How to Read an Eye Prescription

Introduction

When you first receive a glasses prescription, all the numbers and abbreviations can seem overwhelming. However, once you understand what each part means, interpreting your prescription becomes much easier. In this guide, we’ll break down each component of a glasses prescription so you can understand exactly what it means and how it relates to your vision.

1. Basic Components of a Glasses Prescription

A glasses prescription contains several key components, each with a specific purpose. Let’s go over the most common elements you’ll encounter:

OD and OS

• OD (Oculus Dexter): This term refers to the right eye. In Latin, “Oculus Dexter” translates to "right eye."
• OS (Oculus Sinister): This refers to the left eye. In Latin, “Oculus Sinister” translates to "left eye."
• OU (Oculus Uterque): This term is sometimes used to refer to both eyes together.

Understanding whether a particular measurement applies to your right eye, left eye, or both eyes is the first step in reading your prescription.

SPH (Sphere)

This is the primary part of your prescription that indicates the lens power needed to correct your vision. The number in the sphere section shows whether you are nearsighted or farsighted and how strong your lenses need to be.

• A negative (-) number indicates myopia (nearsightedness), meaning you have difficulty seeing distant objects clearly. For example, if you have -2.00 for SPH, it means you may struggle to see things like road signs or distant buildings clearly and need lenses with 2.00 diopters of correction to bring those objects into focus.

• A positive (+) number indicates hyperopia (farsightedness), meaning you have difficulty seeing close objects clearly. For example, if your SPH is +2.00, you might find it challenging to read the text on your phone or a book without holding it further away, and you need lenses with 2.00 diopters of correction to see these nearby objects clearly.

CYL (Cylinder)

This part of the prescription corrects astigmatism, a condition where the cornea has an irregular shape, causing blurred or distorted vision. The CYL value indicates the lens power needed to correct this irregularity. CYL can be either positive (+) or negative (-).

• If you don’t have astigmatism, this part of your prescription will either be left blank or marked with "DS" (Diopters Sphere), indicating that no cylinder correction is needed.

For example, -1.00 CYL means your eye requires 1.00 diopters of astigmatism correction.

Axis

This is the angle at which the astigmatism correction is applied, ranging from 0 to 180 degrees. The axis indicates the direction of the astigmatism and is only relevant if there is a CYL value in your prescription.

• For example, if your prescription says Axis 90, it means the astigmatism correction is oriented along the 90-degree meridian.

If there’s no CYL value, the axis will not be present, as it only applies to those with astigmatism.

ADD (Addition)

This number is used for bifocal or progressive lenses, which help you focus on objects at different distances. ADD represents the additional magnifying power applied to the lower part of your lenses, typically used for reading and other close-up tasks.

• ADD is usually a positive number. For instance, +1.50 ADD means you need an additional 1.50 diopters for near tasks like reading.

If your prescription doesn’t include an ADD value, it simply means you don’t require bifocal or progressive lenses for near vision correction.

PD (Pupillary Distance)

• Pupillary Distance (PD): This measures the distance between the centers of your pupils, in millimeters. PD is crucial for ensuring that your lenses are properly aligned with your eyes to give you the best possible vision.
  • For example, if your PD is 63 mm, it means your pupils are 63 millimeters apart. This measurement is particularly important for people with strong prescriptions or those getting progressive lenses.

2. Prism (Prism Correction)

Prism is a less common component of a glasses prescription, but it’s crucial for those who need it. Prism correction is used to address issues with eye alignment, such as strabismus (where the eyes don’t align properly) or diplopia (double vision).

Key Points:

• Prism Correction: Used to address eye alignment issues like strabismus (misalignment) or diplopia (double vision).
• Prism Diopters (△): Measurement of the prism's strength.
• Base Direction: Indicates which way the base of the prism is oriented to correct the eye alignment.

3. Expiration of Prescription

It’s important to note that a glasses prescription is not valid indefinitely. Most prescriptions have a validity period, typically ranging from 1 to 2 years. After this period, your vision may change, and you’ll need to get a new eye exam to ensure your prescription is still accurate.

Why Prescription Expiry Matters:

• Vision Changes: Over time, your vision can change due to factors like aging, health conditions, or eye strain. Using an outdated prescription can lead to blurry vision, headaches, and eye fatigue.
• Legal and Insurance Reasons: Some regions and insurance companies require a valid prescription before you can order new glasses. This ensures that the glasses you receive match your current vision needs.

If your prescription is close to or has passed its expiration date, it’s time to schedule a new eye exam.

4. Common Terms and Their Importance

Understanding some of the key terms in your prescription can help you grasp the overall picture of your vision correction needs. Here are a few more terms you might encounter:

Hyperopia (Farsightedness) vs. Myopia (Nearsightedness)

Hyperopia (Farsightedness): This condition is corrected with a positive SPH value. If you have hyperopia, you find it easier to see distant objects clearly, while close-up tasks, like reading or sewing, might be challenging.

• Example: Imagine you're at a park trying to enjoy a beautiful sunset. You can see the sunset clearly and appreciate the distant scenery. However, when you try to read a book or check a text message on your phone up close, the text might appear blurry. If your prescription reads +2.00 SPH, it means you need 2.00 diopters of correction to help you see clearly up close.

Myopia (Nearsightedness): This condition is corrected with a negative SPH value. If you have myopia, you can see close objects clearly but struggle with distant vision, making it hard to focus on faraway things like street signs or movie screens.

• Example: Picture yourself at a concert trying to see the band on stage. You can clearly see the details on the stage right in front of you, but the band and the stage might appear blurry or out of focus from where you’re sitting. If your prescription reads -2.00 SPH, it means you need 2.00 diopters of correction to bring those distant objects into clear view.

Astigmatism

• Astigmatism occurs when the eye's cornea is irregularly shaped, leading to blurred vision at all distances. It’s corrected with CYL and Axis values in your prescription.
  • Example: -1.25 CYL with Axis 100 means you have 1.25 diopters of astigmatism correction, oriented at 100 degrees.

Bifocal and Progressive Lenses

Bifocal lenses are designed with two distinct optical powers to help people who need correction for both distance and near vision. They typically have a visible line that separates the two sections of the lens:

• Upper Section: This part of the lens is optimized for distance vision, allowing you to see things clearly when you’re looking at something far away, such as driving or watching television.

• Lower Section: The bottom part of the lens is tailored for near vision, making it easier to read, sew, or work on a computer. This section is specially designed to magnify close objects.

Example: Imagine you're reading a book while sitting at a table and then need to get up to check something across the room. With bifocals, you can look through the lower part of the lens to see the book clearly up close and then shift your gaze to the upper part to view objects at a distance without having to switch glasses.

Progressive Lenses:

Progressive lenses offer a more advanced solution by providing a smooth, gradual change in lens power from the top to the bottom of the lens. Unlike bifocals, progressive lenses do not have a visible line and offer a more natural transition between different vision zones:

• Top Section: This area of the lens is designed for distance vision. It helps you see faraway objects clearly, such as while driving or watching a sports game.

• Middle Section: The middle part of the lens gradually transitions in power and is used for intermediate vision. It’s ideal for tasks like working on a computer or looking at things at arm’s length.

• Bottom Section: The bottom part is for near vision, similar to bifocals. It allows you to see up-close details clearly, such as reading a book or checking your phone.

Example: Consider you’re working at a desk, where you need to look at your computer screen, then glance at a document on your desk, and finally, check a distant object across the room. With progressive lenses, you can smoothly transition your vision from the computer screen through the intermediate zone to see the document and then focus on the distant object without changing glasses or struggling with blurry spots.

Benefits of Progressive Lenses:

• Seamless Vision Transition: They eliminate the visible lines seen in bifocals, offering a more aesthetically pleasing appearance and a continuous range of vision.

• Enhanced Comfort: They provide a natural vision experience, allowing you to look up, down, and sideways with ease, adjusting the focus smoothly as needed.

• Versatility: Ideal for people who need vision correction for multiple distances but prefer a single pair of glasses without the need to switch between different pairs.

5. How to Interpret a Full Prescription

Let’s put it all together with an example of a complete prescription:

• OD (Right Eye): -0.50 SPH, -0.75 CYL, Axis 90
• OS (Left Eye): -0.75 SPH, 0 CYL, Axis 0
• ADD: 0.00
• PD: 61 mm

What This Means:

• Right Eye (OD): You have 0.50 diopters of nearsightedness and 0.75 diopters of astigmatism, corrected at a 90-degree angle.
• Left Eye (OS): You have 0.75 diopters of nearsightedness.
• Addition (ADD): You don't need an extra diopters for reading or other close-up work.
• Pupillary Distance (PD): The distance between your pupils is 61 millimeters, which is essential for the proper alignment of your lenses.

6. Common Misconceptions and Important Tips

Understanding your prescription is the first step, but there are a few more things to keep in mind:

High Prescription Numbers Don’t Equal Poor Vision

• A higher SPH or CYL number simply means your eyes require more correction. It doesn’t necessarily mean you have “bad” vision; it just indicates the strength of lenses needed to help you see clearly.

Pupillary Distance (PD) Is Crucial

• Accurate PD measurement ensures that the lenses align correctly with your eyes. If your PD is measured incorrectly, you might experience blurry vision or discomfort, especially with progressive lenses or higher prescriptions.

Don’t Use an Expired Prescription

• Always ensure your prescription is up to date. Using an old prescription can lead to eye strain, headaches, and suboptimal vision. Regular eye exams are essential for maintaining eye health and ensuring your glasses meet your current vision needs.

7. Conclusion

Understanding your glasses prescription is key to ensuring you get the correct lenses to meet your vision needs. By breaking down each part of the prescription, from SPH and CYL to prism and PD, you can gain confidence in interpreting these numbers. Always remember to have regular eye exams and update your prescription as needed to maintain clear and comfortable vision.

Note: Your Right to Your Eye Prescription

Remember: You Have a Right to Your Eye Prescription

According to the Federal Trade Commission (FTC), you are legally entitled to a copy of your eye prescription after your eye exam. Your eye doctor is required to provide you with this prescription, whether you ask for it or not.

Here are a few important points to keep in mind:

• No Purchase Required: You do not need to buy eyewear from your eye doctor’s office to receive your prescription. It’s your right to get it regardless of where you decide to purchase your glasses or contacts.

• No Fees for the Prescription: You should never be charged a fee just to access your eye prescription. If you’re asked to pay for this information, it’s worth questioning the policy or seeking clarification.

• Keep a Copy: It’s a good idea to keep a copy of your prescription. Having it on hand makes it easier to shop for glasses and contact lenses online, compare prices, and ensure that you’re getting the best deal.