How to Read an Eyeglass Prescription and Measuring you PD?

Prescription Glasses Online
January 21, 2019
Published By Skye Spencer

How to? Your Prescription and What is PD?

Prescription Glasses have evolved over centuries and serve more than just correcting vision. They are a leading fashion accessory that can denote your personality.

Gone are the days when only one pair of designer glasses is all you needed as regulars, now we have several pairs to brace different outfits and occasions. And that is regardless of whether you are nearsighted, farsighted or have astigmatism.

Even readers have got a revamp in the latest designs of mens and womens glasses. But while we indulge in the latest trends of 2019 of prescription eyewear, how often do we think about reading our own prescription?

You might have visited the optometrist a couple of times or perhaps it's your first time, but knowing what those numbers and letters mean on your prescription is something that most of us miss!

Here is all you need to know about reading your prescription and what it means? So, you can buy glasses online as quickly as possible!

Eye Exam:

We all have been recommended to take a yearly eye-exam, just to keep your eyes in check. This happens at your local optician or an optometrist. When you finish your regular eye test, you realize that your eyes needs have changed.

This is where you get a prescription from the eye expert detailing your eye condition. That's when we know if we need to get prescription glasses to correct short/long sightedness, astigmatism or presbyopia.

OS and OD?

This is the primary step to understanding your prescription. OS stands for oculus sinister and OD stands for oculus dexter, which are Latin words that represent your right and left eye.

OS= left eye

OD= right eye

Your prescription may also have OU mentioned, this stands for oculus uterque and means both eyes. While, this is the traditional method of writing an eye prescription some modern doctors use RE and LE, for right and left eye respectively.

Designer Glasses Online

Other Terms on Your Prescription:

These terms on your eyeglass prescription determine how your lenses should be cut, crafted and finished to give you that 20/20 vision. Say if you are getting reading mens glasses online and choose bifocals, you need to know have two powers to input in your buying form. One is for distance and other is your reading prescription.


This is the degree of correction required in your lenses to correct far or nearsightedness. It is measured in DP (diopters). If the number preceding the abbreviation SPH is (+) then you have farsightedness, and if the number is (-) then you are short-sightedness. If there is nothing mentioned before SPH, then you are farsighted. Spherical stands for an equal meridian distance of the eye.


This term CYL is specifically used for people who have astigmatism. It indicated the amount of lens power you need to correct astigmatism. If there is nothing mentioned in the CYL column you either have no astigmatism or it is so less that it need not be mentioned. The correction of astigmatism is based on the meridian and the lens curvature.

Just like SPH, the (+) sign is for far-sighted astigmatism and (-) is for nearsighted astigmatism.


Axis is measured in degrees from 1 to 180. 90 represents the vertical meridian of the eye and 180 represents the horizontal meridian of the eye. This is to determine the orientation axis of astigmatism. If your prescription includes the CYL power then it must also have AXS power, denoted by an ‘x’


ADD is specifically for presbyopia or reading vision that is related to the aging of the eyeball. This is the power used on the bottom lens of a bifocal or multifocal. Since presbyopia is corrected by magnifying vision, it will always be written with a (+) sign and even if it is not mentioned it is taken into as (+) power only.


This is measured in diopters and is measured to cover for any eye alignment concerns. It is written as p.d. Or a superscript triangle is drawn freehand. The amount of prism is indicated by fractional units like ½, and the direction of the prism is noted by reference to the base.


You will see abbreviations like BU (base up), BD ( Base down), BI (Base In) and BO (base out). The out and in are in reference the eyeglass wearers nose for in and ears for out, to calculate direction accurately. This is for the Prism measurement of the eye.

What is PD?

This stands for pupillary distance and measures the distance between the pupil of one eye to the other. In reference to frames or glasses, it is measured from the centre of the prescription lenses to the other, to ensure the prescription glasses correct your vision right and the size is right for you.

Cylindrical, Spherical and Add power are always written in diopters, prism will be written in decimals and axis are whole numbers between 1 to 180.

Now that you know all the terms that are mentioned on your eyeglass prescription, you have learnt the most important part to buy prescription sunglasses UK or prescription glasses online at Perfect Glasses!

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