The minus sign (−) on an eye prescription means you are nearsighted, and the plus sign (+) means you are farsighted. These signs appear before a number measured in diopters, which tells a lens maker how much the lens needs to bend light to bring your vision into focus. The concept is straightforward once you see how it maps to what your eyes actually do, but a full prescription includes several other values that can look like alphabet soup if nobody explains them.
What the Sphere Number Tells You
The most prominent number on your prescription is labeled “SPH” for sphere. It describes the overall optical power your eye needs to see clearly, and its sign tells you the direction of the problem. A minus value means your eyeball focuses light in front of the retina instead of directly on it. Distant objects look blurry because the image forms too early, before it reaches the light-sensitive tissue at the back of your eye. A plus value means the opposite: light hasn’t converged enough by the time it hits the retina, so the focal point lands behind it. Close-up tasks like reading become the main struggle.
Your sphere value is shaped by the physical dimensions of your eye. The overall refractive state depends on the relationship between axial length (how long the eye is from front to back), the curvature of the cornea, and the depth of the space behind the cornea.1PubMed Central. Heritability analysis of spherical equivalent, axial length, corneal curvature, and anterior chamber depth in the Beaver Dam Eye Study An eyeball that grows slightly too long front-to-back pushes the focal point forward, producing a minus prescription. One that is shorter than average leaves the focal point behind the retina, producing a plus prescription.
A minus lens is thinner in the center and thicker at the edges. It spreads incoming light rays apart slightly so they converge farther back, right on the retina. A plus lens does the reverse: it’s thicker in the center, gathering light rays inward so they converge sooner. The number after the sign tells you the strength. A prescription of −2.00 means you need two diopters of diverging power; +3.50 means three and a half diopters of converging power. Larger numbers mean a stronger correction and, generally, blurrier uncorrected vision.
The Cylinder and Axis
Most prescriptions include a second pair of numbers labeled “CYL” (cylinder) and “AXIS.” These describe astigmatism, a condition where your cornea or lens is shaped more like a football than a basketball. Because the curvature isn’t uniform, light bends differently depending on the angle it enters, creating a second focal point. The result is blurred or distorted vision at all distances, not just near or far.
The cylinder value also carries a plus or minus sign, but here the sign doesn’t tell you whether you’re nearsighted or farsighted. It reflects which of two mathematically equivalent conventions your eye care provider used when writing the prescription. Optometrists in many countries default to minus-cylinder notation, while ophthalmologists sometimes use plus-cylinder notation. The two formats describe the same correction and can be converted back and forth with a simple formula your optician handles routinely. Research on patient comfort has found that people tend to accept minus-cylinder prescriptions more readily, which is one reason the minus convention dominates in clinical practice.2Semantic Scholar. Comparative study between plus and minus cylinder prescription in astigmatism
The axis number, written in degrees from 1 to 180, tells the lab which angle to orient the cylindrical correction. Without the axis, the cylinder value is meaningless, because the lens has to be rotated to match the specific angle of your cornea’s irregularity. If you’ve ever noticed that one eye’s axis is 90 and the other is 175, that’s perfectly normal. Each eye has its own shape.
ADD, PD, and Other Abbreviations
If you’re over about 40, your prescription probably has a value labeled “ADD” (short for addition). This is the extra plus power built into the lower part of a bifocal or progressive lens for reading. It’s always a plus number, usually between +0.75 and +3.00, and it’s the same for both eyes in most cases. The ADD compensates for presbyopia, the gradual stiffening of the lens inside your eye that makes it harder to shift focus from far to near. Presbyopia is not nearsightedness or farsightedness; it’s an age-related loss of focusing flexibility that happens to virtually everyone.
You may also see “PD,” which stands for pupillary distance, the measurement in millimeters between the centers of your pupils. This number ensures the optical center of each lens lines up with where you actually look through the glasses. It doesn’t affect your prescription’s power, but getting it wrong can cause headaches and eye strain, especially in stronger prescriptions. Some prescriptions split PD into two numbers (one per eye measured from the bridge of the nose), which is more precise for people whose faces aren’t perfectly symmetrical.
Other abbreviations you might encounter include OD (right eye, from the Latin oculus dexter) and OS (left eye, oculus sinister), or sometimes just R and L. “OU” means both eyes together. “Prism” and “Base” appear only if your eyes don’t align well with each other, and they direct the lab to shift the image position to reduce double vision.
How Severity Is Graded
Eye care professionals group prescriptions into rough severity bands. For myopia, up to about −3.00 is considered mild, −3.00 to −6.00 is moderate, and anything beyond −6.00 is high myopia. For hyperopia, the bands are roughly the same: low is up to about +2.00, moderate is +2.00 to +5.00, and high is above +5.00. These cutoffs aren’t carved in stone, but they matter because the risk of certain eye complications rises as the numbers climb.
Astigmatism has its own scale. A cylinder of less than 1.00 diopter is mild and often goes unnoticed. Between 1.00 and 2.00 is moderate, and above 2.00 or 3.00 is considered significant. Many people have a small amount of astigmatism alongside their sphere correction, so seeing both numbers on your prescription is extremely common.
One thing worth knowing: the severity of your prescription doesn’t always predict how much it bothers you. Someone with −1.50 who spends all day driving may feel more impaired than someone with −4.00 who works at a desk. Context matters as much as the number.
Why Your Contact Lens Prescription Looks Different
If you wear both glasses and contacts, you’ve probably noticed the numbers don’t match. That’s because glasses sit about 12 millimeters in front of your eye, while a contact lens sits directly on the cornea. The closer a corrective lens is to your eye, the less optical power it needs to achieve the same focus. For mild prescriptions (roughly −4.00 or weaker), the difference is small enough that it’s sometimes ignored. For stronger prescriptions, the gap becomes meaningful: a person who wears −8.00 glasses might need only −7.25 or so in contacts.
The conversion applies in the other direction too. A plus-powered contact lens will typically be slightly stronger than the corresponding glasses lens, because the lens is closer and needs a bit more converging power to compensate. Your eye doctor does this math for you, which is why you’ll get a separate contact lens prescription rather than just reusing the glasses one.
Contact prescriptions also include a base curve and diameter that match the shape and size of your cornea. These don’t appear on a glasses prescription at all because they’re irrelevant to a lens sitting in a frame.
Getting the Correction Right
You might assume that a prescription is either correct or it isn’t, but accuracy falls on a spectrum, and small mismatches can affect more than sharpness. Research has shown that the accuracy of spectacle correction influences how well the focusing and convergence systems of the eyes work together in nearsighted people.3Journal of Ophthalmology. Effect of Different Spectacle Correction Status on Binocular Vision Function in Myopia: A Prospective Study Wearing glasses that are too strong, too weak, or out of date doesn’t just make things blurry; it can strain the internal muscles that coordinate your two eyes, leading to headaches, fatigue, and difficulty switching focus between near and far objects.
This is especially relevant for children. A child whose prescription changes rapidly can end up wearing lenses that no longer match, and the resulting strain may affect reading performance and attention in school. Adults tend to tolerate slight mismatches better, but even a half-diopter error in a strong prescription can be noticeable during prolonged screen work or nighttime driving.
When a High Prescription Raises Health Concerns
At the mild end, nearsightedness and farsightedness are optical inconveniences solved with a pair of lenses. At the extreme end, they can signal structural differences in the eye that carry real health risks.
High myopia, generally anything beyond −6.00, means the eyeball is substantially elongated. That extra length stretches the retina and the tissue layer behind it called the choroid. Studies have found that choroidal thinning in highly myopic eyes is linked to axial elongation and to the severity of damage at the macula, the part of the retina responsible for central vision.4PubMed Central. Choroidal thinning in myopia is associated with axial elongation and severity of myopic maculopathy People with high myopia face a higher risk of retinal detachment, myopic macular degeneration, and glaucoma compared to people with mild prescriptions. Regular dilated eye exams become important once your prescription crosses into that territory, because catching structural changes early can preserve vision.
High hyperopia has its own set of concerns. Very farsighted eyes tend to be structurally short, which crowds the internal anatomy. In extreme cases this is associated with narrow drainage angles in the front of the eye, raising the risk of angle-closure glaucoma. A genetic study of a family with very short eyes found hyperopia ranging from about +7.00 to +13.00 diopters, with more than half of the affected members developing angle-closure glaucoma or dangerously narrow angles.5American Journal of Human Genetics. Autosomal Dominant Nanophthalmos (NNO1) with High Hyperopia and Angle-Closure Glaucoma Maps to Chromosome 11 That same research noted that children with very high plus prescriptions who are not diagnosed early can develop amblyopia, sometimes called lazy eye, which can become permanent if the brain never learns to process a clear image from that eye.
None of this means a moderate prescription is dangerous. The structural risks ramp up gradually and become clinically significant mostly at the high end. But it does mean that a prescription isn’t just a shopping list for lenses; it’s also a rough indicator of eye anatomy that your doctor tracks over time.
How Common These Conditions Are Around the World
Nearsightedness and farsightedness are not evenly distributed across the globe. A systematic review and meta-analysis of refractive error prevalence found wide regional variation: myopia prevalence ranged from roughly 5% in South-East Asia to about 18% in the Western Pacific region, while hyperopia ranged from about 2% in South-East Asia to about 14% in the Americas. Astigmatism prevalence ranged from about 10% to 27% depending on the region.6PubMed Central. Global and regional estimates of prevalence of refractive errors: Systematic review and meta-analysis These numbers reflect estimated pooled prevalence across many studies, and they shift depending on the age groups and methods included.
Myopia in particular has been climbing. Projections based on data from 2000 onward suggest that the high-income countries of Asia-Pacific started with significantly higher myopia prevalence than any other region, and that East Asia, Southeast Asia, and high-income North America are expected to narrow the gap by 2050 as myopia rates rise almost everywhere.7Ophthalmology. Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050 The causes behind this trend are still debated, but increased time spent on close-up tasks and less time outdoors during childhood are the leading hypotheses.
Hyperopia, on the other hand, doesn’t get as much attention in the headlines, partly because mild farsightedness is common in young children and usually corrects itself as the eye grows. It becomes clinically relevant again in middle age, when the loss of focusing flexibility from presbyopia makes even a small plus prescription symptomatic.
How Refractive Surgery Changes the Numbers
Procedures like LASIK take the concept behind your prescription and apply it permanently to the shape of your cornea. For a nearsighted person, the laser removes tissue from the center of the cornea, flattening it. This increases the corneal radius and reduces the eye’s total optical power, effectively pushing the focal point back onto the retina.8PubMed Central. Effect of Flat Cornea on Visual Outcome after LASIK For a farsighted person, the laser removes tissue from the periphery of the cornea, steepening the center to add converging power.
Your pre-surgery prescription determines how much tissue needs to be reshaped, and there are limits. Very high minus prescriptions require removing more corneal tissue, and at some point the remaining cornea becomes too thin for the procedure to be safe. Most surgeons set an upper limit somewhere around −8.00 to −12.00 depending on corneal thickness, though the exact cutoff varies by patient. Similarly, correcting high plus prescriptions with a laser is less predictable than correcting minus ones, because steepening the cornea has different biomechanical consequences than flattening it.
After surgery, your prescription ideally reads 0.00 for sphere, meaning no additional correction is needed. In practice, many people end up with a small residual prescription of ±0.25 or ±0.50, which is close enough to sharp vision that glasses aren’t necessary for most activities. The key point is that surgery doesn’t change the underlying length of your eyeball; it just reshapes the front surface so light bends differently. The structural risks associated with a long eye in high myopia remain even after the prescription on paper reads zero, which is why post-LASIK patients with formerly high prescriptions still need regular retinal check-ups.
Prescriptions in Children Versus Adults
A child’s prescription often looks different from an adult’s for reasons beyond just severity. Children have very flexible focusing muscles, which can mask farsightedness during a standard eye exam. That’s why pediatric eye exams frequently involve dilating drops that temporarily relax those muscles, revealing the true underlying prescription. A six-year-old might appear to see fine at the exam chart but actually have a significant plus prescription hidden behind muscular effort. Left uncorrected, that hidden farsightedness can lead to eye strain, crossed eyes, or amblyopia.
Nearsightedness in children tends to increase through the school years, typically stabilizing in the late teens or early twenties as the eyeball stops growing. A child diagnosed at −1.00 at age eight may well reach −4.00 or −5.00 by age 18. The rate of progression varies enormously. Some children add just a fraction of a diopter per year; others add a full diopter or more. Researchers have explored various interventions to slow that progression, including specialty contact lenses and low-dose eye drops, and some show promise, though the field is still evolving.
Adults usually see more stability. A prescription that held steady through your twenties and thirties is unlikely to shift dramatically, though small changes of a quarter-diopter in either direction are normal and not a cause for alarm. The most common change after 40 isn’t a shift in the sphere number but the emergence of that ADD value for reading, as presbyopia sets in. If you’re nearsighted and notice you can read better without your glasses than with them, that’s presbyopia arriving and temporarily working in your favor at close range, though it doesn’t mean your distance vision has improved.