The way our eyes bend or refract light enables us to see the world around us. Refractive errors are due to optical defects that prevent the eye from properly focusing light on the retina, thus resulting in blurred vision. The primary refractive errors are myopia, hyperopia, astigmatism, and presbyopia.
In myopia or nearsightedness, light entering the eye focuses in front of the retina due to a cornea that is too curved, or an eyeball that is too long. As a result, myopic individuals are able to see objects clearly when they are near, but not when objects are at a distance.
In hyperopia or farsightedness, light entering the eye focuses behind the retina due to a cornea that is not curved enough, or an eyeball that is too short. Consequently, hyperopia individuals find objects more difficult to see when they are near, and easier to see when they are at a distance.
In astigmatism, the cornea is curved unevenly. In eyes without astigmatism, the cornea is uniformly curved in all meridians, like a baseball. In astigmatism, the cornea is curved more in one meridian than another, like a football. This irregularity causes light that enters the eye to focus at more than one location, and results in distorted vision for potentially both distant and near objects. Astigmatism often happens in conjunction with myopia or hyperopia.
Presbyopia is a condition that occurs with age and causes progressive difficulty focusing on near objects. This difficulty is often first noted between the ages of 35 to 50 years of age, typically with trouble reading the fine print. While presbyopia causes difficulty with near vision, the ability to see distant objects remains unchanged.