The most well-known part of a comprehensive eye exam is the basic vision test. When you have a general vision test, one of the main conditions the eye care practitioner is checking for is a refractive error. A refractive error means there is an abnormality in the shape of the eye, changing the eye's ability to focus light directly onto the retina.This causes blurred vision and can usually be corrected by wearing prescription eyeglasses, contact lenses and possibly, alternate treatments such as vision therapy, ortho-k, LASIK or refractive surgery such as LASIK.
The term, "refractive error" refers to a problem with the process of refraction that is responsible for sight. Normally, light rays that enter your eye are refracted or bent through the cornea and the lens, and ultimately converge or are focused onto a single point on the retina. From the retina, messages are sent through the optic nerve to the brain which then interprets these signals into the image that we are seeing.
In order for this process to work effectively, the anatomy of the eye including the length of the eye and the curvature of the cornea and the lens must be just right to be able to focus the light onto the retina. When this is not the case, a refractive error will occur.
There are several different types of refractive errors, depending on which part of the eye is affected, and it is possible to have multiple refractive errors at the same time:
Myopia or nearsightedness:
In myopia the length of the eyeball is too long which results in light coming to a focus in front of the retina, rather than on the retina. This allows the individual to see well when objects are close but not clearly when looking at objects at a distance.
Hyperopia or farsightedness:
Hyperopia is when the eyeball is shorter than normal and can result in near objects being blurry. However, people experience hyperopia differently. Sometimes distant objects are clear while other times people may experience overall blurred vision near and far or no problems at all. In children particularly, the lens may accommodate for the error allowing for clear vision but may cause fatigue and sometimes crossed eyes or strabismus. Hyperopia causes eyestrain or fatigue especially when looking at near objects for a period of time. Often people with 20/20 vision may still need glasses at their desk to relax their eyes and improve concentration.
Astigmatism is usually the result of an irregularly shaped cornea (although it can sometimes also be due to a misshapen lens). The cornea, which is normally round, is more football-shaped in an eye with astigmatism, resulting in multiple focus points either in front of the retina or behind it (or both). People with astigmatism usually have blurred or distorted vision to some degree at all distances, near and far.
Presbyopia is an age-related condition which usually begins to appear sometime after 40. As the eye begins to age, the lens stiffens and can no longer focus clearly on objects that are close.
It's important to note that presbyopia is often confused with hyperopia, as both cause problems focusing at near distances. However, high hyperopia can also cause blur at far distances as well, especially in dim lighting, and depth perception problems can result in motor vehicle accidents. In these instances people with hyperopia could use glasses at any distance.
If you are having trouble seeing, it is important to have an eye exam to determine the cause of the problem and to effectively correct your vision. Even if your vision is fine, you should schedule a routine eye exam on a regular basis to ensure that your eyes are healthy and that any potential problems are caught early.