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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.

For more information, give us a call at or visit us in person at , today!

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, tumors, 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.

What to expect from your comprehensive eye exam

Comprehensive eye exams are an important part of keeping your vision clear and your eyes healthy. This is true for children and adults alike, as eye and vision conditions that go undetected and untreated can cause severe difficulties in learning or working, and result in vision loss or even blindness. But what can you expect from a comprehensive eye exam, and how can it help you?

Eye Exams Step-By-Step

There are a number of steps to a standard comprehensive eye exam. These may include:

Measuring visual acuity. This helps your eye doctor see if you need glasses or contact lenses to help you see better. You’ll look at an eye chart and identify different letters printed on a chart positioned about 20 feet away. The letters get smaller as you move down the chart to measure how well you see.

Eye pressure measurements. Your eye doctor may put numbing drops into your eye. They will then measure your inner eye pressure by either using a machine that blows a puff of air into your eye, or performing a test in which they press gently on the eye to observe the amount of pressure it takes to temporarily flatten the cornea.

Eye health evaluation. Your eye doctor may put dilation drops in your eye to make it easier to examine the inside. After these drops take effect, your eye doctor may perform a number of checks and tests using lights and imaging to evaluate the front and inside of each eye.

Other Types of Eye Exams

Your doctor might also use several other tests to check your vision and the appearance and function of all parts of your eyes. These include:

  • Eye muscle tests to evaluate the muscles that control eye movement.
  • Retinal examination and retinal photography to assess the health of the back of your eye.
  • Slit-lamp exam with or without fluorescein dye to examine the eyelids, lashes, cornea, iris, lens and fluid chamber between your cornea and iris.
  • Color vision testing
  • Visual field testing to check the health of your peripheral vision.

At Eye Mechanix, we put your family’s needs first. Talk to us about how we can help you maintain healthy vision. Call us today: 773-857-1260 or book an appointment online to see one of our Lincoln Park eye doctors.

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Q&A

Do I need to bring anything to my eye exam?

You should bring any prescription eyeglasses or contacts you currently use, so that your eye doctor can see if they are still the right prescription for you. You may also want to bring a pair of sunglasses, in case your eye doctor chooses to dilate your eyes for the exam. If your eyes are dilated, you may also want a friend of family member to drive you home.

Is there a difference between a vision screening and a comprehensive eye exam?

Yes. Vision screenings are designed to check 20/20 visual acuity, and will only catch the most obvious cases of conditions such as strabismus or amblyopia. By contrast, a comprehensive eye exam is designed to check your overall vision health and catch a range of potentially sight-threatening diseases that a vision screening would easily miss.