Contact lenses are medical devices that are used primarily for vision correction. They cover a wide range of prescriptions, varying from refractive errors, such as myopia and hyperopia, to astigmatism and the age-related condition, presbyopia.
The contact lens prescription is a mandatory piece of information provided by your optician after your thorough eye check-up, which is followed by a contact lens fitting session. It allows you to buy contact lenses that match your vision and lifestyle requirements. This is why a contact lens wearer must know how to read contact lens prescription correctly.
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Medical terms on a contact lens prescription
A regular prescription provided by an optician includes the specifications, namely:
- Base Curve (BC)
- Diameter (Dia)
- PWR (Power) or SPH (Sphere)
- Rx
- O.S.
- O.D.
- Brand/Material
The prescription must also specify the patient’s name, optician’s name and signature, and the issue and prescription expiry date.
If the prescription is for an individual with astigmatism who requires toric contact lenses, it will include the Cylinder (Cyl) and Axis fields.
For prescriptions to correct presbyopia with multifocal contacts, the fields ‘ADD’, and ‘D’ and ‘N’ may be included.
Rx
Rx is an abbreviation for prescription.
O.D.
An abbreviation for Ocular Dexter, it means the right eye.
O.S.
An abbreviation for Ocular Sinister, it means the left eye.
BC
A short term for Base Curve, BC is the curvature of the back surface of the lens. It determines the correct fit of the contact lens so that it meets the curve of the front surface of the eye. It is usually measured in millimeters (mm). A lower base curve denotes a steeper curve, while a higher base curve indicates a flatter curve.
DIA
A short term for Diameter, Dia is the overall width of the contact lens, which is also measured in millimetres. In case of soft contact lenses, the diameter is generally 2 to 3mm larger than the iris diameter.
PWR/SPH
Abbreviations for the power and sphere, it determines whether an individual has myopia (short-sightedness) or hyperopia (long-sightedness). Measured in the unit of dioptres (D), the power can be a (+) or (-) value. While the plus sign (+) denotes that the prescription is for someone with hyperopia, the minus sign (-) indicates a myopia prescription. The higher the power or sphere, the stronger the need for visual correction.
CYL
A short form for the cylinder, this value is mentioned in the astigmatism-based prescription. It signifies the extra visual requirements needed in toric lenses to correct astigmatism, depending on the severity of the condition. Cyl is normally a minus value and increases in 0.25 steps.
Axis
The contact lens prescription for astigmatism remains incomplete without the axis value, similar to the cylinder. As the astigmatism is caused by the irregular curvature of the cornea or lens of the eye, the axis determines the angle of the correction required to see clearly. It is measured in degrees from 1 to 180.
ADD
An abbreviation for Addition, it is a must-include value in the contact lens prescription for presbyopia. As this condition affects the near vision, the figure has a plus (+) sign and is measured in dioptres. The Add figure determines the additional level of correction needed to see nearby objects clearly. The number is usually between 0.50 and 3.00, indicating the magnifying power in multifocal contact lenses. Sometimes, it can be written as Low, Med or High depending on the brand of the lenses.
D’ and ‘N’
Included only in the prescription for presbyopia, ‘D’ and ‘N’ stand for Dominant and Non-Dominant, respectively. The optician conducts a series of tests during the fitting of multifocal contact lenses and determines the dominant and non-dominant eye. D and N indicate which eye is which.
Frequently Asked Questions (FAQs)
Why do contacts and glasses Rx differ?
The prescriptions for contact lenses and glasses differ because of the different placement of these vision correction devices. While glasses sit at a short distance from the eyes on the nose, contact lenses sit directly on the cornea of the eye. The two different Rx ensure that the light is refracted differently, but achieve the same visual correction with both glasses and contact lenses.
Can I use my glasses Rx to buy contacts?
No, you cannot use your glasses prescription to buy contacts. You need a prescription specifically for contact lenses to buy them.
How often to update contact lens prescription?
According to the NHS , you should get your eyes tested every two years. However, the minimum duration to update the contact lens prescription varies from individual to individual, depending on the health of their eyes and how their eyes have interacted with contact lenses. However, those who wear contact lenses more regularly are usually advised to visit their optician for a check-up on a yearly basis to get their prescription updated.
What are the effects of wearing lenses with the wrong base curve or diameter?
As the base curve determines the right fit of contact lenses, wearing lenses with the wrong base curve can feel too tight or too loose on the surface of the eye, resulting in discomfort. As far as wearing lenses of the wrong diameter is concerned, the contact lenses may keep moving around the eyes, resulting in blurry vision.
Disclaimer: The advice in this article is for informational purposes only and does not replace medical care or an in-person check-up. Please check with an eyecare professional before purchasing any products or remedies. For information on our article review process, please refer to our Editorial Policy.
Tina Patel is a qualified Contact Lens Optician at Feel Good Contacts with over 25 years of experience in the optical industry. She is a member of ABDO and registered with the GOC. Tina holds a BSc. (Hons) in Optical Management from Anglia Ruskin University and qualified as a Dispensing Optician in 2002. In 2007, she further specialised by completing advanced training in contact lenses, gaining her qualification as a Contact Lens Optician. She now shares her clinical expertise through teaching at City, University of London, guiding future opticians.
