There's always been a strong correlation between near-sightedness and reading books or looking at screens in childhood and early adulthood, and it was assumed for a long time that it was because of what you described (focusing on objects too near), but the latest research suggests something much more interesting is happening.
Children who spend a couple hours per day outside have extremely low rates of near-sightedness, so they think there's something about the eyes being exposed to direct sunlight that's necessary for them to grow properly.
Children who spend a couple hours per day outside have extremely low rates of near-sightedness, so they think there's something about the eyes being exposed to direct sunlight that's necessary for them to grow properly.