How come my contact lens and spectacle prescriptions are different?
This can be for several reasons; one is a mathematical formula called effectivity: for spectacle prescriptions more than +4.00 DS or -4.00 DS, when you move the lens from the spectacle plane (where the glasses lens is about 13mm in front of the eye) to the surface of the eye (as in a contact lens), in order to maintain the same relative lens strength, the contact lens prescription must be either raised or lowered to compensate. There are other factors as well dealing with astigmatism, binocularity and simply patient preference that dictate modifying the contact lens prescription from the spectacle prescription.
