normal human eye works to focus or “Refract” light emitted
from objects onto the retina. The light rays first pass through the
clear front windshield of the eye called the CORNEA. The light then
passes through the PUPIL, which is an opening created by the IRIS or
colored part of the eye. Next, the light passes through the LENS and
finally arrives at the RETINA. When these light rays are clearly
on the retina the result is a clear visual image.
Vision is typically measured by Snellen Visual Acuity. The 20/20 description of vision is based on a Snellen Visual Acuity chart that you see in the eye doctor’s office.
Through the years, eye doctors have concluded that a “normal” eye should be able to see a certain amount of detail at a given distance. Typically this distance is 20 feet.
So if a patient’s eye can see the 20/20 line on a Snellen chart that means the eye sees at 20 feet what a “normal” eye should see at 20 feet.
However a 20/40 eye sees at 20 feet what a “normal” eye would see at 40 feet. In other words the 20/40 eye must be closer to an object to see it versus a “normal” eye. Likewise a 20/200 eye would have to be 20 feet away from an object to see it clearly whereas a “normal” eye could see it from 200 feet away!
On the other hand a person with exceptionally good vision might see 20/15. This eye could see detail at 20 feet but the normal eye would have to be only 15 feet away to appreciate the same detail.
Why doesn’t everybody have 20/20 vision? There are multiple reasons. Some of these causes are not correctable such as trauma, amblyopia (lazy eye) and many other diseases such as macular degeneration or diabetes.
There are also correctable causes of decreased vision and the most common of these are called Refractive Errors. The term “refractive” refers to the ability of the eye to properly focus light onto the retina.
As noted above, when light is accurately focused onto the retina clear – often 20/20 vision results. However millions of people are unable to focus clearly without the aid of glasses or contact lenses. These individuals have “refractive errors”. The three main refractive errors are:
· Myopia or nearsightedness – the eye sees better near than at a distance. This is because light is bent too much and focuses in front of the retina.
· Hyperopia or farsightedness – the eye sees better at distance than at near – but typically is not good at either. This is not to be confused with Presbyopia. Hyperopia occurs because light is not bent enough and theoretically, focuses behind the retina.
· Astigmatism – an irregular bending of light rays that results in blurring of vision at all distances.
Presbyopia is not a true refractive error. Presbyopia is the loss of the ability to focus on near objects. This occurs as a consequence of aging often in patients who have had normal vision all their lives. The key is that presbyopic patients had the ability at one time to focus on near objects but slowly lost this ability with age. See CK.
At the Harbin Clinic Eye Center, we offer several options to treat refractive errors (myopia, hyperopia, and astigmatism) that can reduce our patients’ need for corrective eyewear.