Your glasses prescription contains different numbers than your contact lens prescription. Contact lenses can only be prescribed by a licensed eye care professional. To obtain a contact lens prescription, your eye doctor must do additional testing, called a contact lens fitting, during your refractive (glasses) exam. The information from the two types of testing is combined to form your contact lens prescription.
Obtain and verify the glasses prescription, also called the refraction.
Calculate the contact power by converting the dioptres (strength of lenses) with vertex consideration (distance from the eye). Contact lenses sit on your eye and not a centimetre away like glasses do, so the prescription power is different. The amount of astigmatism present should also be taken into consideration.
Use K-readings (corneal curvatures take with a keratometer) to calculate a proper base curve for the contact lenses. Base curves vary because they are determined by trying to match the best curve of your cornea. They average from 8.4 to 9.0mm.
Determine the diameter of the lens. This measurement is from one side of the lens to the other and is also in millimetres. Diameter is important for the fit of your lenses because it determines where the lens will sit on your eye.
Try the lenses as trial fitting. This is the period of time where lenses are worn to see how the eye adapts to them. During this time, which usually takes one to two weeks, the lens may loosen or tighten and will be checked after the trial.
Recheck the fit of the lens and the vision after the trial fitting. Make adjustments to power if necessary and finalise the base curve and diameter fit of the lens.