Which of the following describes the prismatic effect caused by decentering the optical center relative to the pupil?

• A. Displacement of the eye within the socket
• B. Decentration of the optical center relative to the pupil
• C. Changing lens material
• D. Refractive index of the lens

Answer: (B)

When the optical center of a lens is decentered from the pupil, the light rays pass through the lens at an angle rather than straight through the center. This creates a prismatic effect because the lens behaves like a small prism, displacing the image seen by the eye. The magnitude of this prism is roughly the decentration in centimeters multiplied by the lens power in diopters, so more decentration or higher power means a stronger prism effect. This is why aligning the optical center with the pupil is important to avoid unwanted prism, which can cause visual discomfort or diplopia. The other options describe different concepts (eye position, changing lens material, or refractive index) that do not explain the prism caused by decentration.