Disclaimer: This article is for informational purposes only and does not constitute medical advice. The information presented is based on published medical research and clinical practice standards. Always consult with a qualified optometrist or ophthalmologist for interpretation of your individual prescription and eye care needs.
If you’ve ever stared at your eye prescription, feeling like you’re trying to decode an alien language, you’re not alone. Those seemingly random numbers and abbreviations actually tell a precise story about your eyes and how they focus light. Understanding them empowers you to make informed decisions about your vision care.
The Quick Translation Guide
Your prescription contains several key measurements that work together to correct your vision:
- OD and OS: These refer to your right eye (Oculus Dexter) and left eye (Oculus Sinister). Latin terms meaning “right eye” and “left eye.”
- SPH (Sphere): This number tells you how nearsighted or farsighted you are. A minus sign means nearsighted; a plus sign means farsighted.
- CYL (Cylinder): This measures astigmatism when your eye isn’t perfectly round. If this is blank, you don’t have clinically significant astigmatism.
- Axis: This works with the cylinder and shows where your astigmatism is located, measured in degrees from 1 to 180.
- ADD: This is the additional magnifying power you need for reading if you’re over 40. It’s always a positive number.
- PD (Pupillary Distance): The distance between your pupils, essential for properly centring your lenses.
Your Eye Is a Camera That Needs Focus
Your eye works almost exactly like a camera. Light enters through your cornea (the clear front surface), passes through your pupil, gets focused by your lens, and lands on your retina at the back of your eye.
When everything works perfectly, that light lands exactly on your retina, creating a sharp, clear image. Your brain then processes that image, and you see clearly.
But for most of us, our eyes don’t focus light perfectly. The shape of your eyeball, the curve of your cornea, or the power of your lens might be slightly off. That’s where prescription lenses come in. They bend the light just enough to make it land exactly where it should.
Your prescription is essentially a mathematical formula that tells lens makers exactly how much to bend light for your unique eyes.
Breaking Down the SPH: The Myopia Epidemic
The Sphere (SPH) is usually the first number people notice because it’s typically the largest. This measurement indicates the lens power needed to correct nearsightedness or farsightedness, measured in diopters.
The global myopia crisis
By 2050, approximately half of the global population (close to 5 billion people) may be myopic, with about 10% experiencing high myopia. This represents a dramatic increase from historical rates.
In the United States, prevalence among those aged 12 to 54 rose from about 25% in the early 1970s to over 40% by 2004. Some East Asian countries report even more striking numbers, with adolescent myopia rates exceeding 80 to 90%.
Understanding the minus sign
If you see a minus sign before your SPH number, you’re nearsighted (myopic). This means you can see things up close clearly, but distant objects look blurry. Your eyeball is slightly too long, so light focuses in front of your retina instead of directly on it.
A prescription of -2.50 means you need moderate correction. Numbers between +/- 0.25 to +/- 2.00 are considered mild, +/- 2.25 to +/- 5.00 are moderate, and greater than +/- 5.00 are severe.
The serious risks of high myopia
Highly myopic individuals are 20 times more likely to have a retinal detachment in their lifetime than an emmetropic individual. Additionally, pathologic myopia, defined as having high myopia with irreversible retinal atrophy, affects approximately 1% of Caucasians and 1 to 3% of Asians.
Understanding the plus sign
If you see a plus sign (or no sign at all), you’re farsighted (hyperopic). This means distant objects might be clear, but you struggle with close-up vision. Your eyeball is slightly too short, so light would focus behind your retina.
What about zero?
If your SPH reads “Plano,” “PL,” or has an infinity symbol, you don’t need distance correction in that eye. You might still need glasses for other reasons, like astigmatism.
Decoding CYL and Axis: Understanding Astigmatism
Astigmatism is remarkably common. A compilation of many systematic reviews found there was an 8 to 62% prevalence of astigmatism among adults, with an estimated prevalence of 40% worldwide. China reports the highest prevalence at 62%.
What causes astigmatism
Most people think of the eye as perfectly round, like a basketball, but an eye with astigmatism is shaped more like a football. One curve is longer than the other.
The Cylinder (CYL) measurement
The cylinder indicates the amount of lens power for astigmatism, representing the difference in the greatest and weakest powers of the eye, usually separated by 90 degrees. If this column is blank on your prescription, you don’t have significant astigmatism.
Like the sphere, a cylinder can be positive or negative. The higher the number, the more astigmatism correction you need. The prevalence of astigmatism was higher for low levels (less than 1.5 D: 32.7% to 100%) than for higher levels (1.5 D or more: 0% to 39.1%).
The Axis measurement
If you have a CYL value, you must have an Axis value. The axis explains where the astigmatism is located on your cornea, measured in degrees from 1 to 180.
Think of it like a clock face or a compass. If your axis is 90 degrees, the steeper curve of your cornea points straight up and down. If it’s 180 degrees, it points horizontally. Most people have axis values somewhere in between.
Here’s the critical part: the axis number doesn’t tell you how strong your astigmatism is. That’s what the cylinder does. The axis just tells the lens maker where to position that correction.
Eye care professionals often use this analogy: “Imagine you’re hanging a picture on a wall. The cylinder tells you how heavy the picture is, and the axis tells you exactly where on the wall to hang it. You need both pieces of information to get it right.”
Age and astigmatism patterns
The prevalence of with-the-rule astigmatism was higher in the younger population (40 years or younger), while rates of against-the-rule and oblique astigmatism increased with age. The prevalence of astigmatism in the general population varied from 7.6% to 61.7%, with higher rates in older individuals (70 years or older).
ADD: The Universal Challenge After 40
If you’re over 40, you might have noticed something frustrating: you need to hold your phone farther and farther away to read text messages. This is presbyopia, the age-related loss of near focusing ability that affects virtually everyone.
The staggering prevalence
Presbyopia affects 83% to 89% of adults aged 45 and older in the United States, with an estimated 128 to 139 million Americans currently experiencing this condition. Nearly 80% of North American adults ages 45 to 55 experience trouble seeing clearly at close distances.
Globally, the all-ages prevalence of epidemiologically-measured ‘functional’ presbyopia was estimated at 24.9% in 2015, affecting 1.8 billion people, projected to reach 2.1 billion by 2030.
How ADD works
The ADD value is always positive and represents the additional magnifying power needed for reading or close-up tasks. It’s added to your distance prescription to create your reading prescription.
For example:
- If your distance SPH is +1.00 and your ADD is +2.00, your reading prescription is +3.00
- If your distance SPH is -2.00 and your ADD is +2.50, your reading prescription is +0.50
Studies show the ADD prescription increases by about 0.25 diopters every two years from age 40 to 50, then slows to approximately 0.03 diopters yearly after age 50.
Most ADD values range from +0.75 to +3.00. If you need an ADD, you have several options: separate reading glasses, bifocals, trifocals, or progressive lenses that seamlessly blend all distances.
PD: The Measurement Most People Forget
Here’s something most people don’t realize: even if your prescription is perfect, your glasses won’t work properly if the lenses aren’t centred correctly over your pupils.
Pupillary distance measures the distance between the centres of your pupils and is used to centre a prescription lens accurately in your frames.
Why PD matters
Your PD is critical because it determines where you look through the lens. If the optical centre of your lens doesn’t align with your pupil, you’ll experience eye strain, headaches, and blurry vision even with the perfect prescription.
Single vs. Dual PD
You might see one number (like 63) or two numbers (like 32/30). A single PD measures the distance between both pupils. Dual PD measures the distance from the centre of each pupil to the bridge of your nose, with the first number for the right eye and the second for the left eye.
The average adult PD ranges from 54mm to 74mm. Children typically have PDs between 43mm and 58mm.
How to measure your PD at home
If your prescription doesn’t include your PD (many don’t), you can measure it yourself with a ruler and a mirror:
- Stand 8 inches from a mirror
- Hold a millimetre ruler against your brow
- Close your right eye and align the ruler’s zero with your left pupil’s center
- Open your right eye, close your left, and note the millimetre marking at your right pupil’s centre
That number is your PD. Measure several times to ensure accuracy.
Common Prescription Scenarios Explained
Scenario 1: Different numbers for each eye
This is completely normal. It’s actually unusual for both eyes to have identical prescriptions. Your right eye might be -2.00 while your left is -2.50, or one eye might have astigmatism while the other doesn’t.
Scenario 2: Only one eye needs correction
Some people have one eye with perfect vision (Plano) and one that needs correction. Your brain combines the images from both eyes, so you still need glasses for the clearest vision.
Scenario 3: Your prescription changed
Vision changes over time. Children’s eyes typically become more nearsighted as they grow. Adults might see prescriptions stabilize, then change again after 40 with presbyopia. Regular eye exams (every 1 to 2 years) ensure your prescription stays current.
What Your Prescription Doesn’t Tell You
Your prescription numbers only tell part of your eye health story. They don’t indicate:
- Eye diseases: Conditions like glaucoma, cataracts, or macular degeneration require different tests
- Overall eye health: Your prescription won’t show if you have dry eyes, inflammation, or other conditions
- Retinal health: The back of your eye needs a separate examination
- Perfect vision guarantee: A prescription gives you the tools for clear vision, but other factors (like uncorrected eye conditions) might affect results
This is why comprehensive eye exams include much more than just determining your prescription numbers.
Glasses vs. Contact Lens Prescriptions: They’re Not the Same
Here’s a crucial point that trips people up: you cannot use your eyeglass prescription to order contact lenses.
Why? Eyeglasses sit about 12mm from your eyes, while contacts sit directly on the surface of your eyes. This distance difference means the power needed is different. Contact lens prescriptions also include additional measurements, such as base curve and diameter, that glasses prescriptions don’t have.
Never try to convert your glasses prescription to contacts yourself. Always get a proper contact lens fitting from your eye care professional.
Making Sense of Your Next Eye Exam
Now that you understand what those numbers mean, here’s what happens during the exam that determines them:
- Visual acuity test: You read letters on a chart to establish baseline vision
- Refraction test: The doctor uses a phoropter (that big machine with all the lenses), asking “which is better, one or two?” This determines your sphere and cylinder values
- Axis determination: The doctor rotates the cylinder lens to find the exact angle of your astigmatism
- Binocular balance: Ensures both eyes work together comfortably
- Near vision testing: If you’re over 40, this determines your ADD value
The entire process creates the numbers on your prescription, each one crucial to your visual clarity.
Your Next Steps
Understanding your prescription empowers you to:
- Make informed decisions about your eyewear
- Know when something’s wrong with your glasses
- Ask better questions at your eye exams
- Shop confidently for glasses online
- Understand why you need what you need
Those mysterious numbers aren’t so mysterious anymore. They’re simply a precise map showing exactly how to bend light for your unique eyes.
Ready for your next eye exam? Schedule an appointment with your optometrist. Understanding your prescription is the first step to appreciating how amazing your vision can be with the right correction.
Remember: Your eyes are incredible, complex organs that deserve professional care. These numbers are tools to help you see your best, but they’re no substitute for regular comprehensive eye exams.
Have questions about your prescription? Contact your eye care provider. They can explain your specific numbers and recommend the best eyewear options for your needs and lifestyle.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. The information presented is based on published medical research and clinical practice standards. Always consult with a qualified optometrist or ophthalmologist for interpretation of your individual prescription and eye care needs.