What is normal vision?
In order to better understand how refractive errors affect our vision, it is important to understand how normal vision happens. For people 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.
From the cornea, the light passes through the pupil. The amount of light passing through is regulated by the iris, the colored part of the eye.
From there, the light then hits the lens, the transparent structure inside the eye that focuses light rays onto the retina.
Next, it passes through the vitreous humor. This is 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. This is the light-sensitive nerve layer that lines the back of the eye, where the image is inverted.
The optic nerve is then responsible for sending this information to the brain. The brain interprets the impulses it receives into images.
What are refractive errors?
Refractive errors happen when the shape of the eye prevents light from focusing directly on the retina. The following are the most common refractive errors. These errors affect vision and may need corrective lenses or surgery for correction or improvement:
Astigmatism is a condition in which an abnormal curvature of the cornea can cause two focal points to fall in two different locations. This makes objects up close and at a distance appear blurry. Astigmatism 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.
Commonly known as farsightedness, hyperopia is the most common refractive error in which an image of a distant object becomes focused behind the retina. This happens 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. It may cause headaches and/or eye strain.
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.
Commonly known as nearsightedness, myopia is the opposite of hyperopia. It is a condition in which an image of a distant object becomes focused in front the retina. This happens 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.
Another type of farsightedness, presbyopia is caused when the center of the eye lens hardens. This makes it unable to accommodate near vision. This condition eventually affects almost everyone, beginning as early as the age of 35. It even affects those with myopia. Eyeglasses or contact lenses may be prescribed to correct or improve the condition.
December 15, 2017
Visual impairment in adults: Refractive disorders and presbyopia, UpToDate
Fraser, Marianne, MSN, RN,Haupert, Christopher L., MD