This is a very common question on any board exam, and tests your knowledge of** Prentice's rule**:

** PD = h * F**

(where PD is the induced prism; "h" is the distance from the optical center in cm; and "F" is the power of the

lens in diopters)

In this question, PD = 3 prism diopters and "F" is 15. We are solving for "h":

h = PD / F

h = 3 / 15

h = 0.2 cm

** h = 2 mm**

Now we have to determine if we are looking above or below the optical center. A minus lens is basically two prisms placed apex-to-apex with a base up prism superiorly and a base down prism inferiorly. Also remember that the eye wants to move toward the base with induced prism.

Since we want the patient to compensate for a right hypertropia, we want to

**induce a base-down prism effect **by having his right eye look 2 mm

**BELOW** the optical center of the lens. An equivalent way of saying this is we want to move the optical center 2 mm

**UP** in the right lens.

However, this is not an answer choice! Another way of achieving this is to move the optical center up by 1 mm for the right lens and down by 1 mm for the left lens (i.e.

**choice "D"**).