how to read eye prescription

“Could you please clarify which numbers on my prescription correspond to my right eye and which to my left eye?”

When you receive an eye prescription, it contains numbers and abbreviations that can be confusing at first. One of the most important things to understand is which numbers correspond to your right eye and which to your left eye. Knowing this distinction is essential when ordering corrective lenses for glasses or contact lenses.

Here’s a breakdown of how to identify which part of your prescription refers to each eye:

1. Understanding OD and OS:

On your eye prescription, you will typically see the abbreviations OD and OS. These are Latin terms that are commonly used in the medical field to specify each eye:

• OD stands for Oculus Dexter, which refers to your right eye.
• OS stands for Oculus Sinister, which refers to your left eye.

Some modern prescriptions may use simpler terms, like “Right” and “Left” instead of OD and OS, but the principle is the same. Knowing this will help you correctly interpret which values correspond to which eye.

2. Prescription Format:

An eye prescription typically includes several components: sphere (SPH), cylinder (CYL), axis, ADD (for reading prescriptions), and sometimes prism and base. These elements are usually listed in separate rows for each eye, with OD and OS at the start of the rows to indicate whether the following numbers are for the right eye (OD) or left eye (OS).

For example, a prescription might look like this:

In this case:

• The first row (OD) corresponds to the right eye.
• The second row (OS) corresponds to the left eye.

3. Sphere (SPH):

The Sphere (SPH) value in your prescription shows the amount of correction needed for nearsightedness or farsightedness. It’s important to note which number corresponds to which eye. In the example above:

• For the right eye (OD), the SPH value is -2.00, which means you have mild nearsightedness.
• For the left eye (OS), the SPH value is -1.50, indicating slightly less nearsightedness compared to the right eye.

4. Cylinder (CYL) and Axis:

If you have astigmatism, the prescription will also include a Cylinder (CYL) and Axis value, which help correct the uneven curvature of your eye.

• In the example above, for the right eye (OD), the CYL value is -1.00, and the Axis is 90, meaning you have astigmatism that requires correction at a 90-degree angle.
• For the left eye (OS), the CYL is -0.75, and the Axis is 80, so the correction is applied at a slightly different angle.

If you don’t have astigmatism, these fields might be left blank or marked as “DS” (diopters sphere), indicating no astigmatism correction is needed.

5. ADD (Near Addition):

For those who require bifocals or multifocal lenses to assist with both distance and near vision (typically for conditions like presbyopia), there will be an ADD value on the prescription. This shows how much additional magnifying power is needed for near vision tasks like reading.

In our example, both the right eye (OD) and left eye (OS) have an ADD value of +1.50, meaning the same magnification is needed for close work in both eyes.

6. Prism and Base (if applicable):

If your prescription includes values for prism and base, these are used to correct issues with eye alignment (like double vision or strabismus).

The prism value will indicate how much correction is needed, while the base specifies the direction of the prism correction (up, down, in, or out). Like the other elements of the prescription, these values are listed separately for each eye under the OD and OS headings.

7. Pupillary Distance (PD):

Though not always included on the prescription itself, your Pupillary Distance (PD) is another important measurement. PD refers to the distance between the centers of your pupils, measured in millimeters. When ordering glasses, this ensures the lenses are aligned correctly with your eyes.

Some prescriptions may include separate PD measurements for each eye, especially if the distance differs. In this case, it might be listed as “OD PD” and “OS PD.” If it’s a single value, that’s your total PD, and it applies equally to both eyes.

8. Contact Lens Prescriptions:

It’s worth noting that if you wear contact lenses, your contact lens prescription will be different from your glasses prescription. Contacts sit directly on the surface of your eye, while glasses sit about 12 millimeters away. For contacts, you’ll need additional information like base curve and diameter to ensure a proper fit.

Understanding the Eye Prescription Scale

An eye prescription is a set of numbers and terms that describe the type and degree of vision correction you need. The prescription you receive from an eye doctor after an eye exam contains specific values measured in diopters, which represent the strength of lenses required to correct your vision problems. These numbers might seem confusing at first, but they are essential for ensuring that your glasses or contact lenses are tailored to your unique vision needs.

This guide will explain how to understand the eye prescription scale, the meaning of the numbers, and how they correspond to different types of vision problems such as nearsightedness, farsightedness, and astigmatism.

1. Diopters: The Basic Unit of Measure

At the core of an eye prescription is the unit of measurement called the diopter (D). Diopters measure the refractive power of your lenses—the amount of correction needed to focus light properly on your retina, giving you clear vision.

• Positive diopters (+) indicate that the prescription is for farsightedness (hyperopia).
• Negative diopters (-) indicate that the prescription is for nearsightedness (myopia).

The larger the number (whether positive or negative), the stronger the correction needed. For example, a prescription of +1.00D indicates mild farsightedness, while -5.00D indicates more severe nearsightedness. The further the number is from zero, the more vision correction is required.

Example:

• -1.00D: Mild nearsightedness.
• -3.00D: Moderate nearsightedness.
• -7.00D: Severe nearsightedness.
• +1.50D: Mild farsightedness.
• +4.00D: Moderate farsightedness.

2. Key Elements of an Eye Prescription

An eye prescription typically includes several components that detail the specific type and degree of vision correction needed for each eye. These elements include:

• SPH (Sphere): This value indicates the overall strength of the lens needed for your prescription, whether for nearsightedness or farsightedness.
• CYL (Cylinder): If you have astigmatism, the cylinder value represents the amount of lens power required to correct it. Astigmatism is a condition where the eye is not perfectly round, causing blurred or distorted vision.
• Axis: This value describes the angle (from 0 to 180 degrees) at which the cylindrical correction for astigmatism should be applied.
• ADD (Near Addition): For people with presbyopia (age-related difficulty focusing on close objects), this number shows how much additional lens power is needed for reading or close-up tasks.
• Prism and Base: These values are less common and are used to correct problems with eye alignment, such as double vision.

Each of these elements appears as a number on your prescription, and understanding their meanings is important for interpreting your eye prescription correctly.

3. Interpreting the Prescription Scale for Nearsightedness (Myopia)

In people with myopia (nearsightedness), distant objects appear blurry because light entering the eye focuses in front of the retina instead of directly on it. To correct this, lenses that spread (diverge) the light are used, which is why prescriptions for nearsightedness are written with negative (-) diopter values.

The more negative the number, the greater the degree of nearsightedness:

• Mild Myopia: -0.25D to -3.00D
• Moderate Myopia: -3.25D to -6.00D
• Severe Myopia: -6.25D and beyond

Example:

If your prescription reads OD -2.50, it means that your right eye (OD) has mild myopia, and lenses with a power of -2.50 diopters are needed to correct your vision.

4. Interpreting the Prescription Scale for Farsightedness (Hyperopia)

For people with hyperopia (farsightedness), nearby objects appear blurry because light entering the eye focuses behind the retina. To correct this, lenses that converge (focus) light are used, and prescriptions for farsightedness are written with positive (+) diopter values.

The higher the positive number, the greater the degree of farsightedness:

• Mild Hyperopia: +0.25D to +2.00D
• Moderate Hyperopia: +2.25D to +5.00D
• Severe Hyperopia: +5.25D and beyond

Example:

If your prescription reads OS +3.50, it means that your left eye (OS) has moderate hyperopia, and lenses with a power of +3.50 diopters are required to help you see nearby objects more clearly.

5. Astigmatism: Understanding Cylinder (CYL) and Axis

Astigmatism occurs when the cornea or lens of the eye is irregularly shaped, causing light to be focused unevenly. This leads to blurry or distorted vision at any distance. To correct astigmatism, a cylindrical lens is used to compensate for the uneven curvature.

• Cylinder (CYL): This value on your prescription represents the amount of correction needed for astigmatism, measured in diopters. The cylinder can be a positive (+) or negative (-) value, depending on the type of astigmatism. If there is no cylinder value, it means you don’t have significant astigmatism.
• Axis: The axis is a number between 0 and 180 degrees that specifies the orientation of the cylindrical correction. This value is only present if there is a cylinder value. It tells the lens maker where to apply the cylindrical correction on the lens to counteract the uneven curvature.

Example:

If your prescription reads CYL -1.25, Axis 90 for your right eye (OD), it means you have astigmatism in that eye that requires a cylindrical correction of -1.25 diopters applied along the 90-degree axis.

6. Presbyopia: Reading Addition (ADD) Values

As we age, many people develop presbyopia, which makes it harder to focus on close-up objects. For people who require glasses for both distance and near vision, a prescription may include an ADD (Near Addition) value. This is always a positive (+) number that represents how much extra magnification is needed for reading or other close work.

ADD values are typically between +0.75D and +3.00D, and the same value usually applies to both eyes.

Example:

If your prescription includes ADD +2.00, it means that 2 diopters of magnification are needed to help you focus on nearby objects.

7. Prism and Base: Correcting Eye Alignment

Some prescriptions include a Prism and Base value to correct eye alignment issues like double vision or strabismus. The prism value is measured in diopters and indicates the amount of lens power needed to correct the alignment. The base value specifies the direction of the prism correction (up, down, in, or out).

This is less common but important for people who have eye coordination problems that make it difficult for both eyes to focus on the same object.

What Is Considered a “Strong” or “Weak” Prescription?

The strength of your prescription is often a source of concern, but it’s important to note that even high diopter numbers can be effectively corrected with glasses or contact lenses.

Nearsightedness (Myopia):

• Mild myopia: Between -0.25D and -3.00D.
• Moderate myopia: Between -3.25D and -6.00D.
• Severe myopia: Greater than -6.00D.

Farsightedness (Hyperopia):

• Mild hyperopia: Between +0.25D and +2.00D.
• Moderate hyperopia: Between +2.25D and +5.00D.
• Severe hyperopia: Greater than +5.00D.

Astigmatism:

• Mild astigmatism: Cylinder values up to -1.00D or +1.00D.
• Moderate astigmatism: Cylinder values between -1.25D and -2.50D or their positive counterparts.
• Severe astigmatism: Cylinder values greater than -2.50D or +2.50D.

Presbyopia:

• ADD values typically range from +0.75D to +3.00D, indicating how much additional correction is needed for close vision tasks like reading.

So, a higher diopter number (positive or negative) does indicate a “stronger” prescription, but this doesn’t mean your vision is bad or unmanageable. Modern corrective lenses and treatments can accommodate even severe vision issues with ease.

What If You Have a “High” Prescription?

If you have a high prescription, you may worry about your vision deteriorating or whether this means you’ll have long-term problems. While a high prescription does indicate a more significant need for vision correction, it does not necessarily mean your eyes are “bad.” It simply means your eyes require more help to focus properly.

Nearsightedness:

High levels of nearsightedness (greater than -6.00D) are more common than ever, partly due to lifestyle factors like increased screen time and less outdoor activity. While high myopia can increase the risk of certain eye health conditions like retinal detachment or glaucoma, regular eye exams and proper vision correction can help manage these risks.

Farsightedness:

A high prescription for hyperopia (greater than +5.00D) means that nearby objects appear blurry, and you need strong lenses to focus on them. Farsighted individuals with high prescriptions may experience eye strain or headaches when reading without glasses, but corrective lenses can resolve these issues.

Astigmatism:

If you have high astigmatism (greater than -2.50D or +2.50D), it means that the curvature of your eye is significantly irregular. Corrective lenses are designed to address this uneven curvature and help you see clearly. Most modern lenses, including toric lenses for contact lens wearers, can handle even high levels of astigmatism.

Does a “Bad” Prescription Mean Eye Disease?

Many people mistakenly believe that having a “bad” prescription means they’re at risk for serious eye diseases. In reality, while high prescriptions can sometimes be associated with certain conditions (like high myopia and increased risk of retinal problems), a prescription in itself is not a diagnosis of an eye disease. Your prescription simply indicates the level of vision correction you need.

Common Misconceptions:

• A high prescription means you’re going blind: This is a myth. While higher prescriptions may come with some additional risks, regular eye checkups can ensure early detection of any complications.
• If my prescription keeps changing, my eyes are deteriorating: It’s normal for prescriptions to change over time, especially during childhood and early adulthood. Regular eye exams help track changes and adjust your lenses accordingly.

6. How to Take Care of Your Eyes with a Strong Prescription

Even if your prescription is higher than average, there are steps you can take to maintain good eye health:

• Annual Eye Exams: Regular checkups ensure your prescription is up-to-date and can help detect any early signs of eye diseases like glaucoma or cataracts.
• Proper Vision Correction: Always wear the correct lenses as prescribed by your optometrist, whether they are glasses, contact lenses, or specialty lenses like bifocals or progressives.
• Protect Your Eyes: Wearing sunglasses to shield your eyes from harmful UV rays and taking breaks during long periods of screen use can prevent eye strain.
• Healthy Lifestyle: Maintaining a balanced diet, exercising regularly, and avoiding smoking all contribute to better overall eye health.

Conclusion: Your Prescription Isn’t “Bad” — It’s Just Specific to You

The most important thing to remember is that there is no “bad” eye prescription. Your prescription is simply a reflection of the specific vision correction you need, and modern corrective lenses can accommodate a wide range of vision problems. Whether you have mild or severe nearsightedness, farsightedness, or astigmatism, your prescription allows you to see the world clearly and comfortably.

Regular eye exams, proper lens care, and attention to overall eye health will help you maintain the best vision possible, regardless of the strength of your prescription.