The beauty of Auto-ISO...

"...you don't change exposure by changing the ISO."

Except that you do.

No, the metering is changed, not exposure. If you rely on the meter to determine your exposure and you set your ISO higher then you are forcing the camera to select a lower exposure; OTOH, if you choose the best practical EV then you can also choose the best ISO, and if the exposure is low then you can compensate with a higher ISO setting to alleviate that (at least some, and more on some cameras than others). ISO is what you tell your camera to do to the exposure, it is applied after the exposure and is no more a part of exposure than EC which is what you tell the camera's meter to do before the exposure.

Sheesh. 1 EV = one stop of light. I understand what you are saying, but effectively its is a change in exposure even though it's effectuated through sensitivity to light not by the amount of light, but 1 EV still = 1 stop of light.

ISO does not change the sensor's sensitivity to light and by itself it does not change the exposure (again, it changes the metering).

"ISO is what you tell the camera to do in response to the exposure, and not the other way around."

 

They all respond to each other. Change your ISO after you meter, and then meter and check your f-stop and or shutter speed. If you meter for middle gray and get a reading of F8 at 1/125 with ISO set at 200. Then change your ISO to 400 and lets say keep your speed at 1/125. What will you you f-stop be? One stop less of light for the same middle gray. Its not that difficult. and yes it will alter the tonal curve as well, but the exposure will be equivalent at middle gray.

That's applicable to shooting OOC JPEG, but not to shooting Raw. As I wrote above, such an approach makes ETTR inconceivable. When shooting Raw middle gray is where you choose to put it; the implications of raising or lowering ISO has to do with how much headroom you will have and how the exposure is processed by the camera. Again, when shooting Raw ISO is a camera setting and not a camera standard. As I mentioned above, often raising ISO when shooting Raw at a given exposure level can actually increase the dynamic range of the exposure, but nonetheless if you have the same EV but change the ISO you do not change the exposure.

"ISO does not change the sensor's sensitivity to light and by itself it does not change the exposure (again, it changes the metering)."

EV represents a pair of aperture and shutter speed settings which admit the same amount of light (intensity x time). It is not affected by ISO, but which EV you choose for the correct exposure is affected by the ISO setting. The ISO setting does, indeed, affect the sensitivity of the sensor, with or without cooperation of the metering system. What happens to the image if the camera is in manual mode and you change the ISO setting?

To some extent the sensor is a sophisticated coulometer, measuring the charge of a capacitor by its voltage. The charge accumulates proportional to the light reaching the sensor. The ISO setting essentially changes the gain of the voltmeter used to read this charge and report its value to the image processor. In practice, the measuring circuits are inseparable from the sensor, so ISO affects the sensitivity, among other things.

Once the image is available, you can mathematically increase the exposure. This usually affects the noise and possibly the color balance. However if the image quality is the same, whether by setting the ISO or processing the image, the camera is said to be "ISO Invariant."

"ISO does not change the sensor's sensitivity to light and by itself it does not change the exposure (again, it changes the metering)."

 

EV represents a pair of aperture and shutter speed settings which admit the same amount of light (intensity x time). It is not affected by ISO, but which EV you choose for the correct exposure is affected by the ISO setting.

That's approaching the role of ISO backwards. You choose your EV and then you choose your ISO, which is exactly why using Auto-ISO often makes a lot of sense.

The ISO setting does, indeed, affect the sensitivity of the sensor, with or without cooperation of the metering system.

I don't mean to be argumentative, but that is wrong on both counts. More on the definition of a sensor's sensitivity below; and it's not the metering system that is implicated in changing the ISO, it's the Exposure mode (Auto-ISO is just that, an Exposure mode that neither prioritizes aperture at the expense of shutter speed or vice versa, but instead prioritizes both at the expense of ISO). Shooting Raw the only thing that determines where middle gray ends up in my final photograph is how I choose to process the file -- what I have to watch out for is overexposing because I overamplified the data collected by the sensor. In spite of the oversimplified explanations coming from the camera companies (who know better, but their explanations are written for the blissfully uninformed and not by and for engineers) I have not changed the sensitivity of the sensor by changing the ISO (just ask Thom Hogan, Iliah Borg, Nasim Mansurov, et al).

What happens to the image if the camera is in manual mode and you change the ISO setting?

The same thing that happens to a print on the wall when I turn the light shining on it up or down.

To some extent the sensor is a sophisticated coulometer, measuring the charge of a capacitor by its voltage. The charge accumulates proportional to the light reaching the sensor. The ISO setting essentially changes the gain of the voltmeter used to read this charge and report its value to the image processor. In practice, the measuring circuits are inseparable from the sensor, so ISO affects the sensitivity, among other things.

The sensor's sensitivity is its responsiveness to light, and that is immutable. A sensor that detects small amounts of light (photons) is more sensitive than a sensor that requires more light before it is detected. The maximum number of photons a sensor's photodiodes can absorb is its FWC (full well capacity), and base ISO is partially based on the sensor's sensitivity and an arbitrarily determined best use of the FWC, but not exclusively as camera manufacturers leave headroom in the files and take different approaches to determining middle gray (e.g., the default Brightness setting). Rather than having cameras with random base ISO numbers camera manufacturers generally shoehorn the data to make it fit ISO 100 or some other convenient value, and then each subsequent stop is a doubling of the amplification of the data coming off the sensor to match the ISO specification.

Once the image is available, you can mathematically increase the exposure. This usually affects the noise and possibly the color balance. However if the image quality is the same, whether by setting the ISO or processing the image, the camera is said to be "ISO Invariant."

Conversely, you can pull the conversion and in effect be at a lower ISO than the one that you set on the camera. This defines ETTR. As for "ISO Invariant" cameras, they hardly exist -- my D800 has a great deal of exposure latitude, but I can still get marginally better results in the shadows by raising the ISO.

The sensor's sensitivity is its responsiveness to light, and that is immutable.

Technically, you are correct. ISO is implemented as gain imposed on the analog signal from the light sensitive cell, prior to digitalization and image processing. However the analog amplification and digitalization circuits are integrated into the sensor array, prior to scanning, de-Bayering and image processing. In a practical sense, the light sensor plus ISO/gain is analogous to the emulsion of film.