Contact lens parameters and prescription - Guide (2024)

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Shopping for contactlenses online? Great! You’re about to save yourself valuable time andmoney. But first, you need to know which lenses are right for you and how toread that prescription from your optician. As you getstarted withlenses and after you have had an eye exam, your optometrist will suggest aparticular brand and model of contact lenses for you based on thecharacteristics of your eyes and the nature of your vision problems. Not only isthere a difference between daily disposables, bi-weekly lenses and monthlycontact lenses, but also different brands will likely feel different from personto person, so it’s important to follow your optometrist’ssuggestions . Don’t forget you also need an eye exam and prescriptionbefore ordering coloured contact lenses, even those without visioncorrection. And, your prescription for spectacles is NOT the same as aprescripion for contacts, but more on that later. Now, let's break down allthose numbers and abbreviations on your prescription.

Overview ofParameters

Your prescription has three basic components: yourparameters, recommended brand(s) of lenses, and any specialinstructions . To break it down for you, we have created a quickoverview of all the parameters and their common abbreviations.

This tablewill guide you through selecting the correct parameters according toyour optician’s advice . Although it provides many answers, you maystill have questions. Below, we’ve addressed some of our customers’ mostfrequently asked questions.

Right or Left Eye?

Your opticianmay indicate this with a simple “L” and “R” or the Latin terms “OculusDexter” (right) and “Oculus Sinister” (left). If you require a different power for each eye, you’ll need to click “Select attributes forsecond” eye when placing your order on our website. When placingyour order, there is no distinction between left and right, so justthink of them as Eye #1 and Eye #2 . If you have the same prescriptionfor both eyes, there’s no need to select the second attributes! Just fill inyour parameters once, and select the number of boxes you require.

Plusor Minus?

Are you shortsighted (myopic), meaning you can’t see wellin the distance? Then your power will be minus. Farsighted, or struggling to seeclose objects? This condition is also called hyperopia, and it means your powerwill be plus. This value can be indicated on your prescription as either “D”diopter value; “PWR” power; or “SPH” sphere. Be careful when selectingyour power, as a small slip up can cause a big difference in eyesight. If youhave accidentally ordered the wrong value, you can always returnunopened boxes according to our .

Can I use my glasses prescription for contacts?

NO. The parameters for glasses and contacts can be verydifferent , due to where the lenses of each sit in relation to theeyeball. Glasses sit a few centimeters in front of your eyes, while contactlenses fit directly on top of the eyeballs. Your glasses prescription does notcontain the necessary information about the appropriate contact lens base curveand diameter that’s right for you.

How do I choose cylinder and axisfor toric lenses?

Cylinder and axis are two measurements in toriclenses necessary to correct astigmatism. The cylinder is an extra correctionthat denotes the severity of astigmatism and has a minus value expressed inincrements of 0.25. The axis is expressed in degrees between 0° and 180°. Weadvise you to consult your optician if you have any doubts regarding these twoparameters.

What does the ADD power mean?

If you suffer frompresbyopia and require multifocal contacts, your prescription mayinclude an ADD, or additional power to help with near vision . ADDpowers range from +0.50 to +3.00. Some brands of lenses offer ADD power in low,medium or high values. CooperVision lenses have afurther distinction between the dominant and non-dominant eye, so if you haveany questions about your ADD power for each eye, consult with your optician.

Are base curve and diameter important when choosing contactlenses?

The base curve refers to the degree of curvature ofthe contact lens, or how closely it fits against the eyeball . A lowerBC, like 8.40, means the lens is more curved and will fit snugly against theeye. Higher BCs like 8.70 indicate a flatter lens. The diameter refersto the width of the lens and is expressed in millimeters, usually between 13 and15 mm . A slight difference (0.1 or 0.2 mm) will likely not have anynoticeable impact on your comfort or quality of vision. But wearing lenses witha significantly different BC or DIA than you were prescribed can sometimes leadto discomfort and distorted vision, so it’s critical to follow therecommendation on your prescription.

I’ve lost my paper prescription.Where can I find this information?

At Alensa.ie, you may buycontacts without a prescription. If it’s been less than one year sinceyour last eye exam, and you’ve not experienced any changes in vision quality,you can always find the necessary prescription information on your boxof lenses . We’ve decoded it for you here:

Please remember that you should renew yourprescription at least once a year . Even if you haven't noticed anychanges in your vision, it's important to be examined by your optician. Under Irish law, it is the buyer's responsibility to maintain an up-to-dateprescription before purchasing lenses online.

Can I ordertrial lenses?

Please note that we are not authorised to providetrial lenses . Only your eye-care specialist knows your eyes inside andout and can prescribe lenses on a trial basis. Although they’re not consideredtrial lenses, we do offer a Money-back guaranteeon our TopVue lenses. If, for any reason you find them unsuitable, you mayreturn them.