Eyesight is considered bad when your best-corrected visual acuity falls below 20/40, the point where everyday tasks like driving become difficult or restricted. That 20/40 threshold is where eye care professionals begin recommending vision rehabilitation, and it’s the minimum standard for an unrestricted driver’s license in nearly every U.S. state. But “bad” eyesight exists on a spectrum, from mildly blurry to legally blind, and the number on your eye chart is only part of the picture.
How the 20/20 Scale Works
The familiar eye chart in your doctor’s office is called a Snellen chart. The numbers it produces, like 20/40 or 20/200, compare your vision to a baseline. The first number is always 20, representing the 20 feet you stand from the chart. The second number tells you how far away a person with normal vision could stand and still read the same line you’re reading.
So if you have 20/40 vision, the letters you can barely make out at 20 feet are clear to someone with normal sight from 40 feet away. At 20/200, you need to be 20 feet from something that a person with normal vision could see from 200 feet. The higher that second number climbs, the worse your eyesight is.
Levels of Vision Impairment
There’s no single line where eyesight goes from “fine” to “bad,” but clinical guidelines break it into clear tiers. These all refer to your best-corrected vision, meaning the sharpest you can see with glasses or contacts, not your uncorrected vision.
- 20/20 to 20/40: Normal to near-normal range. You can drive without restriction and handle most visual tasks comfortably.
- Worse than 20/40 but better than 20/70: Mild visual impairment. Reading smaller text, seeing road signs at a distance, and driving at night may become noticeably harder. This is the threshold where the American Academy of Ophthalmology recommends discussing vision rehabilitation options.
- 20/70 to 20/200: Moderate visual impairment, classified by the World Health Organization as “low vision.” Daily activities like reading, recognizing faces across a room, and navigating unfamiliar places are significantly affected.
- 20/200 or worse: Legal blindness in the United States. At this level, even the largest letter on the eye chart isn’t readable with your best corrective lenses. This doesn’t necessarily mean total darkness. Many people classified as legally blind still have usable vision.
- Worse than 20/400: The WHO classifies this as blindness, where functional vision is extremely limited.
Your Prescription Numbers Mean Something Different
The 20/20 scale measures how sharp your vision is after correction. Your prescription, measured in diopters, tells you how much correction your eyes need to get there. A person with a strong prescription can still have 20/20 corrected vision, which is why both numbers matter when evaluating how “bad” your eyesight is.
For nearsightedness (trouble seeing far away), the American Academy of Ophthalmology breaks severity into three tiers: mild is less than 3 diopters, moderate is 3 to 6 diopters, and high myopia is anything above 6 diopters. High myopia isn’t just an inconvenience. It increases the long-term risk of retinal detachment, glaucoma, and other structural problems inside the eye, even if glasses or contacts bring your vision to 20/20 right now.
Farsightedness works similarly but in the opposite direction. And starting around age 40, virtually everyone begins losing the ability to focus on close objects, a condition called presbyopia. This is a normal aging process, not a disease. It typically starts requiring reading glasses around +1.00 diopters in your early 40s and progresses to +2.50 or higher by your mid-50s.
Sharpness Isn’t the Whole Story
You can score 20/20 on an eye chart and still feel like your vision is poor. That’s because the Snellen chart only tests one thing: your ability to read high-contrast black letters on a white background in good lighting. It doesn’t measure how well you see in dim light, how you perceive subtle differences between similar shades, or how wide your field of vision is.
Contrast sensitivity, your ability to distinguish objects from their background, plays a huge role in real-world vision quality. Poor contrast sensitivity makes it harder to see curbs, steps, faces in shadow, or cars on overcast days. Research published in PLOS One confirms that contrast sensitivity can be significantly impaired even in patients with normal visual acuity scores. People with inherited retinal conditions, for example, often report poor visual function despite stable scores on the standard eye chart.
This gap between chart results and lived experience is one reason eye specialists evaluate more than just acuity. Scotomas (blind spots in your central or peripheral vision), loss of contrast sensitivity, and visual field narrowing all independently affect how well you function, and any of them can qualify you for vision rehabilitation services.
Peripheral Vision Loss
A normal visual field spans about 160 to 170 degrees horizontally. You’re not consciously aware of everything in that range, but your brain uses it constantly for balance, spatial awareness, and detecting motion. When that field narrows, it creates what’s sometimes called tunnel vision.
Peripheral vision loss is taken so seriously that it has its own criteria for legal blindness, separate from acuity. If your visual field shrinks to 20 degrees or less, even with 20/20 central vision, you meet the U.S. definition of legal blindness. A field of 10 degrees or less meets the WHO threshold for blindness. Conditions like glaucoma and retinitis pigmentosa can narrow the visual field gradually, sometimes without any change in the sharpness of your central vision, which is why regular comprehensive eye exams matter even when you feel like you see fine.
The Driving Threshold
For many people, the most practical definition of “bad” eyesight is the point where it affects their ability to drive. All U.S. states require a vision test for licensure, and all but three set the minimum at 20/40 in the better eye with best correction. If your vision falls below that line, you’ll typically face restrictions (such as daytime-only driving) or may not qualify for a license at all, depending on your state.
Some states also test peripheral vision or have specific requirements for contrast sensitivity, though acuity remains the universal standard. If you’re borderline, your eye care provider can help determine whether updated glasses, contact lenses, or other interventions can bring you back above the cutoff. For many people with mild to moderate refractive errors, corrective lenses are all it takes to return to 20/40 or better. The concern is when best-corrected vision still falls short, which points to an underlying condition that lenses alone can’t fix.