Contact lenses are important trinkets that help you to have clear vision without the restriction of frame glasses. Let's break down what your prescription consists of.
Your prescription can be found on a piece of paper provided by your optician during a check-up, at the end/side of your contact lens box, or on the blister packs that contain your contact lenses.
If you cannot find your prescription, you can contact your optician to learn more about the details of the lenses you wear.
A written prescription will typically include the following specifications: base curve, diameter, power (or sphere), and then additional figures for Cylinder and Axis if you have astigmatism, and Addition and Dominant if you need presbyopia correction. Because your left and right eye prescriptions may differ, they will be listed separately.
When you go to your local optician for a contact lens fitting or check-up, they will give you a copy of your prescription. This is typically found on a form, such as the one shown above.
- What does each prescription figure mean?
Here's what those abbreviations mean:
OD (right eye): Oculus dexter, the Latin term for the right eye
OS (left eye): Oculus sinister, the Latin term for the left eye
PWR/SPH (Power/Sphere): This figure indicates whether you're long-sighted or short-sighted, as well as how much correction your eyes need. If you have long-sightedness (hyperopia), your figure will start with a + (plus sign), and if you have short-sightedness (myopia), it will start with a - (minus sign) (minus sign). This will be followed by a number ranging from 0 to 0.25 dioptres; the higher the number, the stronger the visual correction required (this value can be displayed on the box as "PWR" or "D").
BC (Base Curve): The base curve determines the type of fit required for the lens to meet the curve of your eye; this is usually written in millimeters (usually a number between 8 and 10), but it can also be written as flat, median, or steep. E.g. BC 8.6.
If your brand only has one base curve, your prescription will be devoid of a BC value.
The base curve determines how the lens fits on your eye, and most brands only offer a few base curve options. Just as with clothing, being a BC 8.6 in one brand does not guarantee that you will be the same in another.
DIA (Diameter): The diameter of the contact lens is also measured in millimeters and determines the best fit for your eye (usually a number between 13 and 15).
Brand: The brand of contact lenses that your doctor prescribed for your eyes. E.g. DIA 14.0.
Brand: The brand is part of the prescription and specifies the type of contacts that can be purchased. Each brand uses a unique material and water content.
If you wear colored contacts, you can order any color from the brand you were prescribed.
Expiration Date: A contact lens RX is typically valid for one year. Before you can purchase additional lenses, you must return to your eye doctor when your prescription expires for a checkup on the health of your eyes. Prescriptions for glasses are governed by state law, and the majority of them expire after two years.
- Prescription Figures for Astigmatism
The figures below are only displayed on contact lens prescriptions where astigmatism correction is required and toric lenses are required.
Prescriptions for astigmatism correction include two additional numbers, denoted by the abbreviations:
CYL (Cylinder ): The cylinder is always a negative number that increases in increments of 0.25. The cylinder for toric lenses, like the power/sphere figure on all standard prescriptions, denotes the extra visual requirements required for astigmatism as well as the severity of astigmatism (usually a number between -2.25 and -0.75).
AX (Axis): Astigmatism is caused by an irregular curvature of the eye; the axis is a figure that determines the angle of correction required to see clearly. The Axis always has a value between 0 and 180 degrees.
- Prescription Figures for Presbyopia
The figures below are only displayed on contact lens prescriptions that require presbyopia/multifocal correction.
ADD (Addition): If you have presbyopia, it affects your close vision. The Addition figure determines how much correction you need to see clearly at a close distance. This is a number between 0.50 and 3.00 that is referred to as a high, medium, or low by some contact lens brands.
Dominant: People who wear multifocal or bifocal contact lenses will notice that the correction is determined by the dominant and non-dominant eyes. This is the dominant figure, and it is usually denoted by a "D" and an "N" to indicate which eye is which.
- When does my prescription expire?
According to federal law, contact lens prescriptions have a one-year expiration date. Some states have longer expiration dates, which you should adhere to unless you have a condition that necessitates more frequent eye exams. Never use expired contacts. They can cause eye damage and blurred vision. In most states, a contact lens prescription expires after one year.
When your prescription expires, you won't be able to buy any more contact lenses until your eye doctor updates it. An eye exam is necessary to ensure your overall eye health and that your contact lenses are not impairing your vision. "Just because your contact lenses feel comfortable and appear to be working properly does not imply that your eye health is in good shape," explains an ophthalmologist. "Microscopic issues may exist that can only be seen with a slit lamp, a type of microscope used during an eye exam."
Problems with contacts are unlikely to keep you from wearing contacts indefinitely. Changing to a different type of lens or contact lens solution will usually solve the problem.
A contact lens prescription is not the same as a prescription for glasses.
It is very common for a glasses prescription to be confused with a contact lens prescription, but knowing the difference is critical when ordering contact lenses.
A prescription for glasses cannot be used to purchase contact lenses. A contact lens is placed directly on the surface of the eye, whereas a spectacle lens is placed slightly further away. This difference in position influences the lens's resulting power.
To get the same power in a contact lens, your optician will need to adjust the sphere, cylinder, and axis values found in a glasses prescription. Only your optometrist can perform this calculation for you.
A prescription for spectacles will not include:
contact lens manufacturer/name;
A contact lens prescription will almost never have an axis value other than 5 or 10 degrees.
- Where Can a Contact Lens Prescription Be Used?
Once you have a valid contact lens prescription, you can buy contact lenses from a variety of retailers. This means you can buy lenses from your eye doctor, optical chains, mass merchandisers, and online discount contact lens retailers, but make sure you buy them from legitimate sources.