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The beginner’s guide to reading an eyeglass prescription
Understanding the basics of your prescription

Eyeglass prescriptions are intimidating. They use lots of obscure abbreviations and puzzling numbers that have the capacity to improve your vision. And that, if we’re being honest, might as well be witchcraft.

But fear no more! We put together a short guide that should help you decipher your very own optic magic spell.

1. It is very common (and perfectly normal!) for each eye to have different corrective needs. That is why your prescription needs to specify which eye is in question. The left eye is identified as OS and the right eye as OD (from the Latin Oculus Sinister and Oculus Dexter).

2. The sphere (SPH) refers to the degree of power required for distance or reading glasses. This type of power is distributed evenly all over the lens, and the number will be accompanied by a plus or a minus sign to indicate, respectively, farsightedness and nearsightedness. If your prescription needs to correct farsightedness in addition to nearsightedness, it will be marked as ADD, as in “add power”. Pretty straightforward, right?

3. The cylinder (CYL) refers to the power required to correct astigmatism. Different degrees of power will be assigned to different areas of the lens, along an AXIS. The number associated with the axis refers to the degree of rotation necessary to distribute power with precision.

4. The pupillary distance (PD) is the measured distance between pupils. This information is necessary to align the centre of your lenses with the centre of your eyes.

5. Finally, some prescriptions include the base direction of a PRISM. It is used to align lenses to ensure that both eyes are working together. The different directions are identified as follows: base up (BU), base down (BD), base in (BI) and base out (BO).

Ultimately, each prescription is unique, and you should always turn to a duly registered eye care professional for specific information regarding your own eye health. We hope we were able to demystify some of the main elements of your prescription!

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