In normal vision, light rays travel through the cornea and the lens of the eye and focus on the retina in the back of the eye. Both the curvature of the cornea as well as the size of the eyeball play a vital role in how images are focused on the retina. As light rays pass through the eye they are "refracted" or bent. Refractive disorders, or refractive errors, refer to distortions in the vision when the eye doesn't bend the light properly as it reaches the retina resulting in imperfect vision.
What does seeing 20/20 mean?
In the term 20/20 vision, the first number is referring to the distance between the person that is being tested and the eye chart (which is always 20 feet away). The second number represents the distance that the average person can see the eye chart. A person with 20/20 vision can read a specific "normal" size letter when it is 20 feet away.
A person that has 20/40 vision can only read letters that a person with 20/20 vision could read from 40 feet away, or would require the letters that are twice the size of the 20/20 letters on the eye chart.
Finally, someone that has 20/400 vision sees the chart the way a person that has 20/20 vision and is standing 400 feet from the chart.
There are four types of refractive errors:
- Myopia (nearsightedness)
- Hyperopia (farsightedness)
- Astigmatism (blurry vision)
- Presbyopia (age-related inability to focus up close)
How are refractive errors corrected?
Prescription eyeglasses and contact lenses can be effective tools to correct nearsightedness, farsightedness, and astigmatism. For patients interested in a more permanent alternative to eyeglasses or contact lenses, laser eye surgery may provide the solution they are looking for.
Laser vision correction surgery corrects vision by reshaping the cornea, the transparent layer that covers the outer surface of the eye, thereby changing its focusing power. You should discuss your refractive errors and your lifestyle with your ophthalmologist to decide which corrective surgery will be most effective for you. Learn more about LASIK and Laser Vision Correction options offered at New York Eye and Ear Infirmary of Mount Sinai.