Contact lens parameters and prescription - Guide (2025)

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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’s suggestions. Don’t forget you also need an eye exam and prescription before orderingcoloured contact lenses, even those without vision correction. And, yourprescription for spectacles is NOT the same as a prescripion for contacts, butmore on that later. Now, let's break down all those numbers and abbreviations onyour prescription.

Overview of Parameters

Your prescriptionhas three basic components: your parameters, recommended brand(s) of lenses,and any special instructions . To break it down for you, we have created aquick overview of all the parameters and their common abbreviations.

This table will guide you throughselecting the correct parameters according to your optician’s advice .Although it provides many answers, you may still have questions. Below, we’veaddressed some of our customers’ most frequently asked questions.

Right or Left Eye?

Your optician may indicate this with a simple“L” and “R” or the Latin terms “Oculus Dexter” (right) and “OculusSinister” (left). If you require a different power for each eye, you’llneed to click “Select attributes for second” eye when placing your orderon our website. When placing your order, there is no distinction between leftand right, so just think of them as Eye #1 and Eye #2 . If you have the sameprescription for both eyes, there’s no need to select the second attributes!Just fill in your parameters once, and select the number of boxes you require.

Plus or Minus?

Are you shortsighted (myopic), meaning youcan’t see well in the distance? Then your power will be minus. Farsighted, orstruggling to see close objects? This condition is also called hyperopia, and itmeans your power will be plus. This value can be indicated on your prescriptionas either “D” diopter value; “PWR” power; or “SPH” sphere. Becareful when selecting your power, as a small slip up can cause a big differencein eyesight. If you have accidentally ordered the wrong value, you can alwaysreturn unopened boxes according to our .

Can I use my glasses prescription for contacts?

NO. The parameters for glasses and contacts can be very different , dueto where the lenses of each sit in relation to the eyeball. Glasses sit a fewcentimeters in front of your eyes, while contact lenses fit directly on top ofthe eyeballs. Your glasses prescription does not contain the necessaryinformation about the appropriate contact lens base curve and diameter that’sright for you.

How do I choose cylinder and axis for toric lenses?

Cylinder and axis are two measurements in toric lenses necessary to correctastigmatism. The cylinder is an extra correction that denotes the severity ofastigmatism and has a minus value expressed in increments of 0.25. The axis isexpressed in degrees between 0° and 180°. We advise you to consult youroptician if you have any doubts regarding these two parameters.

Whatdoes the ADD power mean?

If you suffer from presbyopia and requiremultifocal contacts, your prescription may include an ADD, or additionalpower to help with near vision . ADD powers range from +0.50 to +3.00. Somebrands of lenses offer ADD power in low, medium or high values. CooperVision lenses have a further distinctionbetween the dominant and non-dominant eye, so if you have any questions aboutyour ADD power for each eye, consult with your optician.

Are base curveand diameter important when choosing contact lenses?

The base curverefers to the degree of curvature of the contact lens, or how closely it fitsagainst the eyeball . A lower BC, like 8.40, means the lens is more curvedand will fit snugly against the eye. Higher BCs like 8.70 indicate a flatterlens. The diameter refers to the width of the lens and is expressed inmillimeters, usually between 13 and 15 mm . A slight difference (0.1 or 0.2mm) will likely not have any noticeable impact on your comfort or quality ofvision. But wearing lenses with a significantly different BC or DIA than youwere prescribed can sometimes lead to discomfort and distorted vision, so it’scritical to follow the recommendation on your prescription.

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

At Alensa.com, 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 box oflenses . 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 any changesin your vision, it's important to be examined by your optician. Under Irishlaw, it is the buyer's responsibility to maintain an up-to-date prescriptionbefore purchasing lenses online.

Can I order trial lenses?

Please note that we are not authorised to provide trial lenses . Onlyyour eye-care specialist knows your eyes inside and out and can prescribe lenseson a trial basis. Although they’re not considered trial lenses, we do offer a# on our TopVue lenses. If, for any reason you find them unsuitable, you mayreturn them.

Prescription Verification - only applies to the USA andKorea

We strive to provide the best care possible to our customers. Assuch, we have implemented a verification system that is in compliance with theFCLCA and Rules for USA and Korea customers. Under the Fairness to Contact LensConsumer Law (FCLCA), we are required to verify your contact lens prescription.

We provide two easy methods of verifying your contact lens prescriptionduring your checkout process. The most efficient way is to uploadyour prescription slip to your Alensa account.

You can provide usa copy of your valid prescription written by eye care professionals. You caneither scan or snap a picture of your written prescription and send it to us: info@alensa.com . Once weget your prescription, we will keep the record for you, you can check outanytime without the need to re-upload again. Providing invalid or inaccurateprescription may result in order cancellation.