Are contact lens prescriptions the same as glasses?
We get asked a lot of questions by people who are just starting to consider using contact lenses. Some of the most common questions we receive are:
- Can people with astigmatism wear contacts?
- How long can you wear contacts?
- Are contact lens prescriptions different than glasses prescriptions?
To answer your questions –
- Yes, people with astigmatism can absolutely wear contacts. Toric lenses are contact lenses that were designed specifically for people with astigmatism.
- As per the duration of time people can comfortably wear contacts, this usually ranges anywhere between 8 to 16 hours depending on the person.
- And, finally, yes! Contact lens prescriptions are definitely different than glasses prescriptions!
While this last one might surprise you, it actually makes sense - contact lenses and glasses both work toward the common goal of correcting errors in vision. However, while the prescription you receive for your contact lenses and glasses may seem similar, if you use them interchangeably, you'll end up with less visual clarity in one or the other. Read on to see why.
Can you use glasses prescription for contact lenses?
Unbeknownst to most people, the prescription that you’ve received for your glasses should not also be used for your contact lens prescription. This is because the prescription that you receive for eyeglasses is not the same as the one you will receive for your contact lenses.
To understand why, consider where glasses sit on your face when you use them. The position of the glasses in relation to your eyes is much further when compared to contacts, which sit in direct contact over the eye. Thus, to correct vision and refract the light onto the retina in the right position, your doctor would need to conduct two different exams and different kinds of measurements.
Contact lens prescriptions vs glasses prescription
So, what are the major differences between the measurements used in contact lens prescriptions and those of prescription glasses? While there is some overlap between what is measured in prescriptions for glasses and contact lenses, glasses prescriptions take into consideration a series of factors that contact lenses do not.
Prescriptions for glasses take into account additional measurements due to the position of the lenses of the glasses in relation to the eye, and the difference in the size of the lenses. For example, one important distinction is the measure of the distance between the pupils (Pupillary distance, denoted as PD), taken during the eye exam. This measurement is taken in order to configure the shape of the lenses so that the refraction of the light and vision correction as precise as possible.
Of course, there are also things that are common between both kinds of prescriptions. Sphere, or power, measurements, which measure the level of strength needed to correct your vision, is measured for both prescriptions. However, the power measurements may vary significantly depending on the level of refractive error and the type of contacts that you are prescribed.
For people with astigmatism, both their glasses and contact lens prescriptions will account for cylinder and axis measurements in addition to the power of correction, in order to account for the irregular shape of the cornea or lens of the eye.
Why are contact lens prescriptions different?
Because contact lenses sit directly on the eye, there are additional measurements that need to be taken to ensure in addition to the power of the lens correction. In order to ensure that the contact lens sits properly on the eye, your eye doctor will need to match the contact lens base curve to the curvature of your eye. The base curve measurement of your prescription indicates the curvature of the lens in millimeters and is necessary for producing lenses that fit. Similarly, your contact lens prescription will also include measurements for the diameter of the lens, indicating the overall size of the lens and is important for making sure that the lenses fit the way they should.
Hopefully this has helped you to understand why contact lens prescriptions are not the same as glasses!