Your eye prescription is a set of measurements that tells your optician how much vision correction each eye needs. The most common abbreviations include OD and OS for the right and left eyes, SPH for nearsightedness or farsightedness, CYL and Axis for astigmatism, and ADD for reading or multifocal correction.
The numbers may look technical, but each one answers a simple question about how your eyes focus. A current prescription helps your optical team select lenses that support clearer, more comfortable vision for reading, driving, screen use, and everyday activities.
Your prescription can change over time, and vision changes are not always caused by an outdated pair of glasses. A comprehensive eye exam can evaluate both your ability to see clearly and the overall health of your eyes. Schedule an evaluation if your vision has become blurry, your glasses no longer feel effective, or you are experiencing headaches, eye strain, glare, or difficulty seeing at night.
What Do the Numbers on an Eye Prescription Mean?
An eyeglass prescription describes the type and amount of correction needed for each eye. It may include measurements for nearsightedness, farsightedness, astigmatism, reading vision, or eye alignment.
A prescription may look like this:
| Eye | SPH | CYL | Axis | ADD |
|---|---|---|---|---|
| OD | -2.00 | -0.75 | 090 | +2.00 |
| OS | -1.50 | -1.00 | 080 | +2.00 |
Not every prescription includes every measurement. For example, CYL and Axis usually appear only when astigmatism correction is needed.
OD, OS, and OU
These abbreviations identify which eye each measurement applies to:
- OD means the right eye.
- OS means the left eye.
- OU means both eyes.
Some prescriptions use RE for the right eye and LE for the left eye instead.
Your two eyes may have different prescriptions. This is common and does not automatically indicate a medical problem.
Sphere, or SPH
Sphere measures the main strength of your prescription. It is shown in units called diopters and usually includes a plus or minus sign.
A minus sign indicates nearsightedness. This means nearby objects may look clearer than objects farther away.
A plus sign indicates farsightedness. This can make close-up tasks more difficult, although higher amounts may also affect distance vision.
The farther the number is from zero, the stronger the correction generally needs to be. For example, -4.00 represents more nearsightedness than -1.00.
Cylinder and Axis
Cylinder and Axis describe astigmatism correction.
Astigmatism occurs when the cornea or lens has an uneven curve. Instead of focusing light evenly, the eye may focus it in multiple directions. This can cause blurry, distorted, or stretched-looking vision at different distances.
Cylinder, or CYL, shows how much astigmatism correction is needed.
Axis shows the direction of that correction and is measured from 1 to 180 degrees. Axis does not describe the severity of astigmatism. It simply tells the optical laboratory how to position the cylinder correction within the lens.
ADD
ADD refers to additional magnifying power placed in the lower portion of a bifocal, trifocal, or progressive lens.
It is commonly prescribed for presbyopia, an age-related change that makes it harder to focus on nearby objects. You may notice presbyopia when you begin holding books or menus farther away, need more light to read, or struggle to shift focus between near and distant objects.
The ADD measurement is usually written as a positive number and is often the same for both eyes.
Prism
Prism may be included when the eyes need help working together or staying properly aligned. The
Prism correction changes the direction light enters the eye. It may help reduce double vision, eyestrain, headaches, or visual discomfort caused by certain alignment problems.
A prism prescription may also include a direction, such as base-up, base-down, base-in, or base-out.
Pupillary Distance
Pupillary distance, often shortened to PD, measures the distance between the centers of your pupils. This measurement helps the optical team align each lens correctly in front of your eyes.
PD may be reported as a single value or as two separate measurements, one for each eye. Accurate alignment is especially important for stronger prescriptions, progressive lenses, and other customized lens designs.
Is 20/20 Vision the Same as Having No Prescription?
No. A person can have 20/20 vision while wearing prescription lenses, and someone with 20/20 distance vision may still need correction for reading, astigmatism, or eye alignment.
The term 20/20 describes visual acuity, or how clearly you can see from 20 feet away compared with a standard level of distance vision. It does not provide a complete picture of eye health, focusing ability, depth perception, or near vision.
A person may still have an eye condition even if they see 20/20 during a basic vision screening. That is one reason a comprehensive eye exam provides more information than a vision chart alone.
Why Did My Eye Prescription Change?
Common reasons include:
- Normal changes in nearsightedness or farsightedness
- Presbyopia after age 40
- Changes in astigmatism
- Diabetes or fluctuating blood sugar
- Cataracts
- Dry eye
- Certain medications
- Eye strain or increased near work
- Changes in the cornea or natural lens
A small prescription change may simply reflect normal variation. Frequent, sudden, or significant changes warrant a closer look, particularly when accompanied by pain, glare, double vision, flashes, floaters, or other new symptoms.
Eyeglass and Contact Lens Prescriptions Are Different
Glasses sit a short distance in front of the eyes, while contact lenses rest directly on the cornea. A contact lens prescription may include additional information such as:
- Base curve
- Lens diameter
- Brand or lens material
- Replacement schedule
Contact lenses also require a fitting to make sure they sit correctly, provide clear vision, and support healthy oxygen flow to the cornea.
Turning Your Prescription Into the Right Glasses
A prescription provides the measurements, but the lens design and fitting also affect how your glasses perform.
Southwestern Eye Center optical shops offer standard and designer frames, prescription sunglasses, single-vision lenses, bifocals, trifocals, and progressive lenses at participating locations in Arizona. Licensed optical professionals can also help you compare options such as high-index lenses, anti-reflective coatings, scratch protection, polarized lenses, and ultraviolet protection.
The best choice depends on your prescription, work habits, hobbies, screen use, driving needs, and preferred frame style. A person who spends hours on a computer may need different lens features than someone who spends significant time driving or outdoors.
When Should You Schedule an Eye Exam?
Schedule an eye exam when:
- Your vision becomes blurry or distorted
- Your current glasses no longer feel effective
- You develop frequent headaches or eye strain
- You struggle to read or use digital screens
- Night driving becomes more difficult
- You notice glare or halos around lights
- Your prescription changes frequently
- You have diabetes, high blood pressure, or a family history of eye disease
- It has been one or more years since your last prescription check
Seek prompt care for sudden vision loss, severe eye pain, new flashes or floaters, a curtain-like shadow, or a rapid change in one eye. These symptoms may require more urgent evaluation than a routine prescription update.
Get a Prescription Designed Around Your Vision
Understanding your eye prescription can help you make more informed decisions about your glasses, contacts, and everyday vision needs. Still, the numbers are only one part of the picture.
A comprehensive evaluation can determine whether you simply need an updated prescription or whether another eye health issue may be affecting your sight. Schedule an eye exam today with Southwestern Eye Center, then visit a participating optical shop for help selecting lenses and frames that fit your prescription, lifestyle, and personal style.