Eyeglasses 101

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By Linda | February 16, 2008

During his eye exam last week, the Optometrist told my son that his new prescription would correct his vision to better than 20/20. Today when we picked up his new glasses he asked me exactly what that meant. As with many of the questions my kids routinely ask me, I thought it better to research the answer a bit to avoid passing on incorrect or incomplete information. I think that this may be one of those questions that many parents get and don’t know quite how to answer concisely. I therefore give you “Eyeglasses 101″

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What is 20/20 vision?

In the United States, visual acuity (how clear and sharp our vision is) is generally measured using a Snellen Chart. This specially designed eye chart features a series of characters, with the largest at the top. As the person being tested reads down the chart, the letters gradually become smaller.

The Snellen fractions, 20/20, 20/30, etc., are measures of visual acuity. These numbers indicate how well a person can see a particular line on the Snellen Chart at a specified distance. The first number is the distance you stand from the chart (usually 20 feet). The second number is the distance from which people with “normal” eyesight can read that same line on the eye chart.

Another way to say 20/20 would be: Standing 20 feet from the chart, this person can see as well as a person with normal sight can see at 20 feet.

Another way to say 20/15 would be: Standing 20 feet from the chart, this person can see as well as a person with normal sight can see at 15 feet.

Another way to say 20/40 would be: Standing 20 feet from the chart, this person can see as well as a person with normal sight can see at 40 feet.

In other words, if the second number is smaller than the first, the person’s eyesight is better than normal. If the second number is larger than the first, the person’s eyesight is worse than normal.

This measure applies individually to each eye. So you can be 20/20 in one eye and 20/15 in the other.

It should also be noted that contrary to popular belief, 20/20 is not actually normal or average, let alone perfect, acuity. 20/20 was established as a benchmark or reference standard only, but has come to be popularly understood as “normal” vision. the average acuity in healthy adults is actually quite a bit better than 20/20. The maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10

A person with 20/200 vision or less in his or her best eye when wearing corrective lenses is considered legally blind

What do the numbers on my eyewear prescription mean?

Eyeglass prescriptions are measure in diopters. A diopter is a unit of measurement that indicates the amount of correction needed to change your vision to be as close to 20/20 as possible. The greater the diopter, the greater the error and hence the more correction required.

An eyeglass prescription is made up of 3 numbers.
Example : -4.75 -1.75 45

The first number is the correction required for near or farsighted vision. If it has a minus symbol (-) then it’s to correct nearsightedness. In the example above it means myopic (nearsighted) four and three quarters diopter. If it was a zero, the person requires no correction for near or farsighted vision.

The second number is for astigmatism (a defect in the curvature of the eye’s surface).

The third number is the angle of astigmatism, in this case 1.75 diopters of astigmatism at 45 degrees. So the first number shows the correction required for either near or farsighted vision and the second and third number relate to astigmatism. The higher the numbers the greater the correction required.

What’s the difference between an Opthalmologist, an Optometrist and an Optician?

People frequently confuse the three. Here are the definitions of each:

An Ophthalmologist diagnoses and treats eye diseases, prescribes medications, and performs surgery to improve or prevent the worsening of vision-related conditions. An Ophthalmologist will have the initials “M. D.” after his or her name.

An Optometrist diagnoses eye diseases and prescribes eyeglasses, contact lenses, vision therapy, and medications to treat eye diseases. An Optometrist will have the initials “O. D.” after his or her name.

An Optician fits eyeglasses and, in some states, contact lenses, analyzes and interprets prescriptions written by Ophthalmologists or Optometrists, and takes eye measurements to ensure that eyeglass prescriptions are correct and fit properly.

You might also see an Orthoptist who works under the direction of an ophthalmologist to evaluate and treat eye disorders with an emphasis on binocular vision (using both eyes to see) and eye movements; much of their work is with children.

To learn how animal eyesight stacks up against ours CLICK HERE.

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