Understanding Your Eye Prescription: A Comprehensive Guide

Introduction

Have you ever looked at your eye prescription and wondered what all those numbers and abbreviations mean? You're not alone! Eye prescriptions can seem confusing at first, but understanding them is crucial for maintaining good eye health and ensuring you get the right glasses or contact lenses. In this guide, we'll break down the components of an eye prescription in simple terms, explain common vision conditions, and help you decipher your prescription step-by-step.

Key Components of an Eye Prescription

When you receive an eye prescription from your optometrist, it typically includes several key components. Let's explore what each part means.

OD and OS: Understanding Eye Abbreviations

The abbreviations OD and OS are Latin terms used in eye care. OD stands for "oculus dexter," which means "right eye," and OS stands for "oculus sinister," which means "left eye." Sometimes you might also see OU, which stands for "oculus uterque" and refers to both eyes.

SPH (Sphere)

The SPH value indicates the amount of lens power, measured in diopters, needed to correct nearsightedness or farsightedness. A negative number (-) indicates nearsightedness (myopia), meaning you have trouble seeing distant objects. A positive number (+) indicates farsightedness (hyperopia), meaning you have trouble seeing close objects.

CYL (Cylinder) and Axis

The CYL value is used to correct astigmatism, which is an irregular curvature of the cornea or lens. The CYL number can be positive or negative and measures the lens power needed to correct astigmatism. The axis value, ranging from 0 to 180 degrees, indicates the orientation of astigmatism. Together, CYL and axis ensure that the correction is properly aligned with your eye's shape.

ADD (Addition)

The ADD value is commonly found in prescriptions for bifocals or progressive lenses. It represents the additional magnifying power needed for reading or close-up work. This value is always positive and is added to the SPH value for near vision correction.

PD (Pupillary Distance)

PD measures the distance between the centers of your pupils in millimeters. This measurement is crucial for ensuring that the lenses in your glasses are correctly aligned with your eyes, providing clear and comfortable vision. You can use our PD ruler to measure it yourself.

Prism

Prism correction is included in an eye prescription when there is a need to correct alignment problems, such as double vision (diplopia). Prism is measured in prism diopters and helps to shift the image that the eye sees to align it with the other eye. Prism values will be written with a number followed by a direction, such as "BU" (base up), "BD" (base down), "BI" (base in), or "BO" (base out). This tells the optician how to position the prism in your lenses to properly correct the eye alignment.

For example, you might see something like this in your prescription:

• OD: 1.0 BI
• OS: 0.5 BO

This indicates that prism correction is needed for both eyes to help with binocular vision or alignment issues.

Common Vision Conditions

Understanding common vision conditions can help you make sense of your prescription. Here are some of the most frequent conditions:

Nearsightedness (Myopia)

Myopia occurs when the eyeball is too long or the cornea is too curved, causing light to focus in front of the retina. This makes distant objects appear blurry. Myopia is indicated by a negative SPH value in your prescription.

Farsightedness (Hyperopia)

Hyperopia happens when the eyeball is too short or the cornea is not curved enough, causing light to focus behind the retina. This results in difficulty seeing close objects clearly. Hyperopia is indicated by a positive SPH value in your prescription.

Astigmatism

Astigmatism is caused by an irregular shape of the cornea or lens, leading to distorted or blurred vision at all distances. It is corrected with a CYL value and an axis value in your prescription.

Presbyopia

Presbyopia is an age-related condition where the eye's lens becomes less flexible, making it difficult to focus on close objects. This condition usually affects people over 40 and is corrected with an ADD value in the prescription.

Binocular Vision Problems and Prism Correction

Prism correction is prescribed for people with binocular vision issues, such as strabismus (crossed eyes) or diplopia (double vision). These conditions occur when the eyes are not properly aligned, causing them to send two different images to the brain. Prism lenses help to align the images seen by each eye, reducing or eliminating the problem.

Reading Your Eye Prescription

Deciphering your eye prescription can be straightforward if you understand the key components. Here’s a step-by-step guide:

1. Identify OD and OS: Determine which values correspond to your right eye (OD) and left eye (OS).
2. Read the SPH value: Check the SPH value for each eye to see if you are nearsighted (negative number) or farsighted (positive number).
3. Look at the CYL and Axis: If you have astigmatism, find the CYL value and the corresponding axis value. These will help correct the irregular curvature in your eyes.
4. Check the ADD value: If you have presbyopia, the ADD value will indicate the additional power needed for reading or close work.
5. Review Prism Values: If your prescription includes prism correction, note the prism values for each eye, along with the direction (BU, BD, BI, BO). This ensures that your glasses will properly correct any alignment issues.
6. Measure the PD: Ensure the PD is listed, as this measurement is crucial for properly aligning your lenses with your eyes.

Let’s look at an example prescription to put this into practice:

• OD: -2.00 SPH, -1.00 CYL, 90 Axis, 1.0 BI
• OS: -1.50 SPH, -0.75 CYL, 80 Axis, 0.5 BO
• ADD: +1.50
• PD: 63 mm

For the right eye (OD), the prescription indicates moderate nearsightedness (-2.00 SPH) with mild astigmatism (-1.00 CYL) at a 90-degree axis. There is also a prism correction of 1.0 diopters base in (BI) to help with eye alignment. The left eye (OS) shows mild nearsightedness (-1.50 SPH) with slight astigmatism (-0.75 CYL) at an 80-degree axis and a prism correction of 0.5 diopters base out (BO). The ADD value (+1.50) indicates additional reading correction, and the PD (63 mm) ensures proper lens alignment.

How to Convert Axis Values

When you are converting your glasses prescription to a contact lens prescription, particularly if you have astigmatism, you may need to convert the axis value. This is important when dealing with different types of lenses (such as toric contact lenses for astigmatism) or for making other adjustments to your prescription.

Here’s a simple guide on how to convert axis values:

1. Axis Conversion Formula: If the CYL value in your prescription changes from a positive to a negative value, or vice versa, you need to convert the axis by either adding or subtracting 90 degrees.

   • If the original axis is less than 90 degrees, add 90 degrees to the value.
   • If the original axis is greater than 90 degrees, subtract 90 degrees from the value.

2. Example of Axis Conversion:
   Let's say your original glasses prescription has an axis of 30 degrees and a +1.00 CYL value. If you are converting this prescription to a form where the CYL value becomes negative (e.g., -1.00), you would adjust the axis as follows:

   • Original Axis: 30 degrees
   • New Axis: 30 + 90 = 120 degrees

   Now, if your original axis were 120 degrees and you were converting the CYL to a negative value, the axis would be:

   • Original Axis: 120 degrees
   • New Axis: 120 - 90 = 30 degrees

   This conversion ensures that the astigmatism correction remains properly aligned with the irregular curvature of your eye, despite the shift in the sign of the CYL value.

3. Why Axis Conversion Is Necessary:
   This conversion is necessary when switching between different lens types because the orientation of the cylindrical correction (CYL) needs to match the shape of your cornea. Maintaining the correct axis ensures that your lenses can correct the astigmatism accurately, even if the method of correction (such as in contact lenses) is slightly different from that in glasses.

Frequently Asked Questions

What Does It Mean if I Have a Negative or Positive SPH?

A negative SPH value means you are nearsighted and have difficulty seeing distant objects clearly. A positive SPH value indicates you are farsighted and struggle with seeing close objects.

What Is Prism Correction and When Is It Used?

Prism correction is used when the eyes are not properly aligned, causing conditions like double vision. The prism in the lenses helps to shift the image to align properly, ensuring clear and unified vision.

How Often Should I Get My Eyes Checked?

It is generally recommended to have an eye exam every one to two years, depending on your age, health, and whether you wear glasses or contact lenses. Regular check-ups are important for maintaining eye health and updating your prescription as needed.

Can My Prescription Change Over Time?

Yes, your prescription can change over time due to factors such as age, health conditions, and lifestyle changes. Regular eye exams help monitor these changes and ensure your vision correction is up-to-date.