First, let’s talk about the Eye Chart! If you’ve never seen an eye chart like the one pictured, please call an eye doctor immediately to get an appointment, then come back and read this article.
You’ll notice to the right of the eye chart, there are a series of numbers that look like fractions. We’ll start there. The format of those numbers is:
[How far away you can see this line] /
[How far away a “normal” person can see this line].
So for the 20/200 line, the “Big E”, a person with normal vision could legibly read text of that size from 200 feet away. If your vision is “20/200” then you can only see the “Big E” from 20 feet away!
But when you get your eye care prescription from the doctor, you don’t get a slip of paper that says 20/200, you get a decimal number like -3.00.
So how do the two compare?
The Eye Chart measures what is called ‘Visual Acuity’ or ‘How well can you see in real life’. But this is not the only measure that goes into your prescription. If you’ve been to the eye doctor, you’ll know they do many tests to help narrow down your ideal prescription, because of this there is no exact one-to-one comparison between the numbers on the eye chart and your final prescription. There are, however, some vague guidelines that apply to the average eye.
For instance, if your prescription is between -3.5 and -4, you can probably no longer see the “Big E” without correction. By the time you get up to a prescription of -7.0, what other people can see 20 feet away, you can probably only see 5-6 inches in front of your eyes.
And it’s a logarithmic curve! The graph shows a rough representation of how your visual acuity decreases with each prescription increase.
Lets look at it another way. The difference between 20 ft and 200 ft is approximately the difference between an Orca and TWO Blue Whales.
So far we’ve talked about myopia, or nearsightedness. If you are nearsighted, you can only see near you.
It works the opposite way too. 20/10 vision means what normal people can see from 10 feet away, you can see from 20 feet away. This would make you hyperopic, or far-sighted because you can see far. Many farsighted people, however, begin to lose the ability to see what is close to them, especially as their prescription gets stronger. So, they may be able to see two blue whales away, but they can’t read their book!
What are some fun-facts about visual acuity?
Well according to the World Health Organization (WHO), visual impairment is classified in the following buckets based on visual acuity and/or the visual field:
Normal: 20/10-20/25
Near Normal visual impairment: 20/30-20/60
Moderate visual impairment: 20/70-20/160
Severe visual impairment: 20/200-20/400, or 11-20 degrees on the visual field
Profound visual impairment: 20/500-20/1000 visual acuity, or 6- 10 degrees on visual field
Near-total visual impairment: Counting fingers, Hand motion, Light perception, or 5 degrees or less on visual field
Total visual impairment: No light perception
Visual field refers to how “wide” you can see and is largely reliant on your peripheral vision. It is possible for your central vision and peripheral vision to have different abilities and clarity, especially as you get older.
If your visual acuity is 20/40 or less, you are legally not allowed to drive without your visual correction aids (glasses or contacts).
The AOA classifies vision based on the corrective diopters:
Mild +/-0.25 to +/-2.00
Moderate +/-2.25 to +/-5.00
Severe +/-5.00 and up
If we put these two together, you get approximately the following:
Mild +/-0.25 to +/-2.00
Normal: 20/10-20/25
Near Normal visual impairment: 20/30-20/60
Moderate +/-2.25 to +/- 5.00
Moderate visual impairment: 20/70-20/160
Severe visual impairment: 20/200-20/400
Severe +/- 5.00
Profound visual impairment: 20/500-20/1000 visual acuity
Legally Blind
Near-total visual impairment: Counting fingers, Hand motion, Light perception,
Total visual impairment: No light perception
And that's the super basic of what your prescription means! Keep an eye out for our blog in two weeks about how to read your particular prescription to find out where you fall on this scale.
If you have Moderate visual impairment or above, and your main form of vision correction is Daily Contact Lenses, check out our DailyLens to tackle the hassle of your contact lens routine. No mess, no stress, just vision.