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 San Jose 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 San Jose 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 408-293-3730 or visit us in person at Japantown Optometry, 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.