A +1.25D or -1.00D diagnostic lens can be held adjacent to the front of the keratometer to extend the dioptric range of the drum readings.

What is extension Keratometry?

Sometimes a patient has a corneal curvature so steep (keratoconus) or flat (post-LASIK or PKP) that they’re beyond the range of a conventional keratometer. You can extend the keratometer range by taping a trial lens over the central aperture on the patient’s side of instrument (be sure not occlude the mires).

What are the types of keratometer?

There are two types of keratometers: single position Helmholtz keratometers, which are more common, and can adjust image size; and Javal-Schiotz keratometers, two-position machines that adjust object size.