How should you compute the vertical imbalance given the RX: OD +1.00 OS -0.25 Add +1.00?

• A. 1.0 Diopters
• B. 1.25 Diopters
• C. 1.75 Diopters
• D. 2.0 Diopters

Answer: (B)

To compute the vertical imbalance, you first need to understand the role of the prescription parameters provided: the values for each eye (OD for the right eye and OS for the left eye) and the addition (Add).

In this case:

• Right eye (OD) has a refractive error of +1.00 D.

• Left eye (OS) has a refractive error of -0.25 D.

• The Add for the reading portion is +1.00 D, which is applied to both eyes for near vision.

For the left eye, the total power at near would be calculated as follows:

• Near vision for OS = Refractive error + Add = -0.25 D + 1.00 D = +0.75 D.

To find the vertical imbalance, the difference between the two eyes at near vision needs to be computed:

• Difference = Near vision OD - Near vision OS = +1.00 D - +0.75 D = +0.25 D.

This value indicates the amount of imbalance due to the different prescriptions when the patient switches to near vision.

Next, you consider how this vertical imbalance would translate into potential imbalance that one might experience. The typical threshold for vertical imbalance is often