Be aware of vertex distance when fitting a patient with high minus or plus over 5 diopters. In the exam room, the trial lens can be a greater distance from the patient’s pupil than the actual lens in the eyewear. That difference in distance will change the effective power of the lens impacting the patient’s visual acuity. The distance will need to be compensated by using the Vertex Distometer.
With the patient’s eyes closed, allow the cupped tip to rest on the upper eyelid. Extend the second arm until it contacts the minus side of the spectacle lens. Compare the measurement to that taken while the patient was wearing the trial lens on the compensation scale. This determines the appropriate modification to the Rx.