system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why does aperture size effect depth of field?

Why does a wide aperture blur the background while a narrow one keeps it in focus?
gpt
Imagine a light ray emitted/reflected from a specific single point on an object. If your aperture is infinitely small, there's only one position that ray can ever end up after going through the aperture. Do this for all rays emitted from said object, and since there's only one place for each ray to end up, which is related to where it was emitted from, you get an exact replica of the observed object.

If your aperture is bigger however, you could have two rays coming off the same single point on an object, leaving at slightly different angles yet still going through the aperture. Since that single point somehow gets represented in two places after passing through the aperture, blur happens.