What does 20/20 vision actually mean?
03 March 2020
We’ve all heard people mention 20/20 vision, whether it’s your optician, someone boasting about their excellent eyes, or a laser eye surgery provider talking about their services for the year 2020. But how many of us really know what 20/20 vision is, how it came about, and what it actually means in terms of our day to day eyesight?
First, it’s important to note that the “20” refers to 20 feet, which is the distance from which the optician checks your visual acuity. This is based on American practice, while in the UK we tend to go by 6 metres. If you ever hear 20/20 vision being referred to as 6/6 vision, it means the same thing. Your visual acuity is the clarity of your eyesight, measured by how accurately you can identify numbers and letters on an eye chart.
These measurements will always be taken from the same distance while you’re sitting still, and the eye chart will always be in high contrast (e.g. black on white). This means that your official, recorded acuity is somewhat limited by these constraints. It provides no indication of how well you can see colour, moving objects, or differentiate between different levels of brightness. It does, however, show how sharp your vision is and it’s essential for opticians trying to prescribe you glasses. It’s also a crucial measurement to know, in order for laser eye surgery providers to treat you correctly!
The Snellen chart
Everyone has seen a Snellen chart, even if they don’t know the official name for it.
Developed in 1862 by Dutch ophthalmologist, Herman Snellen, the chart is used across the world to measure people’s visual acuity and provide essential information for glasses and contact lens prescriptions. When you have an eye test with a Snellen chart, you will typically be sat 20 feet away from it. The ‘E’ on the first line represents 20/200 vision. If you can read this, but nothing smaller on the chart, you are considered legally blind and see from 20 feet what a person with normal visual acuity can read from 200 feet.
Each line on the chart corresponds to better visual acuity as you go further down. The fifth line represents 20/40 vision which is what you need to reach the driving standard. 99% of Optimax patients achieve driving standard vision following treatment with us. The bottom line on the chart represents 20/10 visual acuity, so it’s certainly possible to have better vision than 20/20 – it’s just very uncommon. Less than 1% of the general population have 20/10 vision, however most people with young, healthy eyes can identify at least some of the letters on the 20/15 line.
Essentially, somebody with 20/10 vision has visual acuity twice as good as the average person, while somebody with 20/40 vision has visual acuity twice as bad as average. 20/20 is what you need to comfortably read the numbers in a telephone book. 20/80 vision means that you can read news headlines with ease, while 20/200 vision (legally blind) means that you can’t see much more than STOP sign letters. On the other end of the spectrum, eagles have between 20/5 and 20/4 vision, hence the common eagle eye saying, while the world record for best visual acuity in a human is an incredible 20/8!
Understanding your prescription
Typically, if you have no prescription at all and don’t require glasses or contact lenses for any tasks, you have 20/20 vision. Prescriptions are measured in dioptres, which is a unit of measurement for refractive errors. Negative numbers indicate that you’re short-sighted, while positive numbers show that you’re long-sighted. Somebody with a -1.00D prescription can see objects at 1 metre clearly, while someone with a -2.00D prescription can only see them clearly at half a metre. The higher your prescription number is, the more trouble you will have seeing clearly without visual aids.
You find out how to read your prescription on our blog and see which treatments at Optimax might be most effective for correcting your vision. Alternatively, you can enter your prescription details into our suitability calculator and check your eligibility that way.
More than 20/20
It’s important to remember that your vision is made up of more than just your ability to read the Snellen chart accurately. You also need to keep an eye (no pun intended!) on your peripheral vision, eye coordination, depth perception, focusing ability and colour vision. These are all crucial elements that contribute towards good eyesight, not just the visual acuity readings that come with a 20/20 measurement.
Make sure you book in for eye tests regularly to assess your eye health. Further to checking your vision, eye tests can also pick up more serious problems like glaucoma, macular degeneration and even brain tumours. Your sight is perhaps the most important of all your senses, so it’s vital that you take every step possible to protect it.
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