by Bob » Thu Sep 12, 2019 10:27 pm
This is a bit of an over simplification, but I hope it will get the idea across. Adjusting the ISO does not change the sensitivity of the sensor. Instead, it amplifies the signal the sensor produces when it is illuminated by a given amount of light. ISO is a gain control. There are actually two amplifiers involved in this. The first is an analog amplifier on the sensor chip. After the onboard analog amplification, the signal is converted from analog to digital and then there is a digital amplifier following that. ISO affects both.
In a dual native ISO sensor, instead of one on-chip analog amplifier, there are two. The second amplifier is stronger than the first and optimized for lower light levels. Which amplifier is used depends on your ISO setting. If it is below the second (higher) native ISO, the lower gain amplifier is used. If it is at or higher than the second native ISO the higher gain amplifier is used. So, you actually do have two different signal paths and two native ISO settings.
As for quality, that's tricky. I don't like the term native ISO as it over simplifies what's going on. Changing the ISO not only affects noise, it affects dynamic range and the distribution of stops that can be recorded above and below middle gray. Native ISO isn't always the best setting for a given lighting situation. I'm not going to go into that aspect. Suffice it to say that as you increase the ISO from the first native ISO towards the second native ISO, the image will be brighter and generally more noisy until you hit the second native ISO and then the second amplifier kicks in and the image quality improves again.