A patient has a prescription of +12.50 diopters with a vertex distance of 12mm but the lens will set 14mm from the eye. What adjustment is needed?

When a prescription is written for a specific vertex distance, it is critical to consider the impact of changes to that distance on the effective power of the lens. In this case, the patient's prescription is +12.50 diopters, intended for a vertex distance of 12 mm. However, due to the lens being set further away from the eye at 14 mm, an adjustment in the lens power is necessary.

As the distance from the eye increases, the effective power of the plus lens decreases. This phenomenon occurs because the light rays coming through a plus lens will diverge less when the lens is further from the eye. To compensate for this reduced effective power, more positive sphere power is needed to maintain the intended optical correction.

Thus, to correct the prescription effectively, the lens must be adjusted to include less plus sphere power. This adjustment brings the effective power back to the originally intended +12.50 diopters at the new vertex distance of 14 mm. Understanding this relationship between vertex distance and lens power is crucial for appropriate lens dispensing to ensure that the patient receives optimal visual acuity.