Introduction to Vision
Video: Nearsightedness, the most common refractive error, explained
Because of the way in which our eyes refract (bend) light, we are able to see the world around us. When a person is experiencing difficulty with their eye sight, it is often due to refractive errors. Refractive errors can be farsightedness, nearsightedness, and astigmatism. When a person is experiencing blurred vision, they are often told they have refractive errors, which are imperfections of the eye‘s optical system and prevent the proper focusing of light.
Refractive errors are not dangerous to your health, and are easily treated. All 3 refractive errors are very common and, if you are diagnosed with one, are nothing to worry about. Treatments include glasses, contact lenses, or LASIK surgery.
The Process of Light As It Travels Through the Eye
While there are many properties of light in its traveling which we do not yet fully understand, we do have a general idea as to how light travels. Light rays can be reflected, deflected, absorbed or refracted, all depending upon the different substances which it encounters.
Reflections are just one of several ways that light rays can react to substances it encounters.
Light travels through many elements. Consider when it travels through water or the lens of a camera, the path of the light is bent or refracted There are certain parts of a human eye that have refractive properties similar to that of a camera lens or water, and that bend the light so that we have clear vision.
Light rays travel through various areas of the eye, such as the cornea (the front, curved, clear surface of the eye), and the eye’s natural lens. The eye’s internal fluids and tear film all have refractive abilities that occur when light travels through them.
How the Eye Sees
Vision is a process that occurs when light rays reflect off objects and travel through the eye’s optical system. The eye then refracts and focuses these light rays into a sharp focus.
When a person has good vision, the light rays focus on the retina. The retina is the tissue at the back of the eye. Just as a camera, the retina captures images through photoreceptors (light-sensitive cells). The eye’s optical nerve then transmits these images to the brain for interpretation.
Through constricting the pupil, the eye is able to control the amount of light that the retina receives, similar to adjusting the lens on a camera for the perfect picture. When a person is exposed to bright light, the pupil constricts; in dark conditions the pupil will widen.
Video: Farsightedness can be caused by an elongated eyeball, or a lens that is too flat.
Three eye features enable the ability to refract light:
- Length of the eye
- Cornea curvature
- Curvature of the lens
Eye Length: If the eye length is not optimum different things occur. If the eyeball is too long, nearsightedness occurs since the light is focused on a point in front of the retina. If the eyeball is too short, farsightedness occurs because the light is focused on a point behind the retina.
Cornea Curvature: A cornea in its ideal state is a perfect sphere. If it is not, light is not refracted properly. This causes astigmatism. Astigmatism occurs in both near- and farsighted persons.
Lens Curvature: Curvature of the eye's lens also can cause nearsightedness or farsightedness. If it’s too steeply curved compared to the eye length and cornea curvature, light will be refracted too strongly and end up in front of the retina. This means you will be nearsighted. The opposite is true also: if the lens is too flat, light won't get refracted enough and end up behind the retina, causing farsightedness.
Video: How vision is affected by astigmatism, which is caused by a misshapen cornea.
There are other types of optical issues as well. Higher-order aberrations occur based on how light rays are focused throughout the eye's optical system. These vision problems can result in abnormal contrast sensitivity. Wavefront analysis, a newer technology, can help identify this issue.
The Treatment and Detection of Refractive Errors
A refraction test is the method used by your eye doctor to determine the refractive error type and degree you are experiencing.
There are two ways the test can be conducted; automated with a computerized instrumentation, called an autorefractor, or manually with a phoropter. In many cases, the automated process is completed by office staff. Then your doctor will further identify your issues using the manual process.
There might be a case where the process identifies multiple refractive problems. You can have, for instance, nearsightedness and astigmatism.
If you want contact lenses, your eye exam will take a little longer, because you will need a contact lens fitting.
As a result of the refraction testing your doctor will provide your glasses prescription. If you are interested in contact lenses, the refraction does not give enough data to complete the prescription for this lens type. A contact lens fitting is also necessary.
The power of both the glasses and contact lenses need to be precise to assure that the light refraction is ideal to compensate for the errors that were identified.
Surgeries, like LASIK, work to correct vision issues. The shape of the cornea is altered which results in the light rays being bent so that they are coming in at a more appropriate focus point.
If you choose to correct your vision with glasses, we're excited to help you.
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