The next step is to over-refract the patient. An over-refraction is a refraction done with the trial lenses still on the eyes. You put the patient back behind the phoropter and refine sphere, cylinder, and axis. Any power you find is added to the power already on the eye. That sum is the power the patient truly needs. This step matters because a contact lens prescription cannot be set until the lens is on the eye. The eyeglass number is only a starting point.
A contact lens sits right on the cornea, while glasses sit in front of it at a vertex distance that averages about 13.5 mm. That gap changes the effective power. The change is small at low powers and larger at high ones. A strong minus spectacle lens usually needs a weaker contact lens power than the spectacle number. The over-refraction catches that shift, plus any leftover astigmatism and any labeling error in the trial lens. The full fitting order is worth memorizing: history and eye exam, then refraction, then keratometry, then choose a lens type, base curve, diameter, and power. Only after the over-refraction does the doctor judge the fit on the eye.
Remember the order of work. You measure first, you teach second, you dispense last. Handling a trial lens fitting in the right order is a core part of the COA contact lens content area.
Incorrect Answers
- A. Dispensing now would skip every check. The final power is not known until the over-refraction is done and the doctor approves the fit.
- B. Removing the lenses first throws away the measurement. The doctor needs to see the lens settled on the eye.
- C. Insertion and removal training is real, but it comes after the power and fit are set, near the end of the visit.
Testing Pearls
- An over-refraction is refracting over a lens that is already on the eye.
- Add the over-refraction result to the power of the trial lens to get the final power.
- A contact lens prescription is only final after the lens is fit on the eye.
- Fitting needs refraction plus keratometry before a lens is chosen.
- High minus powers need a weaker contact lens than the spectacle number.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 14, Principles and Problems of Contact Lenses.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 12, Fundamentals of Practical Opticianry.
