system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do optometrists ask people if they can read better in the red or in the green?
gpt
This is actually a way to fine tune your refraction. Red and green focus at different planes in front of, on, or behind the retina. In order to ensure that you will focus clearly at far distance, i.e., for driving, we will often "keep you in the green". If you see the letters in the red better, you may be focused slightly closer than optical infinity, blurring road signs etc, especially at night.