system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does vision in darkness have grain?
gpt
You have two types of receptors in your retina. Cones are good at picking out fine detail and color while rods are better with picking out contrast and movement. In low-light settings, differences in light coming to your eyes are much more subtle and the cones don't function well. So you don't get fine detail, and what you do see can have some variance, causing your brain to register a grain.