Health Library

Health Library

Refractive Errors

What is normal vision?

In order to better understand how refractive errors affect our vision, it is important to understand how normal vision occurs. For persons with normal vision, the following sequence takes place:

Illustration demonstrating normal vision

  1. Light enters the eye through the cornea, the clear, dome-shaped surface that covers the front of the eye.

  2. From the cornea, the light passes through the pupil. The amount of light passing through is regulated by the iris, or the colored part of your eye.

  3. From there, the light then hits the lens, the transparent structure inside the eye that focuses light rays onto the retina.

  4. Next, it passes through the vitreous humor, the clear, jelly-like substance that fills the center of the eye and helps to keep the eye round in shape.

  5. Finally, it reaches the retina, the light-sensitive nerve layer that lines the back of the eye, where the image appears inverted.

  6. The optic nerve is then responsible for interpreting the impulses it receives into images.

What are refractive errors?

Refractive errors occur when the shape of the eye prevents light from focusing directly on the retina. The following are the most common refractive errors, all of which affect vision and may require corrective lenses or surgery for correction or improvement:

  • Astigmatism. Astigmatism is a condition in which an abnormal curvature of the cornea can cause two focal points to fall in two different locations, making objects up close and at a distance appear blurry. Astigmatisms may cause eye strain and may be combined with nearsightedness or farsightedness. Eyeglasses, contact lenses, or corrective surgery may help to correct or improve the condition.

Illustration demonstrating astigmatism

  • Hyperopia. Commonly known as farsightedness, hyperopia is the most common refractive error in which an image of a distant object becomes focused behind the retina, either because the eyeball axis is too short, or because the refractive power of the eye is too weak. This condition makes close objects appear out of focus and may cause headaches and/or eye strain.

Illustration demonstrating hyperopia

Illustration demonstrating hyperopia corrected

  • Myopia

Simulation photograph: normal vision

Simulation photograph: myopia

Illustration demonstrating  myopia

Illustration demonstrating myopia corrected

  • Presbyopia. Another type of farsightedness, presbyopia is caused when the center of the eye lens hardens making it unable to accommodate near vision. This condition generally affects almost everyone over the age of 35, even those with myopia. Eyeglasses or contact lenses may be prescribed to correct or improve the condition.

Illustration demonstrating presbyopia

Refractive Errors - WellSpan Health

Online Medical Reviewer: Bogus, William J., OD, FAAO
Online Medical Reviewer: Sather, Rita, RN
© 2014 WellSpan Health. All Rights Reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.

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