If the center of the cross hairs falls to the right of the center of the bulls-eye during lensometry for the right eye, what type of prism is in the lens?

• A. BI
• B. BO
• C. BU
• D. BD

Answer: (A)

In lensometry, the prism in the lens shifts the image of the reticle toward the prism’s base because a prism bends light toward its base. So the direction of the reticle’s displacement tells you the base direction of the prism.

Since the center of the cross hairs moves to the right for the right eye, the light is deviated toward the right, which corresponds to a base toward the nose on the right eye — this is Base-In.