A young person's eyeball will grow longer over months to keep near objects in focus if their visual environment is predominantly close-up, potentially leading to myopia.

“if most of your work most of your visual environment is close then the image the light rays from that close object are diverging and so they are falling behind the retina of someone who's perfectly targeted for distance so your eyeball is really smart and it says okay so you're telling me everything is here close up i know i'll just grow longer so that these near objects are in perfect focus and that is exactly what happens over what time scale over a period of months so if you take a young person and you exclusively have them perform near tasks their eye will grow longer so that those near objects are in perfect focus”