ELI5; What causes the “grain effect” in cameras and eyes when trying to look in very low light conditions?

Question

EDIT: So I get the camera part but what about the eyes?

In: Other

Answer 1

The sensor in the camera can be more or less sensitive to light. What we are worried about referred to SNR (signal to noise ratio), the more sensitive the sensor the more of that SNR is actually noise and that noise shows up as a grain. When you have a lot of light available you turn the ISO (image sensor sensitivity) WAY down so the it only picks up obvious signal and not much noise. You have little to no grain and the image looks almost perfect.