Hyperopia is caused when the image is focused behind the retina. For low to moderate degrees of hyperopia, particularly in younger people, the condition can go undetected without a formal eye examination. This is because we are able to adjust the focus to give clear distance vision using the flexible lens inside the eye. The result may be eye strain symptoms including blurred vision or headaches, particularly after prolonged periods of concentration. These symptoms can be relieved with the use of glasses, particularly for close activities. For higher degrees of hyperopia glasses or contact lenses may be used full time to avoid excessive fatigue.