Your Vision: Understanding Visual Acuity
When it comes to understanding your vision, a lot of terms get batted around. One of these terms is “visual acuity”. If you are interested in Laser Eye Surgery treatment, the chances are you will hear this a lot. So, it’s important to understand what is actually being discussed when we talk about your visual acuity. Let us talk you through it…
In short, visual acuity simply refers to the standard measure of a person’s ability to see clearly. For example, when your optometrist measures your visual acuity, they are testing your central vision – that is, your ability to distinguish objects and details from a certain distance.
Another common term we hear when we discuss sight is “20/20 vision”. People commonly refer to this to mean they have “good” vision. And they would be right. Having 20/20 vision means that you can see clearly from 20 feet what a person with normal vision can see at 20 feet. Essentially, it is the average standard of vision.
But it is possible to have above-average vision – that is, better than 20/20.
Measuring Visual Acuity
The most common visual test is one that most people are familiar with – the Snellen Chart (pictured above). This chart is placed at a standard distance (20 feet, or 6 metres). The patient is then asked to read the letters on the chart. The letters on the chart get smaller line-by-line from the top to the bottom.
Towards the bottom of the chart is the ‘20/20’ line – this line features the smallest letters that a person with normal visual acuity is able to read clearly at a distance of 20 feet. Therefore, being able to read this line means you have 20/20 vision or “normal” visual acuity.
If you have a lower standard of vision, you will not be able to clearly distinguish the letters on the 20/20 line. As such, you would receive a different visual acuity measurement. For example, three lines above the 20/20 line is the 20/40 line. The letters on this line are twice as large as those on the 20/20 line. If you cannot read beyond this line, this indicates that you can see at 20 feet what a person with normal visual acuity can see at a distance of 40 feet. To put it simply, you have 20/40 vision.
However, there are also lines beyond the 20/20 line. Some patients with stronger vision may be able to read letters on the 20/16 line. You may have guessed by now that this means they can read at a distance of 20 feet, which people with normal visual acuity would need to approach at a distance of 16 feet.
Measuring Visual Acuity with Visual Aids
Importantly, visual acuity tests don’t only measure a person’s uncorrected vision (without the aid of glasses or contact lenses). They also measure their best-corrected vision – the best vision they are able to achieve with visual aids.
This is important because, while some people may not even be able to read the largest letter ‘E’ on the first line of the eye chart without glasses, they may be able to read up to the 20/20 – and even the 20/16 – line when wearing glasses or contact lenses.
In fact, when visual acuity is mentioned, this usually refers to the patient’s best-corrected vision. The legal driving standard in the UK, for example, is 20/40; however, this accounts for the use of glasses or other visual aids. Therefore, if a person can see at 20/40 or better with glasses, they are legally allowed to drive, regardless of their visual acuity without glasses.
The key aim of Laser Eye Surgery is to get a patient’s vision up to the same visual acuity without glasses or contact lenses as their best-corrected visual acuity (your vision with glasses or contact lenses) before surgery. In some cases, patients can achieve even better vision than this.
Laser Eye Surgery can successfully treat a wide range of prescriptions, including for hyperopia (long-sightedness), myopia (short-sightedness), astigmatism, and presbyopia.