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What Exactly is an Eye Chart?

Eye Chart Snellen

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

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 optometrist uses to assess your eyesight. Based on how well you can see various parts of 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 optometrist uses depends on the personal needs and abilities of the patient. For example, many optometrists 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 standardised eye chart was developed, each eye doctor had their own chart that they preferred to use.

This harmed the quality of vision correction available to patients, as glasses makers lacked the standard measurements needed to accurately design, measure, fit, and adapt glasses according to the patient's needs.

The Snellen eye chart allowed a person to carry over a standardised prescription from any eye care provider they chose to any glasses maker, and get the same results and glasses to accurately correct their vision.

How The Snellen Chart Is Used in Eye Tests

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 optometrist will ask you to take off any vision correction you may be wearing (glasses or contacts) and sit or stand 20 feet away from the chart. They will then ask you to read from the chart.

You’ll be asked to cover one eye and read out the smallest line of letters you can see with just that one eye. Then you’ll be asked to cover the other eye and do the same thing.

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.

So, 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 20/200 vision (the legal definition of blindness), it means you can only see clearly from 20 feet away what a person with normal vision can see clearly from 200 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 test with your local optometrist. Early diagnosis and treatment of eye conditions are essential to ensuring long-term vision and eye health.

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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 from ultraviolet (UV) rays

What health conditions can an eye test detect?

A comprehensive eye test can detect certain underlying diseases that can threaten your sight and eye health, such as diabetes, high blood pressure, high cholesterol, tumours, 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.