Eyeglasses prescriptions differ as a result of ophthalmologists' use of the positive (+) cylinder notation, whereas optometrists utilize the negative (-) cylinder notation.
Ever felt puzzled by your eyeglasses prescription? The mix of numbers, abbreviations, and symbols can be overwhelming. Further, different eye doctors --optometrists and ophthalmologists-- may use different prescription writing approaches, leading to more confusion.Â
Let's unravel the historical and practical reasons behind these differences and explore the key elements of your eye prescription.Â
Eyeglasses prescriptions differ as a result of utilizing different notation systems
When you visit different eye doctors, particularly if one is an optometrist and the other is an ophthalmologist, you may notice variations in your prescription numbers. This could lead to confusion and raise questions about potential mistakes. However, these differences often arise from distinct prescription writing approaches, and the prescriptions may actually be equivalent.
Ophthalmologists commonly use a positive (+) cylinder notation when writing prescriptions, whereas optometrists utilize a negative (-) cylinder notation. This disparity in cylinder notation can result in the mentioned variations. It's important to understand that despite these differences, the vision correction you receive will remain the same.1
The historical and practical reasons behind these two approaches are quite fascinating. In the 1800s, cylinders were denoted with a plus (+) sign because at that time, the cylinder was positioned on the front of lenses, with spheres on the back. However, in the 1900s, it was discovered that optically, it was more effective to have the cylinder on the back of the lenses, leading to the adoption of minus notation for cylinders.1
Fast forward to today, with two systems in place, optometrists generally prefer using negative (-) cylinder notation when prescribing glasses due to the convenience of lens cutting by eyeglass labs and opticians. However, for surgical planning purposes, plus (+) cylinder notation may be more useful. For instance, incisions made at the plus cylinder axis can help reduce astigmatism. This distinction might explain the varying preferences in prescription writing approaches between optometrists and ophthalmologists.1
How to interpret an eyeglasses prescription?
OD, OS, and OU: On your eye prescription, you may come across abbreviations such as OD and OS. OD stands for "oculus dexter," which refers to the right eye, while OS stands for "oculus sinister," representing the left eye. If you see a column labeled OU, it means "oculus uterque," indicating both eyes.
Diopters: Diopters are a unit of measurement used to quantify the refractive power of lenses. When you see the abbreviation "D" on your eye prescription, it refers to diopters. This measurement plays a crucial role in determining the optical correction required to address refractive errors.
SPH (Sphere): When reviewing your eye prescription, you will often come across the term "sphere" or "spherical error." The SPH value indicates the lens power required to correct myopia (nearsightedness) or hyperopia (farsightedness). A negative value (-) indicates myopia, while a positive value (+) suggests hyperopia.
CYL (Cylinder) and AXIS: If you have been diagnosed with astigmatism, your prescription may include the terms CYL and Axis. The cylinder value, also known as cylindrical error, represents the amount of lens power needed to correct astigmatism.
The Axis value, measured in degrees, indicates the orientation of the astigmatism correction. For instance, "against-the-rule" astigmatism is commonly observed in older individuals and those with cataracts. In the (-) cylinder notation, the Axis value typically ranges between 60 to 120 degrees. Conversely, "with-the-rule" astigmatism is often found in younger patients and those with myopia, with an Axis value between 0 to 30 degrees or 150 to 180 degrees.
Note:Â If cylinder correction is not required, a prescription may note âSPH,â âDS,â or âSphericalâ in the CYL column.
ADD: Progressive and bifocal prescriptions may include a value in the ADD column. This value represents the additional power needed, typically for reading or close-up vision. It is commonly included in prescriptions for individuals with presbyopia or age-related farsightedness. The ADD value is usually the same for both eyes and may range from +0.50 to +3.50.
PRISM and BASE: Prism corrections are used in eyeglasses for individuals experiencing double vision due to conditions like strabismus, Graves' disease, or cataracts. Prism assists in aligning the two images so that only one image is perceived. The amount of prism correction is measured in prism diopters (â), and it is accompanied by directions known as the base, such as UP (BU), DOWN (BD), IN (BI), and OUT (BO).
Pupillary Distance (PD): Pupillary Distance, abbreviated as PD, measures the distance between the centers of your pupils in millimeters and is essential for proper lens fitting. The average adult's PD typically ranges between 54-74 mm and can be measured manually or using a specialized device called a pupillometer.
Ensuring an accurate PD measurement could help ensure you will be looking through the ideal spot in your lenses. If the pupillary distance is inaccurate, the prescription power of your lenses will be off-center from your eyes, leading to compromised visual clarity.
Note: In a contact lens prescription, three additional pieces of information are included: base curvature (BC), the diameter of the lens (DIA), and the brand of the contact lens.Â
How to convert an eyeglasses prescription?
To convert an eyeglasses prescription, you can follow a simple three-step process. It's important to note that while prescriptions may be written differently, in the end, they are the same. Here's how to convert a prescription:4
- Combine the sphere and cylinder values to determine the new sphere value.
- Change the sign of the cylinder while maintaining its power.
- Adjust the axis value by adding or subtracting 90 degrees (max. axis value is 180 degrees).
Let's use an example to illustrate this conversion process. Suppose the original prescription for the right eye is written as SPH -2.50 D, CYL -1.00 D, Axis 95 degrees, ADD +1.00 D, PRISM 1.0â, BO. We can convert this prescription as follows:
- Combine the sphere and cylinder values: -2.50 D + (-1.00 D) = -3.50 D.
- Change the sign of the cylinder: +1.00 D.
- Adjust the axis value: 95 degrees - 90 degrees = 5 degrees.
Therefore, the final converted prescription in positive notation would be: SPH -3.50 D, CYL +1.00 D, Axis 5 degrees, ADD +1.00 D, PRISM 1.0â, BO.
Key takeaways
Having a clear understanding of how to read your eye prescription empowers you to stay well-informed about your vision health. Furthermore, it is important to be aware of the two prescription writing approaches commonly used: the negative (-) cylinder notion preferred by optometrists and the positive (+) cylinder by ophthalmologists. Despite these variances, converting between the two notations is a straightforward three-step process. Knowledge of these can enable you to navigate communication with different eye care professionals and facilitate your discussion regarding your vision needs.
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