Refractive Errors

Normal Vision

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:

• light enters the eye through the cornea, the clear, dome-shaped surface that covers the front of the eye.
• the light passes through the pupil. The amount of light passing through is regulated by the iris, or the colored part of your eye.
• the light then hits the lens, the transparent structure inside the eye that focuses light rays onto the retina.
• next, it passes through the virtuous humor, the clear, jelly-like substance that fills the center of the eye and helps to keep the eye round in shape.
• finally, it reaches the retina, the light-sensitive nerve layer that lines the back of the eye, where the image appears inverted.
• the optic nerve is then responsible for interpreting the impulses it receives into images.

Refractive errors

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 - 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.
• Hyperopia - known as farsightedness, it 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.
• Myopia - 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.  
• Presbyopia- A type of farsightedness, presbyopia is caused when the center of the eye lens hardens making it unable to accommodate near vision. This condition affects almost everyone over the age of 50 - even those with myopia.