Can prism be introduced by decentration in aspheric lens?

• A. Yes, by decentration
• B. Prism cannot be used with aspheric lenses
• C. No, ground-in only
• D. Yes, by coatings

Answer: (C)

Prism is a deliberate deviation of light that you want to control in a lens. In aspheric lenses, you can’t rely on decentering the lens to produce a specific, fixed prism because the aspheric surface has a varying curvature across the lens. This makes the way light bends—the prismatic effect—nonuniform and dependent on where the light enters the lens, so the resulting prism isn’t predictable or repeatable. The only dependable way to introduce a known amount of prism in an aspheric lens is to grind it in during fabrication. Coatings don’t add prism, they affect surface properties but not the bending of light in the lens. So, for a defined prism in an aspheric design, it must be ground-in rather than produced by decentration.