During a diagnostic GP lens over-refraction, how should you perform it?

• A. Start with cylinder correction only.
• B. Ignore refraction and rely on keratometry.
• C. Start with sphere; if unable to obtain BCVA, try sphero-cylindrical.
• D. Over-refraction using a fixed power for all patients.

Answer: (C)

The main idea is to refine a GP lens prescription by testing spherical power first and only adding cylindrical power if needed to improve vision. When a diagnostic GP lens is on the eye, you start by checking how well the patient sees with a spherical correction in place. If you can’t achieve the best possible acuity with just sphere, you then try a sphero-cylindrical correction to address any remaining astigmatism revealed under the lens. This approach accounts for how a GP lens changes the eye’s optics and the tear film, so the spherical component is evaluated in the actual optical situation before introducing cylinder.

Starting with cylinder alone would miss the dominant spherical error that may limit vision even with a GP lens. Relying solely on keratometry ignores the lens’ own power and the tear film, which together determine the real refractive outcome. Using a fixed power for every patient ignores individual variations in refractive error and lens fitting, so it won’t reliably give the best acuity.