Both eyes of an athlete need to point straight towards a ball in order to see it clearly. Both eyes need to turn inward to continually see the ball clearly as it travels towards the athlete. Convergence insufficiency (CI) occurs when one or both eyes stop their proper turn inward causing the appearance of the ball to become blurred and doubled.
Athletes with CI usually depend on one eye more than the other to perform the work of seeing. This causes their depth perception to be decreased and they tend to misjudge to position of the ball or other objects in free space.
Eye doctors can prescribe exercises (vision therapy) to help the eye muscles of both eyes work better as a team. This improvement helps athletes reach their genetic potential.
This is a common visual problem with many athletes. Routine vision screenings at pediatricians' offices or in schools do not test for this problem. A comprehensive eye examination at an eye doctor’s office is recommended to detect this condition.
I've included a great video I found on Youtube regarding an eye exercise know as pencil pushups. This is one of the fist line of defenses to help exercise this problem away.