In lensometry, the resultant power measured with all lenses in the trial frame is known as the what?

In lensometry, the resultant power measured with all lenses in the trial frame is known as the what?

• A. front vertex power
• B. apparent power
• C. effective power
• D. back vertex power

Answer: (D)

In lensometry, the reading you obtain reflects the optical power at the back surface of the spectacle lenses—the plane where the eye actually sits. This is the back vertex power. The eye is positioned behind the lenses at the back surface, so the instrument measures the power delivered to the eye at that point. Front vertex power would be the power at the front surface, which isn’t what a standard lensometer reads. Apparent power isn’t a standard term used for lensometer readings, and effective power refers to the power the eye experiences after accounting for the distance between the lens and the eye (vertex distance), which would require adjusting the back vertex reading. If you needed the patient’s actual experience at the eye, you’d convert the back vertex power to effective power by considering the vertex distance, but the lensometer’s direct measurement is the back vertex power.

In lensometry, the reading you obtain reflects the optical power at the back surface of the spectacle lenses—the plane where the eye actually sits. This is the back vertex power. The eye is positioned behind the lenses at the back surface, so the instrument measures the power delivered to the eye at that point. Front vertex power would be the power at the front surface, which isn’t what a standard lensometer reads. Apparent power isn’t a standard term used for lensometer readings, and effective power refers to the power the eye experiences after accounting for the distance between the lens and the eye (vertex distance), which would require adjusting the back vertex reading. If you needed the patient’s actual experience at the eye, you’d convert the back vertex power to effective power by considering the vertex distance, but the lensometer’s direct measurement is the back vertex power.