Eyesight is measured primarily through a visual acuity test, where you read letters on a standardized chart from 20 feet away and your results are recorded as a fraction like 20/20. But that fraction only captures sharpness at a distance. A complete picture of your vision involves several different tests, each measuring a different aspect of how your eyes work.
What 20/20 Actually Means
The Snellen chart is the familiar wall chart with rows of letters that get progressively smaller. You stand 20 feet (6 meters) from it, cover one eye, and read the smallest line you can. Your result is written as a fraction: the top number is your distance from the chart (always 20), and the bottom number is the distance at which someone with normal vision could read that same line. So 20/20 means you see at 20 feet what a normally-sighted person sees at 20 feet. If your result is 20/100, you need to be 20 feet away to see what someone with normal vision can see from 100 feet.
A common point of confusion: 20/20 doesn’t mean perfect vision. It means normal sharpness for distance. You can have 20/20 acuity and still have problems with contrast, peripheral vision, depth perception, or close-up reading.
How an Eye Exam Measures Your Prescription
When you visit an optometrist or ophthalmologist, visual acuity is just the starting point. The exam typically involves two stages of measuring your refractive error, which is the optical imperfection that makes things blurry.
First, an autorefractor gives a quick, objective estimate. You look into a machine that shines light into your eye and measures how it bounces off the back. Within seconds, it calculates a starting-point prescription for nearsightedness, farsightedness, and astigmatism. These machines are precise enough to be used globally as the first step in determining prescriptions, and they’re particularly useful for young children or anyone who has difficulty communicating what they see.
Then comes subjective refraction, the part where the doctor flips lenses in front of your eyes and asks “which is better, one or two?” This is done through a phoropter, the mask-like device loaded with dozens of lenses. The autorefractor result is dialed in as the starting point, and the doctor fine-tunes from there based on your responses. This combination of machine measurement and your personal feedback produces the final prescription.
Reading Your Prescription
Your prescription uses diopters to describe how much correction your eyes need. The sphere (SPH) value corrects nearsightedness (a negative number like -2.00) or farsightedness (a positive number like +1.50). The cylinder (CYL) value corrects astigmatism. A rough way to translate diopters into the 20/20 scale: a sphere of -2.00 corresponds to roughly 20/100 uncorrected vision, meaning without glasses you’d need to be at 20 feet to see what others see at 100.
The key distinction is that diopters measure what your eye needs to see clearly, while the 20/20 fraction measures how clearly you actually see. Most people, once they’re wearing the right prescription, achieve corrected acuity around 20/20.
Near Vision Testing
Distance acuity doesn’t tell you how well you read a book or a phone screen. Near vision is tested separately using a Jaeger chart, a small card with blocks of text in decreasing sizes. You hold it exactly 14 inches from your eyes and read the smallest text you can. Results are recorded as a “J” number: J1 is the smallest print (normal near vision), with higher numbers indicating poorer close-up sight. This test is especially relevant after age 40, when the lens inside your eye gradually stiffens and loses its ability to focus up close.
Contrast Sensitivity
Standard eye charts use black letters on a white background, which is about as high-contrast as it gets. In real life, you often need to see things with much subtler contrast: a gray curb against gray pavement, a face in dim lighting, road markings in fog. Contrast sensitivity testing measures how well you detect objects that barely stand out from their background.
The Pelli-Robson chart looks like a standard letter chart, but instead of letters getting smaller, they get progressively lighter against a white background. You read until the letters fade into the background. Results are scored on a log scale where 2.0 is normal and anything below 1.0 indicates a visual disability. Reduced contrast sensitivity can show up even when your standard acuity is a perfect 20/20, which is why it catches problems that the Snellen chart misses entirely.
Peripheral Vision Testing
A visual field test measures how far up, down, left, and right you can see while looking straight ahead, and how sensitive your vision is across that entire area. This matters for detecting conditions like glaucoma, which silently erodes peripheral vision long before you notice it.
The simplest version is a confrontation test: your doctor holds their hands out to the sides and moves them inward while you stare straight ahead, signaling when you see movement. It’s quick but imprecise. For a more detailed map, automated perimetry uses a bowl-shaped machine that flashes tiny spots of light at different locations and brightness levels while you press a button each time you see one. The machine then prints a map showing any blind spots or areas of reduced sensitivity. Common versions include the Humphrey and Octopus perimetry systems, which are standard tools for monitoring glaucoma and neurological conditions affecting vision.
How Astigmatism Is Measured
Astigmatism happens when your cornea (the clear front surface of your eye) is shaped more like a football than a basketball, creating two different curves that focus light at two different points. Beyond the autorefractor, two specialized tools map corneal shape in detail.
Keratometry measures the curvature of the central cornea along its steepest and flattest points. Corneal topography goes further, creating a color-coded map of the entire corneal surface. Both methods produce results in diopters of astigmatism. Research comparing the two instruments shows they agree closely enough on the amount of astigmatism to be used interchangeably, though they can disagree on the exact angle of the irregularity, which matters for fitting contact lenses or planning surgery.
Testing Vision in Young Children
Children under three can’t read letters, so measuring their eyesight requires different tools. Lea Symbols use four simple shapes (a house, an apple, a square, and a circle) that toddlers as young as two can name or match. The test works the same way as a standard chart: shapes get smaller, and the child identifies the smallest ones they can see, typically from 3 meters away.
Lea Symbols have been shown to get better cooperation from three-year-olds than other pediatric tests, with 92% of children completing the test compared to 85% for letter-based alternatives. For children under about 30 months, monocular testing (covering one eye at a time) has very low cooperation rates, around 8%, so clinicians often rely on other techniques like observing how the child tracks objects or checking how light reflects off each eye to spot alignment problems.
At-Home Vision Tests
Several web-based tools now let you check your visual acuity at home using your computer or phone screen. Research comparing one such tool against in-office testing found that 77% of home results fell within clinically acceptable accuracy, and the average difference between home and clinic measurements was small enough to be statistically insignificant. That’s encouraging, but the remaining 23% of results were outside an acceptable range.
The main factors affecting accuracy aren’t the technology itself but behavior and environment: screen brightness, testing distance, lighting in the room, and how carefully you follow instructions. The study also noted a participation bias toward younger, more tech-comfortable users. If you’re less familiar with digital devices, accuracy drops. Home tests can be a reasonable way to track changes between appointments, but they measure only basic sharpness. They can’t assess your prescription, check eye pressure, examine the retina, or test peripheral vision, all things a comprehensive exam covers.