Glasses prescription vs. contact lens prescription
Contact lens presciptions differ from glasses percriptions
People often notice that the contact lens prescription differs from the glasses prescription in the numbers. This is because glasses sit about 12 mm from the eye, and contact lenses sit right on the eye. In order to account for this distance between the glasses and the eye, a mathematical calculation is made to determine the appropriate contact lens power.
Fitting contact lenes
When contact lenses are fitted to the eye, much more is taken into account than just the numbers from the glasses prescription. The curvature of the cornea (the clear tissue at the front of the eye that the contact lens sits on) is one important measurement that is taken. The diameter of the contact lens with respect to the diameter of the cornea is looked at as well. Position of the eyelids can affect contact lens choice. Additionally, health of the surface of the eye and the surrounding lids is taken into account.
Contact lenses are classified by the FDA as a medical device and are regulated differently than glasses. Because you are actually putting a contact lens on the surface of the eye, there are risks for infections, ulcers, and other complications that are not present with glasses.