Your Eyeglasses Prescription: A Comprehensive Guide on How to Understand


Posted On August 11, 2023

Your eyeglasses prescription is a set of numbers and abbreviations provided by an optometrist or ophthalmologist specifying the lenses you need to correct your vision. Here’s how to read and understand the different components of a typical eyeglasses prescription:

1. Eyeglasses Prescription OD and OS: 

These abbreviations stand for “oculus dexter” (right eye) and “oculus sinister” (left eye), respectively. Right eye notations are generally written at the top of prescriptions, usually abbreviated as OD (ocular dexter, Latin for the right eye). The Latin term for the left eye, ocular sinister (OS), is followed below.

2. Eyeglasses Prescription Sphere (SPH): 

This number indicates the main lens power needed to correct your vision. If the number is positive (+), it means you are farsighted (hyperopic). If the number is negative (-), it means you are nearsighted (myopic).

3. Eyeglasses Prescription Cylinder (CYL): 

This number indicates the amount of astigmatism present in your eyes. Astigmatism is a condition where the cornea is not perfectly round, leading to distorted or blurred vision. If there is no astigmatism, this section will be left blank or noted as “SPH” or “DS” (diopters sphere).

4. Eyeglasses Prescription Axis: 

If you have astigmatism, the axis is a number that shows the orientation of the astigmatism correction. It ranges from 0 to 180 degrees.

5. Eyeglasses Prescription Addition (ADD): 

This is an additional power that might be prescribed for individuals with presbyopia, a condition that affects near vision as we age. The addition helps you see up close. This is common in bifocal or progressive lenses.

6. Eyeglasses Prescription Prism (PRISM): 

A unique kind of lens called a prism changes the direction of the light as it passes through it. The image that is being viewed moves as a result. These lenses are typically prescribed when the eyes are not aligned properly, which, if left untreated, can cause headaches, eye fatigue, or double vision. It’s measured in prism diopters (Δ).

7. Eyeglasses Prescription Base Direction: 

If you have a prism correction, this indicates the direction of the prism, usually noted as “BU” (base up), “BD” (base down), “BI” (base in), or “BO” (base out).

8. Eyeglasses Prescription Pupillary Distance (PD): 

This is the measurement between the centers of your pupils, usually given in millimeters. It’s essential for ensuring that your lenses are correctly positioned in the frames. Additionally, you might occasionally notice things like “segment height” which is the distance vertically between the center of your pupil and the bottom of the lens of your glasses frames. Although  “segment height” and “pupillary distance (PD)” are crucial measurements for eyeglass fitting, they are not necessarily a particular portion of the prescription.


Here’s an example of how a prescription might look:

OD (Right Eye): – Sph: -2.00  Cyl: -1.00  Ax: 45  Add: +2.00

OS (Left Eye): Sph: -1.50 Cyl: -0.75 Ax: 135  Add: +2.00

PD: 63 mm (Right) / 62 mm (Left)

Remember that only qualified eye care professionals can accurately interpret and prescribe eyeglasses based on your individual needs. If you have any questions or concerns about your prescription, be sure to discuss them with your eye doctor or optician.

“Don’t call the world dirty because you forgot to clean your glasses.” — Aaron Hill

How Often Should I Change My Eyeglasses Prescription?

Changes do occur in the eye. This is particularly apparent during the teen years, but as we age, the cornea and lens of the eye also undergo small changes that can cause eyeglasses prescriptions to shift.


The frequency at which you need to renew your eyeglasses can vary based on several factors, including your age, your eye health, and the type of correction you require. Here are some general guidelines for renewing your eyeglasses prescription:

– For adults aged 18 to 60: It’s recommended to have an eye exam and update your eyeglasses prescription every 1 to 2 years, even if you don’t notice any significant changes in your vision. Regular eye exams can help detect any potential eye health issues early on.

– For children and teenagers: Children’s eyes can change rapidly as they grow, so they should have their eyeglasses prescriptions checked more frequently, usually every 1 to 2 years, or as recommended by their eye doctor.

It’s important to note that even if you don’t experience any noticeable changes in your vision, regular comprehensive eye exams are essential for monitoring your eye health. Many eye conditions, such as glaucoma and macular degeneration, don’t always present with obvious symptoms in the early stages, so routine exams are crucial for early detection and treatment.

Always follow the recommendations of your eye care professional regarding the timing of prescription renewals. They can provide personalized guidance based on your specific needs and circumstances. If you experience sudden changes in your vision, discomfort while wearing glasses or contact lenses, or any other eye-related concerns, it’s best to schedule an appointment with your eye doctor for a thorough examination.

You now understand what each number represents. Keep in mind that it’s crucial to routinely evaluate your glasses and contacts to ensure that they are still providing you with the proper fit, comfort, and power.

Has it been a long since your prescription was revised? Schedule a consultation with your optometrist today to ensure that your vision is preserved.

Written by Dr. Okwukwe Davis

Okwukwe Davis is a Nigerian Optometrist with a flair for web design aligned with brand strategy. He loves to help Optometrists and Rising Professionals discover growth through his design & strategy studio - StarRose. In his spare time, he writes engaging content to help online platforms build meaningful relationships.

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