A contact lens sits directly on the surface of the eye, while spectacles sit roughly 12–14 mm in front of it. This distance (the vertex distance) changes the effective power the eye receives.
For higher prescriptions the difference becomes significant, so the contact lens power is recalculated and is not simply copied from the glasses prescription.
Frequently Asked Questions:
Q. Can I order contacts using my glasses prescription?
A. No. A contact lens fitting is required; the powers, base curve and diameter differ.
Q. At what strength does the difference matter?
A. Roughly above ±4.00 D the conversion becomes clinically important.
Q. Do I still need an eye exam for contacts?
A. Yes, a separate contact lens assessment and fitting is needed for safety and comfort.