A refraction is the part of your eye exam that determines whether you need glasses or contact lenses, and if so, exactly what prescription will give you the clearest vision. It’s the test where you look through a series of lenses and tell the doctor which one looks sharper. If you’ve ever heard “which is better, one or two?” during an eye appointment, that was your refraction.
What the Test Actually Measures
Your eye works by bending light through the cornea and lens so it lands precisely on the retina at the back of the eye. When the shape of your eyeball, cornea, or lens is slightly off, light focuses in the wrong spot and your vision blurs. A refraction measures exactly how far off that focus is and calculates the lens power needed to correct it.
The result is your prescription: a set of numbers describing the type and degree of correction each eye needs. If your uncorrected vision is already 20/20, your refractive error is zero and you don’t need a prescription at all.
How the Test Works Step by Step
Most refractions have two parts: an automated measurement and a manual fine-tuning based on your feedback.
The automated portion usually comes first. You look into a machine called an autorefractor, which shines a light into your eye and calculates an estimate of your prescription in seconds. This gives the doctor a starting point, but it isn’t precise enough on its own to write your final prescription. Research comparing the two methods has consistently found that autorefractors are useful for a preliminary reading but aren’t a substitute for the manual step.
For the manual portion, you sit behind a phoropter, the mask-like device mounted on a stand with dozens of lenses inside it. You focus on a letter chart about 20 feet away while the doctor flips different lenses in front of each eye, one at a time, asking you to compare. The doctor adjusts the strength of the lens (the sphere), then checks for astigmatism by testing curved lenses at different angles (the cylinder and axis). Finally, they do a last round of fine-tuning to make sure the sphere power is dialed in.
Your input matters more than you might expect. Because vision depends on how your brain processes the image, not just the optics of the eye itself, your subjective responses are considered the gold standard for determining a prescription in cooperative patients. The machine reading is a guide; your feedback is the final answer. One interesting finding: people tend to concentrate harder and accommodate (unconsciously flex the lens inside the eye) more during the manual test than when staring into an autorefractor, which can slightly shift results. That’s one reason eye doctors rely on specific techniques like fogging, where they deliberately blur your vision with a strong lens and then gradually reduce it, to relax your eye’s focusing muscles and get a more accurate measurement.
Types of Refractive Errors It Detects
A refraction identifies four main vision problems:
- Nearsightedness (myopia): Your eyeball is too long from front to back, so light focuses in front of the retina. Distant objects look blurry while close-up vision stays clear.
- Farsightedness (hyperopia): Your eyeball is too short, so light focuses behind the retina. Near objects are harder to see clearly, though younger people can often compensate by flexing their internal lens.
- Astigmatism: Your cornea or lens has an uneven curve, bending light unevenly so vision blurs at all distances.
- Presbyopia: The lens inside your eye stiffens with age and loses its ability to shift focus between near and far. This is why many people start needing reading glasses in their 40s.
You can have more than one of these at the same time. It’s common, for example, to be nearsighted with some astigmatism, or to develop presbyopia on top of an existing prescription.
Reading Your Prescription
The numbers on your prescription come directly from the refraction. Each eye gets its own line, and there are three key values:
- Sphere (SPH): The main lens power, measured in diopters. A minus sign means nearsightedness; a plus sign means farsightedness. A prescription of -3.00 D, for instance, means three diopters of nearsighted correction.
- Cylinder (CYL): The amount of astigmatism correction needed. If this box is empty, you don’t have significant astigmatism.
- Axis: A number between 1 and 180 that indicates the angle of your astigmatism on the cornea. It tells the lab which direction to orient the corrective curve in your lens.
Together, these three numbers give a lens maker everything needed to grind lenses matched to your eyes.
Why Children Sometimes Need Eye Drops
Children and young adults can unconsciously flex the lens inside the eye during the test, which masks their true prescription. To prevent this, doctors sometimes use dilating drops (called cycloplegic drops) that temporarily relax the focusing muscle. Guidelines recommend that all children with suspected refractive errors undergo at least one refraction with these drops when first identified.
In older children and teens (roughly 11 to 18), a refraction without drops often gives satisfactory results. But drops are still used when a child has crossed eyes, poor depth perception, a family history of conditions like amblyopia (lazy eye), or when the initial refraction doesn’t produce reliable numbers. The drops blur near vision for several hours, so it helps to know this ahead of time if your child’s appointment includes them.
How Often You Need One
The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at age 40 for adults with no symptoms or risk factors. After that, the schedule depends on your age:
- Ages 40 to 54: every 2 to 4 years
- Ages 55 to 64: every 1 to 3 years
- Age 65 and older: every 1 to 2 years
People at higher risk need more frequent exams. African Americans, who face higher glaucoma risk, are advised to start periodic exams before age 40. People with type 1 diabetes should see an ophthalmologist five years after diagnosis and yearly after that. Those with type 2 diabetes should have an exam at the time of diagnosis and yearly from then on.
If you already wear glasses or contacts, you’re likely getting a refraction at every visit since your prescription can shift over time. Presbyopia in particular progresses steadily through your 40s and 50s, so your reading prescription will change even if your distance vision stays stable.
What the Test Feels Like
A refraction is painless and non-invasive. There’s no bright light shining directly into your eye (that’s a different part of the exam), no puff of air, and nothing touching your eye. The only challenge is the decision fatigue of choosing between lenses that sometimes look nearly identical. If two options seem the same, saying so is a perfectly valid answer, and the doctor will simply move on. The whole process typically takes a few minutes per eye, though it varies depending on the complexity of your prescription.
One thing worth knowing: your refraction can fluctuate slightly depending on how tired you are, how dry your eyes are, or even the time of day. If you’ve been staring at a screen for hours before your appointment, your focusing muscles may be fatigued, which can temporarily shift results. None of this is cause for concern, but scheduling your exam when you’re reasonably rested gives the most reliable numbers.