Back to Encyclopedia

Refractive Errors

What is normal vision?

Parts of the eye
Click to enlarge image

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:

  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
Click image to enlarge

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.

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.

Hyperopia
Click image to enlarge

Hyperopia
Click image to enlarge

Eyeglasses or contact lenses may help to correct or improve hyperopia by adjusting the focusing power to the retina. Corrective surgery may also help by changing the shape of the cornea to a more spherical, round shape instead of an oval shape.

Myopia

Example of normal vision
Normal vision

Example of myopia
Myopia

Commonly known as nearsightedness, myopia is a condition in which, opposite of hyperopia, an image of a distant object becomes focused in front the retina, either because the eyeball axis is too long, or because the refractive power of the eye is too strong. This condition makes distant objects appear out of focus and may cause headaches and/or eye strain.

Eyeglasses or contact lenses may help to correct or improve myopia by adjusting the focusing power to the retina. Corrective surgery may also help by changing the shape of the cornea to a more spherical, round shape instead of an oblong shape.

Myopia
Click image to enlarge

Myopia corrected
Click image to enlarge

Presbyopia

Presbyopia
Click image to enlarge

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.

Related Information

Diseases and Conditions

Corrective Eye Surgery for Refractive Errors
View All ( 1 )

Pediatric Diseases and Conditions

Refractive Errors in Children
View All ( 1 )