Contact lenses and glasses are the two most popular forms of vision correction. But, just because they provide good visual clarity and comfort, doesn’t mean their prescriptions are interchangeable.
Glasses and contacts have slightly different prescriptions because of their proximity to the eyes.
Contacts sit directly on the eye’s corneal surface while glasses perch on your nose, resting further away from your eyes, which is why the strength of the numbers needs to be modified accordingly.
Reading a Vision Prescription
To better understand why contact lens prescriptions differ from eyeglasses prescriptions, knowing how to read a vision prescription is helpful.
At first glance, the numbers and values may seem confusing, but they provide valuable insights unique to your vision.
Reading from left to right, a standard vision prescription includes the following parameters:
- Sphere (SPH): This indicates the lens power needed to correct myopia or hyperopia, and is measured in diopters, increasing in 0.25 increments. A negative sign (-) indicates myopia while a positive (+) signifies hyperopia.
- Cylinder (CYL): This indicates the lens power needed to correct astigmatism when your cornea or natural lens is irregularly shaped. If you don’t have astigmatism, this section will be blank. The CYL is also measured in diopters and is typically negative.
- Axis: If you have a cylinder power, the axis specifies the orientation of the astigmatism correction. It’s measured in degrees from 1 to 180 to fine-tune the lens for your specific needs. A CYL power will always be accompanied by an axis power, meaning, if you don’t have an astigmatism, there won’t be an axis power on your prescription.
- ADD power: For those who need help with close-up tasks caused by presbyopia, this value is added to the distance prescription and It’s marked as a “+” number, like +1.50.
For instance, a prescription may appear as, OD (right eye): -3.00 SPH and OS (left eye): -2.50 SPH -0.75 CYL x 180.
Here, the SPH values indicate this patient is nearsighted in both eyes, with more correction needed for the right eye. The right eye has no CYL value, so it doesn’t require astigmatism correction whereas the left eye shows mild astigmatism that requires correction at 180 degrees on the physical lens.
Reading a Contact Lens Prescription
Glasses and contacts incorporate almost the same parameters on their vision prescription, but contacts require additional measurements.
These measurements are used to ensure the contact lenses sit safely and comfortably on the eye’s surface to provide ideal vision correction:
- Base curve (BC): This is the curvature of a contact lens and determines how the lens will fit on your eye. For example, a BC value of 8.6 mm indicates the contact lens curvature appropriate for a particular eye shape and a proper fit.
- Diameter (DIA): This is the edge to edge width of the contact lens, which is important for confirming that the lenses center properly on your cornea. A common diameter might be 14.2 mm for a soft contact lens.
The base curve (BC) and diameter (DIA) are very important, as they are used to customize the lens fit to conform to the unique shape and size of your eyes, which prevents the lenses from shifting. Poorly fitting contact lenses can cause discomfort and may affect visual clarity.
Why are Contact Lens and Eyeglass Prescriptions Different?
Now that we understand how to read a vision prescription, it’s easier to see why contacts and glasses prescriptions differ. When comparing these prescriptions side by side, it’s common to notice a slight difference in the numbers.
The primary reason boils down to how these corrective devices interact with our eyes and the technology behind each form of vision correction.
Glasses rest on the bridge of your nose, sitting about 12 millimeters away from the eyes while contacts are directly placed on the eye’s surface. Their difference in placement means the lens powers have to be adjusted accordingly.
Let’s consider someone with a glasses prescription of OD: -4.00 SPH -1.25 CYL x 145. When we translate this to contact lenses, the prescription may adjust slightly because of the difference in proximity to the eye.
Wearing lenses with a vision prescription that’s too weak or strong can cause eye strain, headaches, and blurry vision.
This is why having separate prescriptions for each form of vision correction is necessary as substituting one for the other, even if the discrepancy is small, may not be accurate.
The Contact Lens Fitting
Since translating a prescription for eyeglasses to contact lenses isn’t always as simple as adjusting the lens power, a contact lens fitting exam is performed so that the contact lens prescription will offer optimal visual clarity and comfort.
During this appointment your optometrist will assess multiple factors, like your corrective needs, the shape and size of your eyes, and the overall health of your corneas. Based on this, a set of trial contact lenses are selected.
Your optometrist will confirm whether the lenses fit safely and provide optimal visual clarity.
Once confident after training on insertion and removal, you will be sent home with a trial set of contact lenses to wear for about a week to experience daily life with contacts. After the trial, your optometrist will evaluate your vision one final time to fine tune the prescription and offer the opportunity to discuss any potential issues concerning your experience with the lenses.
Once you and your optometrist are happy with the visual clarity, comfort and fit of the trial lenses your contact lens prescription is finalized.
How Long are These Prescriptions Valid For?
Since eyeglasses and contacts differ in prescription values and proximity to the eye, so too are how long the prescriptions are valid.
Contact lens prescriptions are valid for just one year. An annual exam is strongly advised because of how contact lenses sit directly in contact with the eye’s surface. This increases the risk of infection and injury and necessitates the extra care and maintenance contact lens wearers need.
However, even if you believe your glasses are working just fine, annual routine eye exams are encouraged to keep your and eye health up-to-date and well-maintained.
Schedule a Visit
Recognizing that eyeglasses and contacts require different prescriptions that aren’t interchangeable allows you to appreciate their benefits.
Connect with our team at Whitby Family Eye Care to schedule a contact lens exam and fitting.
