Why Your Prescription for Contact Lenses Differs from Your Glasses?
You might be surprised to learn that the prescription you get for glasses at the end of your eye exam does not apply to contact lenses! To determine your contact lens prescription, you need a more specific eye exam as well as a fitting.
The reason for this difference is pretty simple. While contact lenses sit directly on your cornea, the lenses of your glasses are usually 12 millimetres away from your eyes (commonly known as the vertex distance). Therefore, depending on your vision problem (myopia, hyperopia, presbyopia, etc.), you’ll need a stronger or weaker prescription in your contact lenses.
Additionally, to fit contact lenses to your eyes, your Optometrist will need to evaluate the condition of your cornea. The corneal curvature and diameter are necessary to ascertain the parameters of your contact lenses. Each patient will have their specific data, like a fingerprint. The quality of your tears, as well as the quantity, will be assessed. The thickness, water content, and oxygen transfer rate of the lens can then be carefully determined by the Optometrist to ensure optimal comfort. Contact lenses are not “one-size-fits-all.”
And that is why the prescription of your glasses is not enough for new contact lenses! Don’t forget to tell your Optometrist if you need both prescriptions next time you have an eye exam.