Is My Eyeglass Prescription the Same for Contacts?
The short answer is: no, your eyeglass prescription is not usually the same as your contact lens prescription. While both correct your vision, the way they do so differs significantly, leading to variations in the prescription needed for optimal vision. This is a crucial point to understand, as using an incorrect prescription can lead to eye strain, discomfort, and even potential harm.
This difference stems from the different positions of the corrective lenses relative to your eye. Eyeglasses sit approximately 12-14 millimeters away from the cornea, while contact lenses rest directly on the cornea. This difference in distance requires adjustments in the lens power (measured in diopters) to achieve clear vision.
Let's delve deeper into the specifics:
Why Are Eyeglass and Contact Lens Prescriptions Different?
This difference in prescription boils down to the vertex distance. Vertex distance refers to the distance between the front surface of the corrective lens (eyeglass lens or contact lens) and the front surface of the cornea. This distance significantly influences the refractive power needed to focus light accurately on the retina. Because eyeglasses sit further away from the eye than contacts, a slightly different power is necessary to achieve the same refractive effect.
The discrepancy isn't always large, but it's crucial for clear and comfortable vision. Ignoring this difference can lead to blurry vision, eye strain, headaches, and even discomfort or irritation.
What About Astigmatism?
The difference is even more pronounced when astigmatism is present. Astigmatism refers to an imperfection in the curvature of the cornea, resulting in blurred vision at all distances. Both eyeglasses and contact lenses correct astigmatism, but the correction will often vary slightly between the two due to the vertex distance. Contact lenses often provide a more precise correction for astigmatism because they sit directly on the cornea, minimizing the impact of the irregular surface.
How Do I Get a Contact Lens Prescription?
You cannot simply use your eyeglass prescription for contact lenses. You need a separate eye exam specifically for contact lenses. A comprehensive contact lens fitting is essential, involving:
- A thorough eye exam: This will assess your overall eye health and identify any conditions that might affect your suitability for contact lenses.
- Corneal topography: This maps the curvature of your cornea, crucial for accurate contact lens fitting, especially for those with astigmatism.
- Contact lens fitting: Your optometrist or ophthalmologist will determine the best type, brand, and parameters (base curve, diameter, and power) of contact lenses for your eyes.
This process is more comprehensive than a standard eye exam for glasses and ensures you have the correct fit and prescription for comfortable and safe contact lens wear. Rushing this process or attempting to self-prescribe based on your glasses prescription can result in significant vision problems.
Can I Use My Old Eyeglass Prescription for Contacts?
No, you should never use an old prescription, whether for eyeglasses or contacts, without a current evaluation from an eye care professional. Your eye health and refractive error can change over time, requiring updated prescriptions to maintain clear and comfortable vision. Using an outdated prescription could lead to vision problems and potential damage to your eyes.
