How do we verify the power of a GP lens?

• A. Lensometry without regard to orientation
• B. Visual estimation
• C. Power measured by alt angle without lens stop
• D. BVP using lensometer (lens placed concave side against the lens stop, power dial adjusted until a clear image)

Answer: (D)

Measuring the back vertex power is the essential step because the power a GP lens provides is the power its back surface delivers when it sits on the tear film over the cornea. To capture that value, use a lensometer and set the lens so that light passes through the back surface as it would in the eye. Place the lens with the concave side toward the lens stop, then adjust the power dial until the mires are sharp. The reading you obtain is the back vertex power, the value that matters for the eye.

Visual estimation or trying to infer power from an alternate orientation won’t reflect how the lens actually behaves on the eye, and measuring with the lens stop ignored or using an alternate angle won’t isolate the back surface’s contribution. So the precise, orientation-corrected measurement with a lensometer gives the true GP lens power.