There is not a more confounding and mysterious element of digital photography than ISO. And just when we've all thought we've figured it out and it starts to make sense, another wrinkle appears and threatens our conceptions all over again.
If you have been struggling to figure out the best ISO to use in low-light situations, then you should definitely check out this video which describes in detail what ISO invariance is, and how to take advantage of it in your own photography.
In the video, astrophotographer Alyn Wallace explains the intricacies of ISO Invariance, how to test your own camera's ISO performance to see if it is ISO invariant or not, and what to do with the information you've collected. The results are eye-opening for sure. So many times we as photographers have been told to limit the ISO in any situation to "get less noise." But the reality can be quite different, depending on which camera you are using.
For example, you might get better results by shooting with a lower ISO and increasing the exposure in post-processing, rather than shooting with a high ISO from the start. The key takeaway is that lower ISOs can help protect the highlights of your scene, whether they be from auroras, streetlights, the moon, distant light pollution, or in deep space objects like the Orion Nebula. Regardless of your camera's ISO invariance, this video will help you understand the best ISO to use for your specific camera, if you follow the instructions he lays out for you to test.