To form an image, your eye relies on the cornea and the lens to focus the light reflected from objects. The cornea is the clear, dome-shaped front surface of your eye. The lens is a clear structure about the size and shape of an M&M's candy. Both of these structures bend (refract) light entering your eye to focus the image on the retina, located on the inside back wall of your eye.

The lens, unlike the cornea, is somewhat flexible and can change shape with the help of a circular muscle that surrounds it. When you look at something at a distance, the circular muscle relaxes. When you look at something nearby, the muscle constricts, allowing the relatively elastic lens to curve and change its focusing power.

Presbyopia is caused by a hardening of the lens of your eye, which occurs with aging. As your lens becomes less flexible, it can no longer change shape to focus on close-up images. As a result, these images appear out of focus.

Oct. 17, 2014