What power of contact lenses do I need?

Since eyeglasses are usually about 12 millimeters from the eye, the prescriptive power of a contact lens — which is positioned directly on the tear film of the eye — will be slightly less nearsighted. This becomes significantly different with prescriptions of 4D or greater. Therefore, in our example, the power of a contact lens will be less than the eyeglass prescription for the right eye.