What Is 20/20 Vision?
Introduction
20/20 vision is a common term used to describe perfect vision. In this blog post we’ll explore 20/20 vision, how it is measured, and what it means for your eye health.
Visual Acuity
When we ask you to read letters on the chart on the wall, we are measuring visual acuity. This is the smallest shape that you can identify at a given distance.
The space between the photoreceptor nerves in your retina limits how sharp your vision is. Research has found that a “perfect” eye can detect the gap between two tiny spots which make an angle of 1/60th of 1 degree at the eye.
We use this to calculate the size of letter that someone with perfect vision could read at 20 feet. That letter size is 8.73mm
Why 20 feet?
We are measuring distance vision and do not want the lens in the eye to do any focussing (called accommodation,) we want it totally relaxed. This only occurs when looking at something 20 feet away or further.
20 feet is also handy as it is a good size for a consulting room. If the room is smaller then we can use a mirror so patient to mirror to test chart is 20 feet.
Why do we use feet?
We don’t!!! 20/20 is an American term. In the UK we use the metric measurement of 6 metres test chart distance. This means that instead of using the term 20/20 we say 6/6. Different ways of saying the same thing. I’ve stuck with feet in the text above as everybody is familiar with 20/20.
What do the bigger lines mean?
This is the part that some find difficult to get their heads around! 20/20 means that the eye can see at 20 feet what it should be able to see at 20 feet.
If it can only read the next line up, the 20/25 line, then they can only read a letter at 20 feet that a perfect eye would be able to read at 25 feet as it is 25% bigger.
20/30 – can only read at 20 feet a letter that a perfect eye would be able to read at 30 feet as it is 50% larger.
And so on.
What if I don’t have 20/20 vision?
If you don’t have 20/20 vision, don’t worry, you’re in the majority! Your optometrist may be able to prescribe glasses or contact lenses to improve your vision. Laser refractive surgery and orthokeratology are other options.
It is also possible that your vision is not up to scratch because of an eye condition such as cataracts. Your optometrist will discuss this with you.
Great, I’ve got 20/20, I’m okay, then?
It is important to remember that visual acuity is only one aspect of good vision. A comprehensive eye exam will evaluate other aspects of your vision, such as near focussing, peripheral vision, binocular vision and a health examination of your eyes to look for any disease or abnormality.
Conclusion
In conclusion, 20/20 vision is a term used to describe perfect vision. However, visual acuity is only one aspect of good vision. Regular eye exams are essential for maintaining healthy eyes and detecting early signs of eye disease.
Here is a more in depth exploration about visual acuity in Wikipedia
At Bellamy Eyecare’s Leicester and Irthlingborough opticians we pride ourselves on looking after people. If you need any advice on designer glasses, eye health, orthokeratology or anything else eye related please do get in touch.