The eye’s ability to refract or focus light sharply on the retina is based on two main anatomic features: the overall length of the eye and the curvature of the eye’s surface or cornea.
- Eye Length: When the eye is too long, images mistakenly focused in front of the retina are out of focus by the time they actually hit the retina. Nearsightedness or myopia then results. When the eye is too short, images never have a chance to achieve focus by the time they hit the retina. This causes farsightedness or hyperopia.
Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without an astigmatism.
As mentioned above, the tear film, crystalline lens, and internal fluids also play a role in focusing an image onto the retina. An irregularly shaped natural lens or defect in the way it functions also can cause focusing problems, leading to blurry or distorted vision.
These various defects in focusing can cause light rays to bend or refract at skewed angles, which means sharp focus cannot be achieved. When abnormalities of this type occur in the optical system, they are known as refractive errors.
More obscure vision errors, known as higher-order aberrations, also are related to flaws in the way light rays are refracted as they travel through our eye’s optical system. These types of vision errors, which can create problems such as poor contrast sensitivity, are just now being detected through new technology known as wavefront analysis.