Which of the following is a reason why a lens power produced via an empirical fit vs a diagnostic fit would not be the same?

• A. inaccurate Ks
• B. inaccurate refraction
• C. lens decentration
• D. lens flexure

Answer: (B)

Empirical GP powers are calculated from corneal data and a refraction, then used to pick a lens without actually testing it on the eye. If the refraction used in that calculation is off, the resulting spherical power will be off too. The diagnostic approach, on the other hand, involves wearing a trial lens and refining the refraction with the lens in place to see how the eye actually responds. This real-on-eye measurement tends to reveal the true required power, so it won’t match what was predicted empirically if the refraction used in the prediction was inaccurate.

While lens position or deformation can alter how the lens’s power is experienced on the eye, the fundamental reason for a mismatch between empirical and diagnostic powers is an inaccurate refraction used in the empirical calculation.