EDIT: So I get the camera part but what about the eyes?
In: Other
In video it’s from high ISO, which is needed in low light situations. It’s similar to cranking up the volume of a microphone if the source is too far away. Eventually you’ll pickup the source you want, but now you’re also amplifying all of the other ‘noise’ around it.
In video, when you crank up your sensor sensitivity (high ISO) you do the same thing. The sensor is now picking up al of the ‘noise’, but it’s represented as a grainy effect.
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