You may not know what a Snellen chart is, or how to read an eyeglasses prescription, but if there’s one thing you’re sure of, it’s that you want 20/20 vision. After all, 20/20 is a “perfect” score, right? 

Actually, it’s more of an average score, says Brian Rashid, O.D., an optometrist at Buckeye State Optometry inside America’s Best Contacts & Eyeglasses in Columbus, Ohio. What it means, he says, is that if you were looking at an eye chart, you would have to stand 20 feet away from it to read the letters — exactly the same distance that the average person has to stand to read the same chart. 

And that’s where the nuances begin: If you have 20/15 vision, for example, you have better-than-average vision. That means you can read the eye chart from 20 feet away, but the average person needs to be 15 feet away — 5 feet closer to the chart.  

“The [smaller] the bottom number, the better the vision,” says Dr. Rashid. 

So far, so good. But what happens if the bottom number is bigger — say 20/40? In that case, it means your vision is weaker than the average person’s. But that’s not necessarily as bad as you might think. 

Read on to learn more about 20/40 vision, including what you can do to correct it, and when it’s a problem.    

What is 20/40 vision? 

Let’s go back to the Snellen chart — aka an eye chart. A person with 20/40 vision would be able to see the letters on the chart clearly from 20 feet away — but the average person can make out the chart from 40 feet away. 

Sounds problematic, but Dr. Rashid explains “there would be a little bit of blur, but it’s really not that bad.” 

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Can I drive if I have 20/40 vision? 

In most states, you need to have at least 20/40 vision or better to be able to drive without corrective lenses. (Check your state’s motor vehicle department for the rules that apply to you. Some states may allow you to drive if one eye is better than 20/40, even if the second eye is 20/40 or worse, for example.) 

Even if you’re allowed to drive without corrective lenses, Dr. Rashid says it’s important that you feel comfortable and confident in your ability to see the road and your surroundings while you’re behind the wheel.  

“For people who can’t pass their driver’s test because they have 20/40 vision or worse, I give them glasses or contacts and 99% of the time, they can pass, no problem,” says Dr. Rashid. He adds that he recommends most people with 20/40 vision have a pair of glasses for times when clear vision is crucial — like driving. 

“People with 20/40 vision could probably read some road signs, but they would have to be the large signs, like a highway sign or a speed limit sign, and they’d have to be pretty close to you,” says Dr. Rashid. “They wouldn’t be able to read a smaller, neighborhood road sign, though.” 

Is 20/40 vision considered blind? 

No. Legal blindness is defined as 20/200 vision corrected. This means that even with glasses or contacts, you have to be at 20 feet to see something that the average person can see at 200 feet.  

“When people say they’re legally blind without their glasses, that’s not a real thing,” says Dr. Rashid. “In order to be legally blind, it’s best corrected to 20/200.” 

What causes 20/40 vision? 

Many people who have 20/40 vision simply have a refractive error: They’re either nearsighted (can’t see distances) or farsighted (can’t see up close). The good news about refractive errors is that “these things can be fixed,” he says. 

But for the small percentage of people whose vision can’t be fixed with eyeglasses or contacts, they likely have an eye condition that has caused some vision loss, says Dr. Rashid. Diabetic retinopathy or cataracts, for example, can cause people to lose vision.  

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How do glasses and contact lenses fix 20/40 vision? 

If your 20/40 vision is being caused by a refractive error, it means the light isn’t entering your eye correctly, Dr. Rashid explains. Ideally, light will enter your eye through the cornea (the outer layer of the eye) and land directly on your retina (the thin layer of the back of the back of the eye).  

People who are nearsighted, though, have a steeply curved cornea, which causes the light to land in front of the retina, he says. People who are farsighted have a flatter cornea, which causes the light to land behind the retina. In both cases, this light-focusing error results in an inability to see from a distance of about 20 feet away. 

“Glasses and contact lenses refocus the light directly on the retina, making it perfectly clear,” says Dr. Rashid. 

Your eye doctor can determine your prescription by asking you to read an eye chart. First, you’ll cover one eye and read the smallest line of letters you can see, then you’ll repeat the process on the other side. They’ll also measure the curvature of your cornea. 

Press play to see how eye doctors check for refractive errors, and more, during an eye exam:  

Even people who are very nearsighted or farsighted will likely be able to have better than 20/40 vision with the right glasses or contacts.  

“We can grind pretty much any lens for glasses, and contacts are very technologically advanced now,” he says. “For glasses, the sky’s the limit. For contacts, the clouds are the limit.” 

Medically reviewed by Brian Rashid, O.D. 

See our sources: 
State vision rules for driving: PreventBlindness.org 
Definition of legal blindness: Social Security Administration