Refraction refers to the process by which your eye focuses light. The eye’s refractive ability is determined by three factors: the cornea’s curvature, the overall eyeball length and the power of the lens. Ideally all of these factors work together to focus the light sharply onto the retina to allow clear vision at all distances without glasses or contact lenses. Many people experience a refractive disorder resulting in a blurred image at some or all distances.
Myopia, or nearsightedness, occurs when the cornea’s curvature is too steep or the eyeball is too long, causing light to be focused in front of the retina, rather than directly on the retina. Objects are easy to see if they are close, but those in the distance are blurry.
Hyperopia, or farsightedness, occurs when the cornea’s curvature is too flat or the eyeball is too short, causing light to be focused behind the retina, rather than directly on the retina. Objects are easy to see if they are far away, but close up objects are blurry.
Astigmatism occurs when the cornea is shaped irregularly and often accompanies myopia and hyperopia. Light rays are not focused, but scattered, so that they focus behind and in front of the retina, rather than directly on the retina. Vision is blurred at all distances.
Presbyopia is a vision condition that affects people after about age 40, regardless of whether they have experienced eye problems in the past. The lens of the eye loses flexibility, making it difficult to focus on close objects.