Your eyes aren't static objects and can change the lens shape and your focal distance to an extent. The theory is that by not forcing your eyes to strain and ex

Your eyes aren't static objects and can change the lens shape and your focal distance to an extent. The theory is that by not forcing your eyes to strain and exercise towards seeing things at a farther distance they don't get the very exercises that may mitigate or delay further degradation or change in focal length in the opposite direction. I don't know of any research offhand that definitely says it does or does not permanently effects your vision, but it isn't hard to get confusing or contradictory results if studies aren't well prepared specifically because your eyes can change focus slightly through concentration and can also get tired and will effect the maximum focal change possible at different times.