ISO invariance is a property of camera sensors where there is little difference in noise between a picture shot at one ISO setting and then brightened in post processing by some amount and another picture shot at a much higher ISO and not brightened. For example, with an ISO invariant sensor, shooting at ISO 100 and brightening by 4 EV in post should result in about the same noise as shooting at ISO 1600 without brightening in post. The advantage of shooting at a lower ISO is that more detail is preserved in the bright areas. Very few cameras have sensors that are truly ISO invariant. But most modern cameras have sensors that are almost ISO invariant at higher ISO setting, for example, ISO 3200 and above.
I use a low ISO (typically 1600 at F/4 and 10 seconds) which results in a very dark image out of the camera. But after stacking and post processing I get an image that has low noise and color in the bright stars. Due to ISO invariance, the noise is about the same as I would get at higher ISO, but with the advantage of more color in the bright stars.
Last updated December 2023.