC does not operate on different parts of the matrix separately, but processes all the values ​​at once, the answer to your question in a comment to chromimon 's answer is "no, you can't". But this is not connected with any mythical "tension", about which he invents something along the way. As well as about "squeezing" something more out of the matrices. The only thing a camera manufacturer can do better or worse with a particular sensor is to not mess up what it delivers. That is, influence the result indirectly - do not allow it to overheat (by placing it in a more massive case), do not use any stupid noise reduction algorithms (to please the user who does not want / does not know how to use programs for working with RAW data), do not make "fake sensitivity values ​​and so on.

Answer 2

The tension is changing.
Surely the ADC is calibrated differently.

Answer 3

The gain of the ADC matrix changes, as a result of which the noise increases, since the signal-to-noise ratio is constant, and the signal, for example, in low light conditions, can be small.
You can read more about camera exposure settings in my blog about shutter speed, aperture and ISO .