> But there is no grain in reality, so it does the opposite
Well ackchually -- illumination is inherently random, so all time-bounded captures of a scene (including what your eyes do) are subject to shot noise: https://en.wikipedia.org/wiki/Shot_noise
You misackchuallied grain with any noise, for example, the bottom right square of the image at the wiki page is not grainy despite being technically shot-noisy
Well ackchually -- illumination is inherently random, so all time-bounded captures of a scene (including what your eyes do) are subject to shot noise: https://en.wikipedia.org/wiki/Shot_noise