How can prism be introduced in an aspheric lens?

• A. By decentration within the frame
• B. Prism cannot be used with aspheric lenses
• C. Prism by special coating
• D. Ground-in prism only, not prism by decentration

Answer: (D)

Aspheric lenses have nonuniform curvature designed to reduce aberrations. Introducing prism by simply decentring an aspheric lens in the frame relies on the lens’s power changing with position, so the actual prism effect becomes inconsistent and can introduce unwanted astigmatism or aberrations as you look through different parts of the lens. Ground-in prism, on the other hand, is incorporated into the lens during manufacturing, so the prism’s magnitude and base direction are fixed and aligned with the lens design. This makes the prism effect reliable and predictable for an aspheric lens. Therefore, prism should be added by grinding it into the lens, not by decentration.