system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does my body know if my vision is in focus or not? What tells my eyes to adjust?
gpt
Your brain knows from experience what things should look like and what things look like when they're out of focus.  When you see something that your brain thinks looks out of focus, it controls muscles in your eye to change the shape of the lens on the front of your eye until they appear correctly.  

This is why when you see a picture that is slightly out of focus, it makes your eyes feel funny -- because your brain keeps trying to focus on something that can't be focused on.

This is all done subconsciously like hearing, breathing, etc., so you don't have to think about it.  Vision is controlled by the Occipital Lobe and Visual Cortex parts of the brain.