This tip covers ISO in technical depth.
For the basics of ISO, go to ISO.
ISO stands for International Organization for Standardization.
This organizations publishes the specifications for many items.
ISO as a term, isn't good.
A self-explanatory term would be better, such as Photon Amplification.
That term isn't scientifically precise, as it's voltage that's getting amplified.
But, photon sounds cooler.
The sensor is covered with photosites.
They collect the photons (light).
Each photosite consists of:
• A lens to gather light from many angles.
• A color filter used to determine the eventual color of the pixel.
• A photodiode that collects photons—and converts them to electrons.
• A well in which the electrons are stored as a voltage.
• An amplifier that may boost the voltage if the light is dim.
The voltage is sent to the Analog to Digital Converter.
There, the analog voltage becomes digital.
It becomes a number between 0 and 4095 (12 bit).
Only so many electrons can be stored in each well.
This amount of electrons, the voltage, is the native ISO of the sensor.
If you use an ISO setting other than the native ISO setting, the voltage is amplified.
You're turning the "volume up" on millions of amplifiers on the sensor.