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The Ultimate Guide To Actually Understanding Your Eye Prescription

You will be able to buy your favorite glasses and googles with utmost confidence after reading this thorough breakdown of all the technical terms on your eye prescription.


Reading Time: 10 minutes


If you’re in a rush, click one of the titles below now to get the exact information that you need:



Let’s start with the basics…


  1. ?



  • Example of a standard eye prescription. 


Eye prescriptions reflect the eyes measurements needed to build bespoke corrective lenses to fit your vision.


Lens manufacturers use your eye prescription, written by eye doctors, to make lenses that correct deficiencies in your eyesight.


You’ll struggle to see through a friend’s prescription glasses – especially, if they have very bad eyesight – because it was made for them, not you.




The measurements displayed on an eye prescription are taken from a prior eye examination operated by your eye doctor (also called an orthoptist, optometrist, optician or an ophthalmologist)



Standard eye examinations consist of numerous tests made by your ophthalmologist to gain a general assessment of your vision and the health of your eyes.


Everyone is encouraged to have regular eye exams to help avoid eye diseases and vision loss. Eye exams can test your visual acuity, pupil function, extraocular muscle motility, visual fields, intraocular pressure and ophthalmoscopy through a dilated pupil.


Only half of the estimated 61 million American adults deemed at high risk for vision loss have visited an eye doctor in the past 12 months, according to the Centers for Disease Control and Prevention (CDC).


  • ?   



  • The O.D. and O.S. categories are highlighted in yellow.  


O.D. is your right eye and O.S. is your left eye. Both are Latin abbreviations, O.D. means ‘Oculus Dexter’ and O.S. stands for ‘Oculus Sinister’.  Usually, O.D. and O.S. will appear on all your eye prescriptions, whether it be for glasses, goggles, contact lenses or medication. Recently, some doctors have started to issue prescriptions with ‘R’ and ‘L’ or ‘RE’ and ‘LE’, meaning right eye and left in plain English.


O.D. will be mentioned first on your prescription followed by the O.S., as standard practice. This is because, during eye examinations, your eye doctors will check your right eye (from their left side) first.


  • O.D means right eye and O.S. refers to the left eye


Sometimes, you may instead see O.U., which means ‘both eyes’ and is an abbreviation of ‘Oculus Uterque’. In other words, the measurement given applies to both eyes equally.


  • The Pupillary Distance (PD) is highlighted in yellow.


The pupillary distance, also known as the PD, concerns your pupils – the black, rounded center of each eye.  



The PD is the exact distance measured in millimeters between the center of the pupil in your left and right eye. This is also known as the Binocular Pupillary Distance or ‘Binocular PD’ and is represented by one number. In the example above, the PD is 58mm.


If your PD column includes two numbers then your eye doctor has measured your Monocular Pupillary Distance (also known as Monocular PD). Ideal for fitting eyewear to asymmetric faces, this measures from the pupil’s center to your nose’s bridge.


Getting your PD measurement right is essential for selecting the correct prescription eyewear. The wrong PD can lead to an uncomfortable and disorientating viewing experience, and headaches. 


From a sporting point of view, it can make reacting to a ball in tennis and basketball very difficult, and in heavy-contact sports such as rugby and football, it can have dangerous consequences.  




Therefore, measuring your pupillary distance should always be done by a professional optometrist.   


  • Comparison – Binocular Pupillary Distance versus Monocular Pupillary Distance


  • A guideline on how to read the PD value on your eye prescription


A guideline on how to read the PD value on your eye prescription




  • 68 - Single Number – You have a binocular measurement of 68 millimeters

  • 34/34 – Two Equal Numbers – The monocular distance for both eyes are identical.

  • 34/32 – Two Different (Low) Numbers – One eye is slightly further away from the centre of the nose than the other, which is very common.

  • 65/62 – Two Different (High) Numbers - The first number is your Distance PD and the second number is your Near CD (explained below).




Prescription eyewear is made to ensure the lens center is perfectly aligned with the center of the pupil to give the most precise vision. As you move away from pupil’s center, the prescription worsens. Therefore, if the PD isn’t 100% accurate, and you have a high prescription, it will feel like you’re looking through the wrong prescription because the lens center is not focused on the right part of the eye.


The pupil passes light through to the retina, enabling us to see. The extent to which our pupil widens (dilating) or narrows (constricting) is largely controlled by the iris, the eye muscle surrounding the pupil.  


In dark conditions, our pupils dilate to allow more light to enter the eye to help us see more clearly. By contrast, during a sunny day, the pupil doesn’t open as wide because it is already receiving ample light to see correctly.  




The pupillary distance will change, depending on whether you're focusing on an object that's close to your face or far away.


Your prescription may show ‘Dist PD’, or distance PD, the pupillary distance measured when you are looking at an object from a long distance. The ‘Near PD’ is the PD value when you’re focusing on something in front of you.  


Typically, the distance PD is 3 to 4 mm greater than Near CD. For example, (55/52) values show the distance PD to be 55mm and the Near CD to be 52mm.





The Optical Center (OC) highlights the importance of wearing specialized prescription eyewear for each given sport.


Compared to the pupillary distance, which locates the lens center by measuring horizontally across the face, the optical center measures from the top of the pupil to the bottom on each eye.


A Sight for Sport Eyes usually only require an OC measurement on very high prescriptions or for progressive bifocals.  But if people get their glasses and it is blurry, it is usually an OC problem because they are looking through the wrong part of the lens.  Unfortunately, you only find that out once you’ve tried on the frames, as it also depends on how the glasses fit on their nose and ears.


A Sight for Sport Eyes makes the OC the center of the frame. But someone with a larger or thinner nose or “high” or “low” ears may experience the frame sitting high or low on the face thus causing the pupils to be more above or below that center line. 


For sports use, this presents a problem. For cyclists for instance, they sit on their bike and look through the top of the lens - no longer looking at the center.  


However, if we adjust the OC to help them cycle, a similar issue will occur when they use the same glasses for other pursuits such as driving or reading books. Therefore, it is essential customers invest in appropriate eyewear for each sport and activity.



  •  The Sphere (SPH) category is highlighted in yellow.  


On your prescription, the Sphere or ‘SPH’ is the measurement of how long sighted (hyperopic) or short sighted (myopic) you are.


This is the main part of your prescription and most of your power lies here.


Diopters  or ‘D’ is the measurement unit and the number is a two-point decimal such as 2.50 or 4.00 with either minus (-) or a plus (+) sign as its prefix. If you are short sighted, your SPH value will have a minus (-) sign before it, and a plus(+) If you are long sighted. The PI or infinity symbol indicates you are neither long nor short sighted.  


The associated numbers signify the strength of lens you require to achieve good vision. Most values range from 0.00 to +/- 20.00 and go up by 0.25.


The associated numbers signify the strength of lens you require to achieve good vision. Most values range from 0.00 to +/- 20.00 and go up by 0.25.




  • (-/+) 0-3.00D SPH VALUE - A Mild Prescription

  • (-/+) 3.00 to 6.00D SPH VALUE – A Moderate Prescription 

  • (-/+) 6.00D and above SPH Value - A High Prescription 


For buying sports eyewear, these sphere values will largely influence what strength lenses you need for your googles and glasses. Some sports lenses like prescription swim goggles are made in half diopter steps (0.50) only, so you may need to round up or down to find right strength to fit both eyes.


  •  A guideline on how to read the PD value on your eye prescription





This is the additional magnifying power needed for your corrective lenses.



The ADD values inform the lens manufacturer how much extra magnifying power should be added to the bottom part of the lenses to correct presbyopia – a difficulty seeing small print clearly.


Therefore, this is most commonly used for reading glasses, as well as progressive and bifocal lenses.


The ADD numbers are also always the same for each eye and are a "plus" power, even if it is not preceded by a plus sign. Typically, ADD powers range between +0.75 to +3.00 D.



  •  The Cylinder (Cyl) and Axis are highlighted in yellow.  


Cylinder (Cyl) and Axis are like ‘bread and butter’, you cannot have one without the other.


Their presence means you have an astigmatism – a common condition that causes blurred vision and discomfort. The Cylinder measures the severity of the astigmatism and the Axis shows its location on the eye.


Astigmatism is caused by the irregular shaping of either your cornea, the eye’s front cover, or the eye lens. A person with astigmatism typically has an eye shaped more like a football than a round (like a basketball).


You can still see objects, but they will seem very hazy at any distance. This is because an egg-shaped eye restricts light from focusing properly on the retina, preventing a sharp view. The higher the astigmatism, the more halo affect you will have around objects. 


Measured in diopters, the Cylinder parameters can be in either plus (+) or minus (-) form and function to counter-act the blur caused by the astigmatism.


The Axis tells us the exact angle of the eye the cylinder power needs to be applied. It’s measured on a range from 0-180 degrees, like a protractor.


How does this relate to sports-specific glasses?


A Sight for Sport Eyes consider minor astigmatism anything below 1.00. 


Minor astigmatism is usually not corrected in contact lenses, and thus for step diopter sports goggle purchases, it would not need to be corrected either.


When you order your glasses, we will combine each eye’s sphere value with half of the respective Cylinder value to produce your "effective" power number. 


In the example above, the O.D.’s (right eye) SPH value is -2.50 and it’s Cyl is +1.75. Therefore, -2.50 + 1.75 = -0.75 (the effective power number).


If you’re unsure, simply copy the prescription into the comments or contact us and we will make the determination for you.


If your second number (CYL) is larger than -1.00, A Sight for Sport Eyes suggest you go into a custom-made product instead of a step diopter product for the best vision.



  • The Prism and Base are highlighted in yellow.


The Prism describes an adjustment made to corrective lenses to solve double vision.  


People with double vision – also known as diplopia – see two separate images of one object. The addition of the prism power to lenses aligns the two images into one by redirecting the incoming light into the right area of the eye.


Double vision can be caused by numerous severe health issues such as eye muscle problems, head injuries, strokes and nerve issues.


Measured in diopters and written in decimal form, the direction of the prism is indicated by noting the relative position of its "base" (the thickest edge of the lens).


Four abbreviations are used for prism direction: BU = base up; BD = base down; BI = base in (toward your nose); BO = base out (toward your ear).


Now you have an understanding of your eyes (and your prescription), find the perfect specialized prescription and non-prescription lenses for you at A Sight for Sport Eyes.    

12 Sep 2019

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