How To Read Your Eyeglass Prescription | Prescription Guide
Estimated Reading Time: 5-10 minutes
If you have gotten an eye exam in the past, either from an optometrist or ophthalmologist, you’re usually given a piece of paper (at least the good ones) at the end of the exam. This paper contains a bunch of abbreviated terms and numbers that they call your eyeglass prescription.
For most people, you look at it and you have no clue where to begin.
You may have heard the terms nearsightedness, farsightedness, astigmatism, and progressives but have no way to determine how that relates to your prescription.
This article will provide a quick and easy guide for reading your eyeglass prescription. Hopefully, it will give you a better understanding of what’s going on between the lines.
Right and Left eye: How can I tell which is which?
When you look at your prescription, you’re probably wondering where to start. Some people may not even bother and let their Eye Care Professional deal with it.
But clearly, if you’ve actively clicked this link then you’re a little bit curious or even adamant at finally being able to read your prescription (kudos to you).
The first thing is to determine the Right and Left eye prescriptions. If you look on the left-hand side of the sheet, there are abbreviated terms. You will see O.D., O.S., and sometimes O.U.
These terms are indicators for the right and left eye:
O.D. = “oculus dexter” Latin for RIGHT eye
O.S. = “oculus sinister” Latin for LEFT eye
O.U. = “oculus uterque” Latin for BOTH eyes
Why not just label them left and right?
Similar to many terminologies in medicine, anatomy, and law, they are Latin based subjects that still use the original terms even to this day.
There are some optometry practices that have opted out of Latin based terminology and have made it simpler by using terms like RE for “right eye” and LE for “left eye”
Farsighted, nearsighted, and astigmatism: the other parts of your eyeglass prescription
Now that you can tell the difference between your left and right eye, you can now begin to figure out what the numbers depict in your prescription.
This portion will determine if you are nearsighted or farsighted. The numbers will be displayed as either "+" or "-". A "+" will indicate that you are farsighted and conversely, a "-" will indicate nearsightedness.
The number that follows the "+/-" symbol is the amount of lens power needed to correct your vision. The 0 acts as a baseline and indicates no spherical correction is needed. The farther the number deviates from 0 (either "+/-") the stronger the prescribed power. Lens powers are measured in diopters.
This section of your eyeglass prescription will identify the astigmatism correction you may need. There will be cases where this portion will be left blank or will have the terms “Sph” (only spherical correction needed) or “DS” (diopter sphere). This just means that you have little or no astigmatism that needs correction.
The axis value works in conjunction with your cylinder power. The axis value ranges from 1-180 (measured in degrees) and signifies how much the lens should be rotated to correct blurriness caused by astigmatism.
Correcting vision problems associated with astigmatism requires a corresponding axis value. Without the axis value, the lens power will not be in the correct position.
Rotating the lens 90 degrees away from the axis value will give you the total power of the lens. You can then identify the cylinder power by subtracting the sphere power to the total power of the lens.
Cylinder Power = Total Power - Sphere Power
For the average person, you will rarely have the opportunity to measure your lens power with a lensometer.
I mean why would you?
It’s more for the purpose of awareness with your prescription. If you see that there is a value placed in the cylinder but none in the axis (and vice versa) then there must be an error somewhere in the way the prescription is written.
Presbyopia patients require both distance and reading corrections. The ADD or Addition is the prescribed magnifying power for the lower portion of a multifocal lens. The value is always a "+" power and is implied even when there is no "+". The ADD power will always match for both eyes.
Nowadays most people will opt-in for a pair of progressive lenses also known as "no-line bifocals". Progressive lenses are versatile because they have 3 distance zones (distance, intermediate, and reading); perfect for day-to-day activities. They are also popular because they have better aesthetics with no hard lines visible on the lens.
Other multifocal lens designs:
This section will usually be left blank or omitted for most people. Only a small percentage of prescribed vision correction will include prism.
It represents the prismatic power needed to correct eye alignment issues. It’s measured in prism diopters and may appear as “p.d” or a superscript triangle followed by a numerical value.
While sphere and cylinder are measured in increments of 0.25D; prism is listed in increments of 0.5D. Prism values are followed by a direction that indicates where the lens should be placed for optimal correction.
Prism directions are abbreviated and are listed as follows:
BU = Base Up
BD = Base Down
BI = Base In (towards the nose)
BO = Base Out (towards the ear)
Contact Lens Prescriptions
There is a distinction between a contact lens prescription and an eyeglass prescription. A common misconception is to assume that they are interchangeable. You cannot purchase contact lenses with your eyeglass prescription.
The distance of the corrective lens relative to the position of the eye is important
The distance between your eye and the corrective lens, whether it's a contact lens or eyeglass lens, makes a big difference in refractive power. Contact lenses rest directly on your eye while your eyeglass lenses are typically 12mm from your eye.
Contact lenses will also have unique information such as the base curve and diameter of the lens. During a contact lens exam, your eye doctor will take measurements of your eye to ensure that the lens fits on your eye comfortably.
Your eye doctor will usually include recommendations for your lenses included in your eyeglass prescription. These can be specific lens brands, material, design, and treatments.
For the most part, unless otherwise stated by your doctor, they are suggestions on products that will give you the most comfort when wearing your glasses.
You may choose to opt-out of these suggestions due to economical restrictions or personal preference. Always consult with your eye doctor first before making these decisions.
Your eyeglass prescription is yours to keep after a comprehensive eye exam. The Optometry Practice is required by law to provide you with a copy of your prescription after the eye exam.
Your eye doctor should give you a copy of your prescription regardless of whether you have asked for it or not.
You are not obligated to purchase prescription eyewear directly from your eye doctor and they do not have a right to charge you for the release of your prescription.
This gives you the freedom to choose the optical retailer you would like to buy your glasses from.
Getting a better understanding of your eyeglass prescription will help you in making an informed decision about your corrective eyewear. It can also make the process of getting glasses a lot smoother and more enjoyable. (Because we all love shopping for glasses!)
It will save you a lot of time and potential frustration by being aware of key elements to consider when shopping for ophthalmic frames and prescription lenses. You will also be able to identify potential errors in the prescription which happens from time to time. (We’re all human)
It’s always best to open the conversation with your eye doctor, optician, and optical retailer to ensure you are getting the best vision correction for your specific needs. They will be more than happy to help and with a deeper knowledge of your prescription you can engage in a better dialogue.