We have all heard of 20/20 vision as the benchmark of perfect vision. But what does it actually mean and how do these numbers break down?
Visual acuity is defined as clarity and sharpness of vision and the ability to discern shapes and details of things you can see.
When an optometrist measures your visual acuity in a comprehensive eye exam, your visual acuity score is expressed as a fraction, such as 20/20. But what does that number represent? Let’s break it down.
The numerator, or first number 20 in the fraction 20/20, represents the distance between a person and a standard eye chart. When an optometrist performs visual acuity testing, 20 feet (6 meters) is always the standard viewing distance and this number does not change.
The denominator, or second number in 20/20, has to do with the size of the letter you are viewing. If you can clearly view a set of eye chart letters at 20 feet, which are deemed by an optometrist as what should normally be seen at that distance, then you have 20/20 vision.
However, if you can see smaller letters more clearly or you’re unable to differentiate larger numbers at a distance of 20 feet, then you do not have 20/20 vision.
Your vision may be better or worse than 20/20 and is therefore expressed as different fractional numbers.
Let’s start with the benchmark of 20/20 (normal vision).
Remember, the top number or numerator does not change – the standard viewing distance remains stable. However, the denominator, or bottom number, is variable, and changes depending on how well you can see a certain row on the eye chart.
The larger the bottom number is, the worse the vision. So if 20/20 is considered normal vision, 20/40 is slightly worse, 20/80 is worse still and so on.
However, people can see better than 20/20. This means they are able to see smaller numbers clearly on a standard eye chart. Many people have 20/15 vision indicating excellent vision.
So how does an optometrist determine this number? The two most popular visual acuity tests are the Snellen and Random E tests:
If you have ever gotten your eyes checked, you will recognize the Snellen Eye Chart. It is a large chart consisting of letters of different sizes arranged in a series of rows and columns.
During the test, you will stand 20 feet from the eye chart. (If the chart is not physically 20 feet away, the distance is manipulated by mirrors for accuracy). You will then cover one eye at a time and read the letters aloud. This allows the optometrist to see how small of letters you can clearly distinguish.
In the Random E chart test, you will try to identify which way the letter E is facing – up, down, left, or right. This test is helpful for patients who cannot read or young patients who don’t yet know the alphabet, although it is used by some optometrists in addition to Snellen Eye Chart testing to test visual acuity.
Vision is more than just seeing clearly. While it is important to know your clarity and sharpness of vision, visual acuity tests are just one part of a comprehensive eye exam. Your optometrist will also test:
Your optometrist will thoroughly assess the health of your eyes. Preventative care is important, and serious eye conditions such as glaucoma, macular degeneration, and cataracts must be diagnosed and managed as soon as possible.
If it is determined that you have worse than 20/20 vision, you may suffer from a refractive error. This means your eye cannot focus images directly on the retina due to atypical eye shape. Your optometrist will conduct refraction tests that will determine what prescription is needed to correct your vision. Then provide and fit you with glasses or contact lenses to correct the error.
In addition to visual acuity, eye health, and refraction, your optometrist will perform tests that measure the following:
All of which contribute to your overall eye health and visual ability.