A vision prescription is a shorthand formula that tells a lens maker exactly how to bend light so it focuses correctly on your retina. Once you know what each abbreviation and number represents, the whole thing stops looking like a secret code. The key values are sphere (your basic correction), cylinder and axis (for astigmatism), and add power (for reading help as you age), with a few extra entries that appear on some prescriptions but not others.
OD, OS, and What “Sphere” Actually Means
The first thing you’ll notice is that your prescription is split into two rows. OD stands for “oculus dexter,” the Latin term for your right eye, and OS stands for “oculus sinister,” your left eye. Occasionally you’ll see OU, meaning both eyes together. Each row has its own set of numbers because your two eyes rarely need the same correction.
The most prominent number is the sphere value, sometimes labeled SPH. This is the main correction for how your eye focuses. A minus sign in front of the number means you’re nearsighted: you see close objects well but distant ones are blurry. A plus sign means you’re farsighted: distance vision may be reasonable, but close-up work is harder. The number itself is measured in diopters, which is simply the unit eye care professionals use for lens power. A prescription of −2.00, for example, indicates moderate nearsightedness, while +1.50 indicates mild farsightedness.
The physical reason behind these numbers comes down to the shape and length of your eyeball. Nearsighted eyes tend to be longer from front to back, so light focuses in front of the retina instead of on it. Farsighted eyes tend to be shorter. One study measuring eye dimensions found that myopic (nearsighted) eyes averaged about 25.2 mm in axial length, while hyperopic (farsighted) eyes averaged about 22.6 mm, a difference of roughly 2.5 mm that is enough to significantly shift where light lands inside the eye.1Journal of Vision. Myopic versus hyperopic eyes: axial length, corneal shape and optical aberrations The lens in your glasses or contacts compensates for that mismatch by redirecting light to hit your retina squarely.
The higher the sphere number, the stronger the correction you need. Someone with −0.50 might barely notice a difference without glasses, while someone at −6.00 would struggle to read a clock across the room. Don’t compare your number to someone else’s as a measure of eye “health,” though. A high sphere value just means your eye’s natural optics deviate further from the ideal shape; it says nothing about the health of the retina, optic nerve, or anything else inside.
Cylinder and Axis
If your prescription has numbers in the cylinder (CYL) and axis columns, you have some degree of astigmatism. This is extremely common, and mild amounts often go uncorrected. Astigmatism means that your cornea or lens is shaped more like a football than a basketball, so light bends differently along one meridian than another. The result is that images look slightly stretched or smeared in one direction.
The cylinder number, also measured in diopters, tells the lens maker how much extra correction is needed to compensate for that uneven curvature. It can be written in minus cylinder form (more common in the United States) or plus cylinder form (more common in the United Kingdom and parts of Europe). These are mathematically equivalent, just expressed differently, so don’t panic if a new provider’s prescription looks different from your old one. The key numbers haven’t changed; the notation has.
The axis is a number between 1 and 180 that describes the angle, in degrees, at which the cylindrical correction should be oriented. Think of it as telling the lab which direction to tilt the extra correction so it lines up with the way your cornea curves. An axis of 90 means the correction is oriented vertically; an axis of 180 means it runs horizontally. If your cylinder value is zero or the column is blank, you don’t have clinically significant astigmatism and the axis doesn’t apply.
One thing that confuses people is that their sphere and cylinder seem to change in opposite directions when they switch providers. This usually happens because one doctor wrote the prescription in minus cylinder form and the other in plus cylinder form. The total optical correction your eyes actually receive is the same. If you’re ever unsure, an optician can convert between the two formats in seconds.
The Add Power
Starting somewhere around your early to mid-forties, you may notice a new entry on your prescription labeled ADD. This stands for addition power and represents the extra magnification built into the lower portion of your lenses for reading and other close-up tasks. It’s always a plus number, typically ranging from +0.75 to +3.00 in most adults.
The reason you need it is a process called presbyopia. The natural lens inside your eye gradually stiffens over the decades, losing its ability to flex and change focus. Research on human crystalline lenses has confirmed that younger lenses undergo significant changes in focal length when stretched, whereas lenses from people older than about 60 show essentially no change at all.2PubMed. Presbyopia and the optical changes in the human crystalline lens with age This loss of flexibility is progressive and universal. It doesn’t matter whether you were previously nearsighted, farsighted, or had perfect vision; presbyopia happens to everyone.
The add value is the same for both eyes in most prescriptions. It’s combined with your distance sphere to produce the reading zone in bifocals, trifocals, or progressive lenses. So if your distance sphere is −3.00 and your add is +2.00, the effective power in your reading zone is −1.00. You don’t need to do this math yourself; the lab handles it. But understanding it helps you see why your progressive lenses have different strengths at different vertical positions.
Progressive, Bifocal, and Trifocal Lenses
If your prescription includes an add power, your eye care provider will typically discuss whether you want bifocals, trifocals, or progressive lenses. Bifocals have a visible line separating the distance zone from the reading zone. Trifocals add a third strip in between for intermediate distances like a computer screen. Progressives eliminate the visible lines entirely by blending the power gradually from top to bottom.
Progressive lenses have seen the most development over the past several decades. The design challenge is complex: engineers must create a smooth gradient of power across the lens surface while minimizing distortion in the peripheral areas. Modern “free-form” manufacturing techniques have made significant advances in reducing the swim and blur that plagued earlier generations of progressives.3PubMed Central. Progress in the spectacle correction of presbyopia. Part 1: Design and development of progressive lenses If you tried progressives years ago and hated them, newer designs may be worth a second look.
One practical note: the corridor of clear intermediate and near vision in a progressive lens is relatively narrow. You’ll find you need to point your nose more directly at what you want to read, rather than just glancing with your eyes. This adaptation period is real but usually passes within a couple of weeks.
Prism Corrections
Some prescriptions include a prism value, measured in prism diopters, along with a base direction (base up, base down, base in, or base out). Prism is prescribed when your eyes don’t naturally aim at the same point, causing double vision or eye strain. The prism in the lens bends the image just enough to align what each eye sees, so your brain can fuse the two images into one.
Prism prescriptions are less common than sphere and cylinder, but they can be dramatic when needed. A study of patients with double vision from various causes found that about 88% reported complete or partial resolution of their diplopia with prism glasses, and roughly 89% of those patients continued using prisms rather than opting for surgery.4PubMed. Effectiveness of prisms in the management of diplopia in patients due to diverse etiologies Even patients who needed more than 10 prism diopters of correction saw good results. Research on how prism lenses affect eye alignment has shown that base-in prisms shift alignment in one direction and base-out prisms shift it in the opposite direction, with medium to large effect sizes in most conditions tested.5PLoS ONE. Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity
If you see prism on your prescription and you’ve never had it before, ask your provider to explain why it was added. Prism is sometimes prescribed temporarily after a concussion or stroke, or it may be a long-term correction for a muscle imbalance. The base direction matters just as much as the number, and it must be made correctly by the lab.
How a Contact Lens Prescription Differs
A glasses prescription and a contact lens prescription are not interchangeable. Glasses sit roughly 12 millimeters in front of your eye, while contacts rest directly on the cornea. That small gap changes the effective power of the lens, especially at higher prescriptions. Your eye care provider adjusts the sphere value to account for this, which is why your contact lens power often doesn’t match your glasses power exactly.
Contact lens prescriptions also include parameters you won’t find on a glasses script. Base curve (BC) describes the curvature of the back surface of the lens, and diameter (DIA) describes the overall width. These values matter because the lens needs to sit properly on your cornea: too flat and it slides around or pops out, too steep and it grips the eye uncomfortably and restricts oxygen flow. Research has shown that when you change the base curve by a small amount, you need a corresponding change in diameter to maintain a similar fit. Specifically, a 0.4 mm change in base curve calls for roughly a 0.2 mm change in diameter to keep the on-eye behavior consistent.6PubMed. Inter-relationship of Soft Contact Lens Diameter, Base Curve Radius, and Fit
Material matters too. Silicone hydrogel lenses, which are now the most widely prescribed soft lens material, may need steeper base curve selections than older conventional hydrogel materials to achieve comfortable wear.7PubMed Central. Base Curve Influence on the Fitting and Comfort of the Senofilcon A Contact Lens This is one of the reasons you can’t simply order a different brand of contacts and expect the same fit; the base curve and diameter that work in one brand’s silicone hydrogel may not work in another’s.
If you wear contacts for astigmatism (toric lenses), your prescription will also include cylinder and axis values, just like your glasses prescription. Toric contacts have a built-in orientation system, usually a slight weighting at the bottom of the lens, that keeps the cylinder correction aligned as the lens settles on your eye.
How the Numbers Are Determined
You’ve probably sat behind a large instrument during an eye exam that flashes lights and prints out numbers automatically. That’s an autorefractor, and it gives your provider a starting point by estimating your refractive error based on how light bounces off your retina. But it’s just a starting point. The final prescription almost always comes from subjective refraction, the “which is better, one or two?” process where you compare lenses in a trial frame or phoropter.
The two methods generally agree on the overall ballpark, but they can disagree on the fine details. A study comparing autorefraction to subjective refraction found that the spherical equivalent values were reasonably close, but there were significant differences in the cylinder component between the two methods.8PubMed Central. A Comparison of Autorefraction and Subjective Refraction in an Academic Optometry Clinic In other words, the machine may get your general nearsightedness or farsightedness about right, but the astigmatism correction often needs fine-tuning by a human. Another study of healthy young adults confirmed that while average spherical equivalents were close between the two methods, there were statistically significant differences, with autorefraction tending to give slightly different cylinder readings compared to subjective testing.9PubMed Central. Subjective versus objective refraction in healthy young adults
This is why the subjective step matters. A prescription isn’t just an objective measurement of your eye’s optics; it’s the correction that gives you the clearest, most comfortable vision when your brain is in the loop. Your visual system has preferences and adaptations that a machine can’t fully capture.
Why Your Prescription Changes
If you’ve been told your prescription is “stable” and then find it’s shifted at your next exam, that’s normal. Refractive error isn’t a fixed trait like your blood type. It shifts with age, and the pattern of change differs depending on your starting point.
For most people, nearsightedness progresses through childhood and the teenage years, then typically stabilizes in the mid- to late twenties. Research tracking refractive error across age groups has found that myopic individuals are responsible for most of the overall refractive change that occurs over a lifetime, and that variability in refractive error does not increase with increasing farsightedness the way it does with nearsightedness.10PubMed Central. Refractive error magnitude and variability: Relation to age In practical terms, if you’re farsighted, your prescription is more likely to remain relatively steady over the years, though it may shift slightly with age. If you’re nearsighted, you can expect more fluctuation, especially before age 30.
After your distance prescription stabilizes, the add power is the number that will keep climbing. As presbyopia progresses through your forties, fifties, and sixties, the add creeps up until it maxes out around +2.50 to +3.00 for most people. That’s the point where the natural lens has lost essentially all of its flexibility.
Certain medical conditions can also change your prescription unexpectedly. Uncontrolled diabetes can cause temporary swings in blood sugar that physically alter the shape of the lens inside your eye, shifting your prescription by a diopter or more until blood sugar is brought under control. Cataracts, which cloud the natural lens, often produce a gradual shift toward more nearsightedness before they’re removed surgically. If your prescription changes dramatically between exams, your provider should look for an underlying cause rather than simply writing a new script.
What Your Prescription Leaves Out
A standard prescription captures your eye’s focus errors in terms of sphere, cylinder, and axis. Optically, these are called “lower order aberrations,” and they account for about 90% of the total optical imperfection in most eyes. But the remaining portion comes from higher order aberrations: more complex distortions that warp light in ways a simple lens can’t fix. These include effects like coma, which smears points of light into comet-like tails, and spherical aberration, which causes light passing through the edge of the pupil to focus differently from light passing through the center. A review of these aberrations notes that standard sphero-cylindrical lenses can correct the second-order modes (sphere and cylinder) but that higher order modes in the third radial order and above historically were not correctable with ordinary glasses.11African Vision and Eye Health. A review of higher order aberrations of the human eye
For most people, higher order aberrations are too small to notice in daily life. But they become more relevant if you have large pupils (which let more peripheral light in), if you’ve had corneal surgery like LASIK, or if you have a corneal condition like keratoconus. Custom wavefront-guided contact lenses and some newer LASIK platforms can address these aberrations, but they require specialized mapping of your eye that goes well beyond a standard prescription.
Your prescription also tells you nothing about the health of your eye. Glaucoma, macular degeneration, diabetic eye disease, and other conditions are assessed during the examination but don’t appear on the prescription form. You can have 20/20 corrected vision and still have a serious eye disease developing quietly in the background. That’s why the exam itself matters, not just the slip of paper you walk out with.
Pupillary Distance and Frame Measurements
You may notice a number labeled PD on your prescription, or you may have to ask for it separately. PD stands for pupillary distance, the distance in millimeters between the centers of your two pupils. It’s used to position the optical center of each lens directly in front of your pupil. If it’s wrong, you end up looking through an off-center part of the lens, which can cause eye strain, headaches, and blurred vision, especially in stronger prescriptions and progressive lenses.
PD is sometimes given as a single number (the total distance between both pupils, typically somewhere between 54 and 74 mm for adults) or as two numbers (one for each eye, measured from the bridge of the nose). The dual-number format is more precise because most people’s faces aren’t perfectly symmetrical. If you’re ordering glasses online, you’ll need your PD, and some providers include it on the prescription automatically while others require you to ask.
Segment height is another measurement relevant to bifocal and progressive wearers. It’s the vertical distance from the bottom of the lens to the point where the reading zone should begin, and it depends on both your anatomy and the frame you choose. This measurement is frame-specific, so it’s taken after you’ve picked your frames, not during the exam. It won’t appear on a prescription from most doctors’ offices.
Expiration Dates and Legal Details
In the United States, federal law (the Fairness to Contact Lens Consumers Act and related FTC rules) requires your eye care provider to give you a copy of your prescription automatically after an exam. Glasses prescriptions typically expire after one to two years depending on state law, while contact lens prescriptions usually expire after one year. The expiration exists because your prescription can change over time, and because conditions like glaucoma or retinal damage need periodic monitoring that wouldn’t happen if you simply reordered the same lenses indefinitely.
You’re legally free to take your prescription to any provider or online retailer you choose. Your doctor cannot charge you extra for releasing it, and they cannot condition the release on buying lenses from them. If you shop online, make sure the retailer asks for the complete prescription including all values your provider specified. Skipping the cylinder or axis values, for instance, would produce lenses that correct only half your problem.
One thing worth keeping in mind: the prescription is the minimum information needed to make your lenses. It is not a complete set of instructions for a finished pair of glasses. Frame fit, lens material, coatings, and progressive corridor design all affect how well you see and how comfortable the glasses feel. Two pairs made from the same prescription can perform quite differently depending on these choices. The prescription gets the optics right; everything else determines whether you’ll actually enjoy wearing them.