We all hear the term 20/20 vision thrown about to mean anyone who has healthy eyesight. But what does it mean? And how does it convert to the lens prescription your optometrist gives you, measured in diopters? By the end of this article you’ll be an expert on the definition and understanding of 20/20 eyesight.
What is 20/20 Vision?
20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see
clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.
Having 20/20 vision does not necessarily mean you have perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance. Other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and color vision, contribute to your overall visual ability.
Can You Have Better Than 20/20 Eyesight?
Yes, it's indeed possible to have sharper than 20/20 vision. In fact, most people with young, healthy eyes are capable of identifying at least some of the letters on the 20/15 line or even smaller letters on the Snellen chart.
This may be due in part to better printing methods available today vs. those in the 19th century when Snellen was determining the smallest letters a person with normal vision should be able to discern. So a case could be made that "normal" visual acuity today is an ability to identify letters that are a bit smaller than those on the 20/20 line of a traditional Snellen eye chart.
On the other hand, people are living longer today than they did in Snellen's era. Normal aging changes in the eye, such as early cataracts, could justify considering somewhat larger letters than those on the 20/20 line as being indicative of "normal" vision among adults in their 60s or older.
Some fighter pilots use eye exercises to attain 20/10 vision, while the world record belongs to a man naturally achieved 20/8 focus by training his eyes for better vision. Incidentally, eagles have natural vision of 20/2 – hence the expression eagle eye, which means an exceptional eye for detail.
What are Diopters?
If you are nearsighted (anything worse than about 20/50 vision), your optometrist will write you a prescription to improve your long distance vision. This describes, in a measure called diopters, the refractive power of your new glasses or contacts.
You will get a separate diopter value for each eye – often one eye is slightly weaker than the other. If the measurement has a minus (-) before it, you are nearsighted and have trouble seeing things in the distance. If there is a plus (+) before it, you are farsighted and have trouble seeing things up close.
The second column on the prescription is labelled “Cylinder” and refers to the degree of Astigmatism in diopters. For example, you might have 0.5 diopter Astigmatism.
The third column is labelled “Axis” and refers to the angle in which you have Astigmatism. This will be described in degrees. For example, 85°
Mild Astigmatism is about 0.5 diopter Medium Astigmatism is around 1.5 diopters. Severe Astigmatism is more than 2 diopters – this is also much fewer cases.
This column describe the power of the prism element that may be included in the perscription and is lables Prism. The last column indicate the angle or axis in which the prism element is incerted.
How to Convert Diopters to 20/20 Vision
While the 20/20 vision system measures how well you can see compared to a healthy benchmark, diopters measure the focusing power of your glasses.
There is a relationship between the two, shown in the table below. This gives you a very rough conversion between diopters and the 20/20 measurement – but bear in mind there is a lot of variation between individuals.
*Higher diopter values are harder to calculate because it depends on the person – some sources cite a whole range between -4.00 and -6.00 to reflect 20/400 vision. So these figures are an approximation. Note how the 20/20 measure doesn’t increase consistently, and may actually increase exponentially beyond -6.00 (severe myopia).
Where to Get Expert Advice
The first step to maximizing the clarity and comfort of your eyesight in all situations is to see a qualified optometrist or ophthalmologist for a comprehensive eye exam and vision evaluation.