It’s a term often used to describe perfect vision. But what does 20/20 vision actually mean? Can you have better than 20/20 vision? And, what other factors impact the quality of your eyesight? If you are looking to improve your vision, it’s important to know how your vision and eyesight is tested so you are aware of what improvement treatment will bring. And, what these improvements will mean to your quality of life.
The simple definition of 20/20 vision:
“20/20 vision represents normal or ‘best corrected’ vision acuity”.
This simple definition immediately raises a few more questions.
What is Visual Acuity?
Visual acuity relates to sharpness of vision and vision clarity. Visual acuity It is a combination of three main functions within the eye:
- How well the cornea and lens focus light on the back of the retina.
- How well the retina converts light into images.
- How well the optic nerve and brain transmit and interpret the image sent from the eye.
What conditions can affect visual acuity?
Refractive errors are where the light passing through the cornea and lens at the front of the eye do not focus correctly on the retina at the back of the eye. Refractive errors are what are commonly overcome with glasses, contact lenses or laser eye surgery.
Corneal diseases often cause a cloudiness or swelling at the front of the cornea which can affect the way light passes through the eye and how it is focused on the retina. Different corneal diseases require different types of intervention, monitoring and in some cases corneal transplant.
How is 20/20 vision measured?
The measurement 20/20 is known as a Snellen fraction. This is named after Herman Snellen, the Dutch inventor of the common eye test and measurement system.
The first number in the Snellen fraction represents the distance (20 feet) that the person stands from the eye chart. 20/20 vision is also referred to as 6/6 vision which is simply the metric equivalent distance of 6 metres that the person completes the test from.
The second number of the Snellen fraction relates to the last line of letters on the eye chart a person can read. Based on the chart, the ‘20/20’ line contains letters that are expected to be read with someone with normal vision. Which is why 20/20 vision relates to normal (not perfect) vision.
What is better or worse than 20/20 vision?
People who are young and healthy often score better than the 20/20 line on a standard eye test. For example, someone with 20/16 vision would have a better visual acuity. Conversely, vision that is worse than 20/20 is where the second number is higher than 20. For example, a person with 20/40 vision means that they need to be twice as close to an object to see it as clearly as someone with normal (20/20) vision.
What other factors can affect vision?
Visual acuity is just one aspect of overall vision quality and eye health. Things the Snellen eye test don’t measure are colour contrast, tracking of moving objects, colour vision, depth perception and focusing speed. Also, many other eye conditions like Glaucoma and Cataracts can affect vision. So, it’s true that someone with 20/20 vision could still have trouble or imperfect vision due to other conditions.
If you feel your vision is not clear or if the sharpness of your vision has changed, it’s a good idea to have an eye test. An ophthalmologist can test your vision and recommend the best treatment to ensure your vision is optimised to see the world as clear as possible.