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What Exactly is an Eye Chart?

If there’s one aspect of optometry that everyone recognizes, it’s the traditional eye chart, with its rows of big letters on top, which gradually become smaller the farther down you go. This chart is usually known as the Snellen chart.

Yet how much do you really know about this eye chart? Are all eye charts the same? How are these eye charts used? And when were they invented?

Here’s everything you need to know about eye charts and more!

What is an Eye Chart?

An eye chart is one of the tools your eye doctor uses to assess your eyesight. Based on how well you can see various letters on the chart, your optometrist will determine whether you have myopia (nearsightedness), hyperopia (farsightedness), presbyopia (age-related farsightedness) or astigmatism, and will measure the prescription that will give you the clearest, most comfortable vision.

Are All Eye Charts The Same?

There are a number of variations to the standard Snellen eye chart. The one an eye doctor uses depends on the personal needs and abilities of the patient. For example, eye doctors will use charts with pictures or patterns for younger children who may not have learned to read or identify letters and numbers.

There are also certain charts that specifically measure distance vision, while others are better for measuring near vision.

History of the Snellen Eye Chart

The Snellen eye chart was developed by Dutch eye doctor Hermann Snellen in the 1860s. Before this standardized eye chart was developed, each eye doctor had their own chart that they preferred to use.

Having so many different eye charts made it impossible to [standardize] the vision correction available to patients. Eyeglass makers didn’t receive the defined measurements they needed to accurately design, manufacture and measure the optical prescriptions their patients needed.

For the first time, the Snellen eye chart allowed a person to provide a standardized prescription from any eye care provider they chose to any eyeglass maker, and get the same optical lenses to accurately correct their vision.

How The Snellen Chart Is Used in Eye Exams

The standard Snellen chart displays 11 rows of capital letters, with the first row consisting of a single large letter. The farther down the chart you go, the smaller the letters become.

Your eye doctor will ask you to look through a phoropter – an instrument used to test individual lenses on each eye during an eye exam – and look at the Snellen chart placed 20 feet away. Your eye doctor will prescribe the lenses that provide you with the clearest and most comfortable vision.

In many offices, where 20 feet of space may not be available, you’ll be asked to view the chart through a mirror. This provides the same visual experience as if you were standing 20 feet away.

If you have 20/20 vision, it means you can see what an average person can see on an eye chart from a distance of 20 feet. On the other hand, if you have 20/40 vision, it means you can only see clearly from 20 feet away what a person with perfect vision can see clearly from 40 feet away.

If you have 20/200 vision, the legal definition of blindness, this means what a person with perfect vision can see from 200 feet away, you can see from 20 feet away.

Does 20/20 Visual Acuity Mean Perfect Vision?

No. While eye chart tests identify refractive errors, they can’t detect signs of visual skill deficiencies or diseases such as glaucoma, cataracts or macular degeneration. These are diagnosed using advanced equipment as part of a comprehensive eye exam with your local eye doctor. Early diagnosis and treatment of eye conditions are essential to ensuring long-term vision and eye health.

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Q&A With Your Local Optometrist

How do you keep your eyes healthy?

You only have one set of eyes – don’t take them for granted!

Make sure to implement the following habits for healthy eyes (and body). These include:

  • Eating a balanced diet rich in fiber, fruits and vegetables
  • Drinking plenty of water to hydrate your body and eyes
  • Not smoking, and avoiding 2nd-hand smoke
  • Wearing sunglasses to protect your eyes from ultraviolet (UV) rays
  • Maintaining normal BMI with regular exercise
  • Regular visits to your eye doctor as recommended

What health conditions can an eye exam detect?

A comprehensive eye exam can often detect certain underlying diseases that can threaten your sight and eye health, such as diabetes, high blood pressure, high cholesterol, [tumor}s, autoimmune conditions and thyroid disorders. This is why having your eyes checked regularly is key. The earlier the diagnosis and treatment, the better the outcome and the higher your quality of life.

Dry, Red Eyes?, Dr. Yamanaka – Los Alamitos Optometry, Los Alamitos, CA

Either:

The eyes don’t make enough tears, and the eye cannot be comfortably hydrated

OR

The eye produces tears which are flawed. They often lack one or more essential parts normally included in tears to allow them to properly coat and hydrate the eye.

Red Eyes

Red eyes are generally not as worrisome as dry eye syndrome and you shouldn’t worry too much about them. In most cases, red eyes are caused by allergens or foreign substances, which can cause your eyes to become irritated. Small blood vessels throughout your eyes then become inflamed and enlarged, becoming much more visible and turning your eyes a reddish hue. This is where the term “red eyes” comes from.

The leading defense against either of these forms of dry eye syndrome is a type of specialty eye drops called “artificial tears.” These special eye drops combat the uncomfortable symptoms of dry eye syndrome by imitating real tears as closely as possible. In order to do this in the best way possible, there are many different formulations of artificial tears. Each formulation addresses a different underlying cause of your dry eye. Some help to to address the issues of dry eye syndrome in which tears are lacking in quantity, and others will add one or more building blocks to your tears to help them better perform their intended function., Dr. Yamanaka, Los Alamitos eye doctor.

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A not of caution, however: The great number of conditions that can cause dry eyes makes it very hard to know exactly what the cause may be sometimes. If your red eyes are accompanied by a high fever, headache and/or a great deal of eye pain, go to the doctor immediately. This note aside, red eyes are almost always minor and should not cause anxiety unless accompanied by these additional symptoms.

Many times, red eyes will heal up and return to normal without any medical attention at all.

Even with a brief but comprehensive explanation such as this, it can sometimes be hard to tell the difference between these two similar looking conditions.

Please don’t forget to make an appointment to see our Eye Care Specialist

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