A while back I was commenting on the different between digital and film and ISO intolerance, which basically means there is no change in ISO in digital.
WAIT you say. How come there is an ISO dial on my digital camera? It is a throwback to film days a way of maintaining understanding. It is useful in calculating the outcome in JPG images and live view. In reality it is just a gain dial. When you use RAW it is not as important to the final image.
So here is an exercise, shoot a scene at high ISO then reshoot using the same Aperture and Shutter settings but drop the ISO to a low value. The post processing will be different but normalize the exposure to look the same in both images. The final images will look remarkably the same. There will be a very slight difference in the highlights. The lower ISO will look better. This difference has to do more with the storing of data than the ‘ISO’ of the chip.
With the image above I shot it with the same Aperture and shutterbug also wit a number of different ISO starting at ISO 400 and going to ISO 6400. There was no difference in the final processing. In the field I found ISO 1600 best only because it allowed my camera to see Jupiter in real time and thus focus on it easier. The sensor received the same amount of light in all shots the ISO selection was just a gain to allow me to see the Image in the camera.