A child presented with difficulty in seeing the blackboard at school. His teachers say that he frequently 'squint' his eyes. Most probable cause is:

For the myopic or nearsighted individual, objects nearby are in focus; those at a distance are blurred. This is because the plane of focus is anterior to the retina. The onset is typically at about age 8 years and may progress throughout adolescence and young adulthood. A myopic person may squint to produce a pinhole effect, which improves distance vision. Divergent lenses provide clear distance vision. Ref: Braverman R.S. (2012). Chapter 16. Eye. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.