system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does the eye-sight-measuring machine at the glasses shop actually give you the exact number of your eyesight?
gpt
Lenses work both ways. The machine is basically getting a camera trained on your retina and then ‘auto-focusing’ until it sees a clear image of your retina. Software then calculates the properties of your lens based on the amount of focusing it required to get that clear picture of your retina.