Good eyesight (visual acuity) is often described as 20/20 vision and generally means the ability to see objects clearly. Most people are familiar with the Snellen test — but what actually is it and how does it measure visual acuity? Here, we take a closer look.
What is visual acuity?
Visual acuity is a measure of your ability to see objects clearly with glasses or contact lenses, if required. Visual acuity will vary from person to person depending on their age, eye shape and general eye health. A person with 20/20 visual acuity is the benchmark for what optometrists consider ‘normal’ vision or ‘normal’ visual acuity though the reality is that many patients will be able to achieve better than 20/20.
How do you check visual acuity?
The Snellen test, or letter chart, is the most commonly used piece of equipment to test visual acuity and is also the test most people think of when they think of eye tests. Other tests that could be performed during a routine eye test are a refraction (to determine your prescription) and an eye health check dilated eye exam or an automated refraction eye test.
What is the Snellen test?
Developed by Dr Hermann Snellen in the 1860s, the Snellen letter eye test chart features capital letters in rows of descending sizes and is used by opticians to test visual acuity. In a Snellen chart each row of letters represents the minimum size of letter that a person with normal vision would be able to see at six metres, nine metres and various intervals up to 60 metres. In the US the six-metre distance is calculated in feet, so people will view the chart from 20 feet – which is where we get the term 20/20 vision from.
Of course, not all clinics are big enough to place the chart six metres away from the person being tested, so digital screens are now used that can be calibrated depending on how far away the person is sitting from the chart.
In some Specsavers stores and NHS hospitals, optometrists use LogMAR visual acuity charts which are considered even more accurate than Snellen tests.
How do you do a Snellen test?
The Snellen letter eye test is a normal part of your routine eye appointment. Your optician will present the chart containing rows of letters of varying sizes, and ask you to read the letters aloud. They might also place lenses in front of your eyes to see if the chart becomes clearer for you.
What do the results of a Snellen Test mean?
In the Snellen eye test results, the term 20/20 is used to describe a ‘normal’ visual acuity score. Interestingly, it’s a term only used in North America. In Europe, 6/6 is used in its place as most visual acuity testing is done at 6 metres.
The first number refers to the distance at which the chart is viewed (20 feet or six metres) and the second number refers to the distance at which a person with ideal vision can see a letter clearly. A person with 6/6 visual acuity (20/20) is the benchmark for what optometrists consider ‘normal’ vision. It means that they can see what an average person can see on a Snellen eye test chart when they are standing 6 metres away.
If you can read only the big letter at the top of the chart, it means you can see at 6 metres what people with good eyesight can see at 60 metres.
On the other hand, your eyesight can be better than 6/6. – if your visual acuity is 6/5 you can see a line of letters from 6 metres , which someone with normal eyesight can only see when they are 5 metres away.
If you can’t see the 6/6 line without glasses, you might require spectacles. Your optometrist will place lenses in front of your eyes until the chart becomes clearer. This is one of the ways they will determine your prescription so that your new glasses or contact lenses will give you 6/6
It’s important to remember that only an optician can interpret your Snellen test results, so you should always book an eye test if you’ve noticed a change in your vision.
What are the different types of eye test charts?
A Snellen test chart is just one way to measure visual acuity — however they are not always suitable for everyone. Sometimes they are too young, or maybe they don’t use the Latin alphabet in their native language — for example Japanese or Arabic. In these cases, we have a number of other tests that our opticians can perform to measure your visual acuity. Some of the most used are the Random E or Tumbling E test.
What is the Random E test?
When the Snellen test chart isn’t appropriate to use, we might use the Tumbling E or Random E test instead — which was also created by Dr Snellen. For this, the person being tested will be asked to say or point to the direction the ‘prongs’ of the E are facing – up, down, left or right.
The results of this test will be calculated in the same way as the Snellen test with the chart at a set distance and the size of the Es getting smaller on each line.
What is a Near Vision reading chart?
A Near Vision reading chart is used to test your near vision. Held at a comfortable reading distance (about 40cm or 14 inches), it features a series of paragraphs that the person being tested can read out loud.
The Jaeger eye chart works on a similar principle to the Snellen chart in that the text used in each paragraph gets smaller and smaller. Your optometrist will ask you to read what you can at a comfortable distance. When you start to struggle with the font size, they may ask you to move the chart closer or further away to see if that helps. The font size is categorised from N4 to N24 and N6 is the near vision equivalent to 20/20 on the Snellen chart; N8 would be about the same size as newspaper print, and N24 is the largest print.
Much like the Snellen test, the optometrist will determine which, if any, lenses would help you to read the paragraphs more clearly. They will do this if they consider you to be long-sighted or have presbyopia (age-related changes to your vision).