Ever wonder how the sensor in your camera or iPhone works? Specifically, how does ISO, or sensor sensitivity, work? If so, go grab the beverage of your choice, get comfortable and spend ten minutes in the fine company of Dylan A Bennet, as he explains it all.
Dylan gets a little carried away with coloring in his drawings, which slows things down bit, but the tutorial is very easy to understand. In short, it involves two things: light and magnetic fields. The electricity running through the sensor’s circuits generates tiny magnetic fields. As the wires criss-cross each other, these fields induce currents in other wires. This tricks the sensor into thinking that light is hitting it.
This causes noise.
When light levels high, this is drowned out. But when there’s not much light, the noise is significant and can still be seen. And when the camera amplifies the sensor info (The ISO dial is effectively the same as a volume knob on an amplifier), it also amps up the noise. Thus, high ISO gives noisier photos.
This won’t help you get rid of noise, but at least you now know what causes it.