Your visual system can undergo tremendous stress these days.
Students now read three times the number of textbooks their grandparents did.
Adults constantly use their near vision at their work. And the growing use of computers has engaged a growing number of workers in prolonged, near-vision tasks. Human beings weren't designed to do this stressful seeing less than arm's length away. We have hunter eyes for survival, spotting game and enemies at a distance. Only in the last half century have so many people been forced to deal with sustained, near visual tasks.