When we have an eye examination, we may hear references to errors in the way our eye refracts light. This is because the eye’s ability to refract or “bend” light is also its ability to focus light, which determines the sharpness of our vision.
The normal eye can refract or focus light without the help of any other lenses such as glasses or contacts. If the eye cannot focus an image sharply and requires another lens to assist it, then the eye is said to have a refractive error.
The procedure to determine a prescription for eyeglasses or contact lenses is called a refraction. Eye surgeons who use vision correction procedures are also referred to as refractive surgeons.
But what does it really mean when we’re told that our eye has a refractive error?
We all know that, in order to see, we must have light. While we don’t fully understand all the different properties of light, we do have an idea of how light travels. A light ray can be deflected, reflected, bent, or absorbed, depending on the different substances it encounters.
When light travels through water or the curved glass of a lens, for example, its path is bent or refracted. Certain eye structures have refractive properties similar to water or lenses and can bend light rays into a precise point of focus essential for sharp vision. As the light rays are bent, so is the image from which they originate.
Most refraction in the eye occurs when light rays travel through the curved, clear front covering (cornea). The eye’s natural (crystalline) lens also bends light rays. Even the eye’s tear film and internal fluids (aqueous humor and vitreous) have certain refractive properties.
Light rays from an image traveling through the eye’s optical system are refracted and focused into a point of sharp focus that ideally should center on the retina. The retina is the tissue that lines the inside of the back of the eye, where light-sensitive cells (photoreceptors) capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eye’s optic nerve to the brain for interpretation.
Just as a camera’s aperture (called the diaphragm) is used to adjust the amount of light needed to expose film in just the right way, the eye’s pupil widens or constricts to control the amount of light that reaches the retina. In dark conditions, the pupil widens. In bright conditions, the pupil constricts.
Refractive Errors in the Eye’s Optical System
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.
Vision Correction for Refractive Errors
A refraction from an eye care practitioner helps determine the type and degree of refractive error, which may be addressed with glasses, contacts, or refractive surgery.
An eye care practitioner has various methods of determining refractive errors in an eye, including use of this phoropter fitted with various lenses.
Eyeglass lenses and contact lenses are fabricated with precise curvatures that help offset flaws in our eye’s optical system. These lenses intercept and bend light rays, such that they achieve a more precise point of focus on our eye’s retina.
Many vision correction surgeries such as LASIK also aim to correct refractive errors by changing the shape of our eye’s front surface (cornea), so that light rays are bent into a more accurate point of focus.