Your vision is measured by how clearly you can see at a standard distance, usually 20 feet, and your eyeglass prescription is a written set of numbers that tells a lens maker exactly how to bend light for each eye. The first number (sphere) sets the basic focusing power. The second (cylinder) and third (axis) correct astigmatism. A fourth number, add, is for reading. Once you know what each column means, the sheet stops looking like code.
How To Know What Your Vision Is And Read Your Rx
Vision and prescription are two different things, and mixing them up causes most of the confusion.
Vision is a measurement of what you can actually see. It is written as a fraction, like 20/20 or 20/40. The top number is the testing distance. The bottom number is the distance at which a person with typical vision could read the same line you read. So 20/40 means you need to be 20 feet away to read what a typical eye reads at 40 feet. A smaller bottom number means sharper vision.
Your prescription is not a measure of how well you see. It is a set of instructions for making lenses. Two people can both see 20/20 with glasses and have very different prescriptions. Someone with a strong prescription can have 20/20 vision once corrected.
Think of it this way. The 20/20 line on the chart is the score. The prescription is the recipe that gets you there.
What Do the Numbers on an Eyeglass Prescription Mean?
A prescription is written in a grid. There is one row for each eye, labeled OD and OS.
- OD stands for oculus dexter, Latin for right eye.
- OS stands for oculus sinister, Latin for left eye.
- OU means both eyes together.
Each row has up to four columns. Here is what they do.
Sphere (SPH) is the main lens power. It corrects nearsightedness or farsightedness. The number is measured in diopters and usually comes in quarter steps, like -2.00 or +1.25. A minus sign means you are nearsighted and need help seeing far away. A plus sign means you are farsighted and need help with close work. Zero means no correction is needed in that column.
Cylinder (CYL) corrects astigmatism, which is when the cornea or lens is shaped more like a football than a basketball. That uneven curve bends light differently in different directions. The cylinder number tells the lens how much extra correction to add. It also uses a minus or plus sign.
Axis is a number from 1 to 180. It is not a power. It is a direction, in degrees, that tells the lens maker where to place the cylinder correction. An axis of 90 and an axis of 180 point the correction in completely different directions.
Add is the extra magnifying power added for reading or computer work. It appears on prescriptions for people who need help with close vision, often after age 40. The add is usually the same for both eyes.
What Does 20/20 Vision Actually Mean?
20/20 is a reference point, not perfect vision. It simply means your clarity at 20 feet matches what is considered typical.
The fraction only measures sharpness at a distance. It says nothing about peripheral vision, color vision, depth perception, or how well your eyes work together. You can have 20/20 central vision and still have a serious eye disease. Glaucoma, for example, often damages side vision first and can leave central vision sharp for a long time.
Some people see better than 20/20. A result of 20/15 means you can read at 20 feet what most people need to be at 15 feet to read. That is normal variation, not a sign of anything wrong.
| Vision | What it means |
|---|---|
| 20/20 | Typical distance clarity |
| 20/40 | You must be at 20 feet to read what a typical eye reads at 40 feet |
| 20/200 | Vision at or below this level with best correction meets the legal definition of blindness in most US states |
That last row is worth pausing on. Legal blindness is defined by corrected vision, meaning the best you can see with glasses or contacts, not without them.
Why Is My Prescription Different From My Friend’s?
Prescriptions vary because eyes vary. The shape of your cornea, the length of your eyeball, and the flexibility of your lens all affect how light lands on the retina.
A longer eyeball tends to cause nearsightedness, because light focuses in front of the retina instead of on it. A shorter eyeball tends to cause farsightedness. An uneven cornea causes astigmatism. These are structural traits, not habits or failures.
Age changes prescriptions too. Many children become more nearsighted through their teen years. Around age 40, the lens stiffens and close vision blurs, a normal change called presbyopia. That is when an add number first shows up. It happens to nearly everyone eventually, even people who never needed glasses before.
Two people with the same 20/20 vision can have prescriptions that look nothing alike, because the numbers describe the correction needed, not the outcome.
How Often Should You Get Your Eyes Checked?
A vision screening and a full eye exam are not the same thing. A screening checks how well you see. A full exam also looks at the health of your eyes.
For adults with no known eye problems, most eye care professionals suggest a complete exam every few years in your 30s and 40s, and more often after 60. If you wear glasses or contacts, have diabetes, have a family history of eye disease, or take certain medications, your eye doctor may want to see you more frequently.
These intervals are general guidance, not fixed rules. Your own schedule depends on your eyes and your risk factors, and your eye doctor is the right person to set it.
One point that gets lost. Many serious eye conditions, including glaucoma and diabetic eye disease, cause no symptoms in their early stages. You cannot feel high pressure inside your eye. An exam can catch problems before you notice anything, which is when treatment tends to work best.
Can You Read a Prescription Without a Degree?
Yes, for the basics. You can look at your sheet and know whether you are nearsighted or farsighted, whether you have astigmatism, and whether you need reading help.
What you cannot do is judge whether the prescription is right for you, or whether a change from last year is meaningful. Small shifts are normal. Large jumps may need a conversation.
Be careful with online sellers that let you type in numbers without a valid prescription. A prescription is a medical document, and in the US, contact lenses specifically require a valid prescription from an eye doctor. That rule exists because a poorly fitted contact lens can damage the cornea.
If your glasses arrive and the world looks tilted, doubled, or gives you a headache, that is worth a call to your eye doctor. It does not always mean the prescription is wrong. Sometimes it means your eyes need time to adjust. But it is not something to simply push through for weeks.
What the Numbers Do Not Tell You
A prescription is a snapshot of refraction. It is not a report on eye health.
Your eye doctor checks eye pressure, examines the optic nerve, looks at the retina, and tests side vision during a full exam. None of that shows up on the prescription sheet. That is why a strong prescription is not the same as an unhealthy eye, and a mild prescription is not the same as a healthy one.
If you only ever read the numbers and skip the exam, you are reading the recipe and ignoring the kitchen.
Frequently Asked Questions
What does 20/20 vision mean in plain terms?
It means your distance clarity at 20 feet matches what is considered typical. It measures sharpness only, not eye health or peripheral vision.
Is a higher prescription worse for your eyes?
No. A strong prescription means you need more lens correction, not that your eyes are unhealthy. Eye health is judged by a full exam, not by the numbers on the sheet.
What does the axis number on my prescription mean?
Axis is a direction from 1 to 180 degrees that tells the lens maker where to place the astigmatism correction. It is not a measure of power.
Can my prescription change over time?
Yes, and it often does. Age, especially around 40 and beyond, commonly shifts close vision and adds a reading number. Regular exams track those changes.
