system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a camera only focus on one distance at a time, and what occurs inside to change that focal distance? Does the same principle apply to our eyes?
gpt
When an image is formed by a lens, it's only sharp in a narrow range.  That range defines the focal point of the optical system.  Hold your finger an arm's length in front of your face and shift you focus from your finger to a distant object.  When you focus on one, the other becomes out of focus.