The Snellen Chart: Understanding a Common Eye Test
The words “test” and “exam” are often enough to make people shudder. You might be transported back to beige classrooms and sports halls with the sound of the clock noting the passage of time with its tick-ticking as grumpy invigilators shuffle between row after row of miserable students… A chilling memory indeed.
Perhaps these connotations are enough to put us off tests, even as adults – even those that aren’t testing our intellect! If that’s the case, then putting the word “eye” in front of it isn’t likely to make much of a difference. When faced with the Snellen chart in a routine eye test, many of us every back to the competitiveness of our schooldays: Will I pass? Can I get all the way to the end?
What is the Snellen Chart?
The Snellen chart – or Snellen scale – is a visual acuity test used to measure how well you can see objects at a distance. It features capital letters in rows of gradually decreasing sizes which your eye doctor will ask you to read – until you can no longer distinguish them.
The Snellen chart is named after the Dutch ophthalmologist, Herman Snellen, who invented the chart in 1862. Today, the test remains the most common way for eye doctors to measure visual acuity.
How does the Snellen chart work?
As mentioned above, each row of letters on the Snellen chart differs in size. The very top row features only one very large letter – usually an E, H, or N. The next line features two smaller letters, the line after that has three even smaller letters, and so on, and so on.
Each of these lines represents a measurement of visual acuity. The first line is the 20/200 line: this means the reader can see at 20 feet what someone with “normal” visual acuity would be able to see from 200 feet.
The 8th line – which features 8 letters – is the 20/20 (or 6/6) line. People who can read this line are considered to have normal visual acuity. However, the chart goes beyond this – all the way to 20/10.
The traditional Snellen chart only used ten letters – C, D, E, F, L, N, O, P, T and Z. But modern charts add other letters into the mix – even though some are considered harder to distinguish than others such as P versus F; and G, Q’s and O’s.
Despite being the go-to test for visual acuity, the Snellen chart has faced some criticism over the years. Some point out the fact that the number of letters increases while the size decreases which introduces two variables. Moreover, it is technically possible to cheat on the test by memorising the sequence of the letters – but let’s face it, faking having good vision is a lose-lose situation!
Is it time to get tested?
Many of us have a natural aversion to tests of any description. And yet, eye tests are much more important than many of us give them credit for. In fact, the NHS recommends that most people get their eyes tested every two years.
Regular eye tests not only make sure your prescription is up to date, but they can also be invaluable at identifying the early signs of eye conditions and diseases – some of which can threaten your sight!
If you’d like to learn more about eye health or the treatments offered at the London Vision Clinic, get in touch with one of our friendly clinic coordinators today. Alternatively, Book a Consultation to kickstart your journey to improved vision.
