What does your glasses prescription tell you?
Introduction:
If you have ever undergone an eye examination, you should have been given a copy of the prescription at the end of the exam. It will be filled with an array of numbers, abbreviations, and symbols which may initially appear confusing. This explanation should help you understand it a little better. I firmly believe that optometrists should explain as much as possible at the end of the eye examination and include the client in decision making. That includes an explanation of any differences in spectacle prescription that we find.
Sphere (SPH):
The “SPH” value indicates if you are short-sighted (myopic) with a – value, or long-sighted (hypermetropic) with a + value. The higher the number the higher the strength of lenses required. For the more technical minded, this is measured in dioptres (D) which is a reciprocal metre scale https://en.wikipedia.org/wiki/Dioptre Everybody else can happily ignore that!
Cylinder (CYL):
The “CYL” component accounts for astigmatism, a common condition resulting in blurred vision due to an irregularly shaped cornea. Think of the surface of a rugby ball with different curvature horizontally and vertically. Again, the higher the value the greater the astigmatism you have.
Axis:
The “Axis” measurement complements the CYL value and denotes the direction of the astigmatism correction. To carry on the rugby ball analogy, think of the ball lying flat – that’s 0 degrees; if the ball (eye’s astigmatism) is not flat then the axis measures the angle to the horizontal.
Add or Near Vision Prescription:
For those (usually aged 45 and over) requiring vision correction specifically for reading or close-up tasks, an “Add” value is found in the prescription. This indicates the extra power that needs to be added to the SPH to see close up. This can be provided as the lower portion of bifocal or progressive lenses or as separate single vision glasses that the client can only use for close work as distance vision will be blurred with them. Important to note that this is not long sight, but for the condition called presbyopia.
Prism (PRISM):
On occasion, a prescription may include a “PRISM” value. Prism prescriptions address issues of eye alignment, such as imbalances in the eye muscles or double vision. The PRISM measurement is denoted in prism dioptres (Δ) and describes the direction and magnitude of the prismatic correction needed.
What is not included – the pupillary distance measurement (Pd):
Occasionally we get asked for this by clients who want to buy glasses online. It stands for (inter) pupillary distance and is used to position the optical centre of the spectacle lens correctly. However, it is only one of several measurement that are taken which are unique to the frame that is going to be used. Or to put it another way, until we see the frame that you’re going to wear we cannot measure it accurately so the Pd cannot be part of the prescription.
Conclusion:
Understanding your glasses prescription helps you to make well-informed decisions regarding your vision correction needs. It will help you effectively communicate with your optometrist and ensure the provision of the best eyewear for your eyes. Remember, your prescription is distinctive to you, and a professional eye examination is vital for precise measurements and diagnosis.
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