To understand the difference between a small pupil and a large one, it's useful to understand how "aperture" settings work on a camera: https://en.wikipedia.org

To understand the difference between a small pupil and a large one, it's useful to understand how "aperture" settings work on a camera: https://en.wikipedia.org/wiki/Aperture https://en.wikipedia.org/wiki/Aperture Specifically, bright light results in a small pupil which allows for better focus. And low light results in a large pupil which reduces focus.