A Guide to Understanding Glasses (or Contact Lens) Prescriptions

If you have had an eye exam recently, your eye doctor may have given you a prescription for either glasses or contact lenses. The prescription describes in numerical terms how the lenses of your new eyewear must be made to allow you to see clearly when wearing them.

Each field of an eye prescription covers a specific aspect of your vision. One eye can need one set of optical corrections. The other can need the same set – or something different.

Although they can be similar, eyeglasses prescriptions and contact lens prescriptions are not interchangeable. If you want to wear contact lenses instead of glasses, let your eye doctor know before your eye exam begins. This will let them know to make a few additional measurements during your exam.

What Does My Eye Prescription Mean?

Eye prescriptions follow a standard framework used by all eye doctors. Some fields can be noted several different ways, but they all mean the same thing.

OS and OD

OS and OD are Latin abbreviations for each of your eyes.

• OS is short for oculus sinister, or the left eye.
• OD is short for oculus dextrus, or the right eye.

You may also see these notes written as LE and RE, which stand for left eye and right eye, respectively. Which one of these two sets of terms appears on your prescription depends on what your eye doctor prefers.

Sphere (SPH)

Sphere indicates the amount of lens power you need to correct either farsightedness or nearsightedness. This figure is measured in diopters and can be either positive or negative:

• Positive numbers correct farsightedness
• Negative numbers correct nearsightedness

Cylinder (CYL)

Cylinder indicates the amount of lens power you need to correct astigmatism. It is also measured in diopters.

• Positive corrects for farsightedness
• Negative corrects for nearsightedness
• No numbers mean you have no astigmatism

Axis (AXIS)

Axis indicates the lens meridian that has no cylindrical power. It is only used when astigmatism is present and helps opticians get the curvature of the lens correct.

Addition (ADD)

Addition is only used when prescribing progressive lenses or bifocals. It indicates additional lens power needed to correct farsightedness when a person struggles with both close and distance vision. This additional power is measured in diopters.

Prism/Base (PRISM/BASE)

Prism and Base both measure corrections for double vision. Prism is measured in diopters while Base indicates the direction that the thickest part of the corrective lens should face. If the patient does not experience double vision, these fields are left blank.

Near/Far PD

PD stands for pupillary distance, or the distance between your pupils as measured in millimeters. In layman’s terms, PD measures how far apart your eyes are from each other. This is important because it tells the person fabricating your eyeglasses where the center of each of your lenses should be.

Some glasses prescriptions include just one number for your pupillary distance. Others split this number into two figures: near PD (or the distance between your pupils when you are looking at something that is close to you) and far PD (the distance between your pupils when you are looking at something that is far away).

Not all eye doctors will not include your pupillary distance on your prescription at all unless you ask for it. Some will also charge an additional fee for this information.

However, it is just as important as any other part of your prescription. You should never purchase eyeglasses without having your pupillary distance professionally measured.

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Example Eye Prescription Chart

To better understand what eye prescription fields really mean, we can look at a sample prescription chart.

	SPH	CYL	AXIS	ADD	Far PD	Near PD	PRISM/BASE
OS	-1.25	-1.5	120		33	31
OD	-1.0	-0.5	80		33	32

This person is nearsighted and has astigmatism in both eyes. However, the left eye is significantly more impaired than the right.

They have no difficulty seeing at a distance and they do not have any double vision, but they do require correction for close vision.

This person’s pupils are 66 millimeters apart when they are looking at something at a distance. Their pupils tilt 3 millimeters inward when looking at something close-up. The effect is slightly more pronounced in their left eye than their right.

This patient will be prescribed eyeglasses to help them see better when reading, writing, and doing other close work. The lenses will be different on each side, but together, they will allow this person to see normally at all distances.

How Prescriptions Are Measured

Eyeglasses and contacts prescriptions are usually measured using a tool called a photoropter. During your eye exam, your eye doctor will use this tool to circulate a series of lenses in front of your eyes. They will then ask you to look at an eye chart posted on the wall in front of you and read out the letters you see on it.

This method is called subjective refraction. Because it involves input from the patient, it is considered the best way to measure eyewear prescriptions.

In some cases, your eye doctor might use tools like an autorefractor or retinoscope during your eye exam. These tools help them observe how light behaves when it hits your eye.

Measurements from these tools can be used to issue an eyeglasses prescription on their own, but the results are not usually as accurate as prescriptions based on subjective refraction.

After taking these measurements, eye doctors often test their patient’s vision using a photoropter to refine the results.

For young children, people with special needs and other patients who have difficulty participating in subjective refraction exams, autorefractor and retinoscope results can be used without further refinement.

How Do You Read a Contact Lens Prescription?

Contact lens prescriptions are similar to eyeglasses prescriptions. They contain two additional fields:

• BC (base curve, or the outside curvature of your eye).
• DIA (diameter, or the distance across the lens surface of your eye).

These measurements indicate the curvature and size of your contact lens respectively. Both are measured in millimeters.

All other contact lens prescription measurements are close to eyeglass prescription measurements. However, the two types of prescriptions are not interchangeable.

If you are interested in getting either eyeglasses or contacts, be sure to have an eye doctor measure your current prescription for the appropriate type of eyewear.

Why Are Glasses and Contact Lens Prescriptions Different?

Prescriptions for glasses and contact lenses are different because of their different shapes and functions.

Glasses sit in front of your face and do not make direct contact with your eye. While wearing glasses that are not made to fit your prescription is not recommended, doing so will not harm your eyes.

Contacts are used in a completely different way. Instead of sitting in front of your eyes, they sit directly on the surface of your cornea. This means that each pair must be made to fit the exact eye size and shape of the person they are intended for.

If your contacts do not fit your eyes properly, they will not stay in place while you are wearing them. Not only will this make it difficult to see, but it will also put you at risk of eye infections and injuries.

How Often Do I Need a New Prescription?

Most people need a new eyewear prescription every two years. Some people with severe refractive errors or other eye conditions may need their prescriptions updated sooner (usually on a yearly basis).

Your eye doctor will use your prescription’s expiration date to indicate how quickly they expect your prescription to change. When your prescription expires, it is time to get a new one.

References

1. Patient fact sheet. (2022). California Optometric Association.

2. Healthy Vision and Contact Lenses. (2017). North Carolina Optometric Society.

3. How to Read Your Eyeglass Prescription. Canadian Association of Optometrists.

4. Let the Buyer Beware: A Closer Look at Ordering Eyeglasses Online. (March 2015). Southern College of Optometry.

5. Retinoscopy. (January 2020). American Association for Pediatric Ophthalmology & Strabismus.

6. What Is Prism Correction in Eyeglasses? (March 2021). American Academy of Ophthalmology.

7. How to Read an Eyeglasses Prescription. (May 2020). American Academy of Ophthalmology.

8. Patient FAQs: Eye Exams & Vision Care. (2022). College of Optometrists of Ontario.

9. Eye exam. (April 2021). Mayo Clinic.