What is Visual Acuity and How Does it Work?
Visual acuity refers to the measurement of your central visual field and describes how sharp and clear your vision is. A visual acuity test is performed by an optometrist in an eye examination and is represented as a fraction such as 6/6.
Keep reading to find out more about visual acuity and the testing process.
What is visual acuity (VA)?
Visual acuity is a measurement of the sharpness and clarity of your eye sight. Many people would have had a visual acuity test as it is performed during an eye test. A person with normal visual acuity would have 6/6 vision, which is a term you are likely familiar with.
Visual acuity measures your central vision only and can be tested for both distance and near vision.
Is visual acuity important?
Visual acuity is an important part of determining the clarity of your vision but it is only one aspect of a comprehensive eye exam. While a visual acuity test can measure how well you see at a distance, there are other factors that indicate whether you have ‘good’ vision.
Other aspects of ‘good’ vision include:
- Peripheral vision - The area of sight that is outside the central visual field (the sides of your vision).
- Depth perception - The ability to judge how far one object is away in relation to another object, and also from the viewer. Involves perceiving the world in three dimensions.
- Colour perception - Your ability to distinguish between frequencies of light to see different hues.
What is 20/20 vision (normal visual acuity)?
20/20 vision is a measure of normal visual acuity using letters on an eye chart. If you have 20/20 vision, it means you can see objects clearly from 20 feet away. This measurement is referred to as 6/6 vision in Australia as 20 feet converts to 6 metres.
In a visual acuity test, if you have 6/6 vision, you will be able to read letters that are a height of 10 millimetres at 6 metres away.
If you have 6/6 normal vision, it means you are likely to have healthy eyes.
What is 6/12 vision?
6/12, means your vision isn’t as clear and sharp as someone with 6/6 vision. Essentially, what you can see clearly at 6 metres , a person with 6/6 vision only needs to be 12 metres away to see.
What about 6/3 vision?
In terms of visual acuity, a person with 6/3 vision has better eyesight than someone with 6/6. People often believe that 6/6 is perfect vision but that is not true. You could have 6/3 vision, meaning you can see clearly at 6 metres what a person with normal vision would need to be at a distance of 3 metres to see.
Visual acuity testing
A visual acuity test is a very common eye test that is performed by an optometrist in a comprehensive eye exam. Therefore, you may have had this test if you have been asked to read letters off a chart at a given distance.
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How is visual acuity tested?
Visual acuity is tested using a Snellen eye chart set at a testing distance of 6 metres away and is used to measure the sharpness and clarity of your vision. It is performed by an optometrist or an ophthalmologist.
The test will begin with you sitting across from the Snellen chart set to 6 metres away. Your optometrist will get you to cover one eye and read letters that get progressively smaller until you are unable to read the letters clearly. This will be repeated with the other eye covered and then with both eyes.
The further down the chart you can see, the better your visual acuity is. Whatever the lowest line is you can read is expressed as a fraction and can be used to describe your visual acuity.
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What is a Snellen Chart?
A Snellen chart is a vision chart that is used in most facilities to test visual acuity. It contains rows of letters that decrease in size as you descend. It is meant to be read at a distance of 6 metres.
The testing process: What to expect
A visual acuity test is a non-invasive procedure that will usually take 5 minutes. Your optometrist will encourage you to relax and blink regularly while you are reading the letters.
There is no preparation needed for a visual acuity test and you should feel no discomfort.
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Visual acuity test interpretation
Once you have finished the test, your results will be expressed as a ratio, such as 6/6 or 6/12. The first number refers to the distance at which you were tested and is shown in metres. The second number represents the line that contains the smallest letter you could read. This is known as the Snellen fraction.
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Visual acuity testing in children
A visual acuity test for children is different to those performed on adults. For school age children, they can be tested using the Snellen letter chart but for younger children, they will be tested with a modified version consisting of a small selection of letters or symbols that they can identify.
Do I need my visual acuity tested?
Yes! Visual acuity is an important part of your eye health and helps to determine if you need vision correction. A visual acuity test will be performed as part of a complete eye exam.
We recommend that you have a regular eye test once every 2 years or as directed by your optometrist to take care of your eye health. If you are over 65, it is recommended you have an eye exam annually.
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