20/20 vision is a term that is used regularly to describe ‘good’ eyesight or good visual acuity, but what does it really mean? How is it measured using a Snellen test? Let’s take a look.
What is considered good visual acuity?
Visual acuity is the ability to see objects clearly. It’s a technical term used to describe the result of an eye test that determines your prescription and whether you need glasses or contact lenses to see clearly. Visual acuity will vary from person to person depending on their age, eye shape and general eye health.
How do you assess visual acuity?
Many of the tests we perform in your eye appointment will determine how well you can see, whether that is direct vision, peripheral vision or colour recognition. We’ll also perform tests to check the overall health of your eye and look for factors which could impact your eyesight.
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.
What is the Snellen test?
Developed by Dr Hermann Snellen in the 1860s, the Snellen letter chart features capital letters in rows of descending sizes.
Each row 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 most Specsavers and NHS hospitals, optometrists use LogMAR visual acuity charts which are considered even more accurate than Snellen tests.
What does 20/20 vision mean in an eye test?
This term indicates that an individual has a visual acuity of 20/20. It’s a term only used in North America - in Europe, 6/6 notation is used. These figures are based on Snellen letter charts which are used in the standard eye test.
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 20/20 vision is the benchmark for what optometrists consider ‘normal’ vision. It means that they can see what an average person can see on a Snellen chart when they are standing 20 feet away.
So, if you have 20/40 (or 6/12) vision then you’ll just be able to see something from a distance of 20 feet (six metres) what a person with perfect eyesight would be able to see from 40 feet (12 metres).
If you can read only the big letter at the top, it means you can see at 20 feet what people with good eyesight can see at 200 feet. Conversely, your eyesight can be better than 20/20 – if your eyes are 20/15 you can see the line of letters at 20 feet, that someone with normal eyesight can only see when they are 15 feet away.
If you can’t see the 20/20 line without glasses, you might be short-sighted. 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 20/20 vision.
What is a Random E test?
Of course, not everyone can read the Snellen chart, 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 might use the Tumbling E or Random E test, also created by Dr Snellen. 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 chart with the chart at a set distance and the size of the Es getting smaller on each line.
What is the Jaeger eye chart?
The Jaeger eye chart or card 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 J1 to J10 – J1 is the near vision equivalent to 20/20 on the Snellen chart; J7 would be about the same size as newspaper print, with J10 being 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).