Do plus lenses effectively become weaker or stronger as they approach the cornea?

Do plus lenses effectively become weaker or stronger as they approach the cornea?

• A. Stronger
• B. Weaker
• C. No change
• D. Undefined

Answer: (B)

Vertex distance changes how much power a lens actually provides at the cornea. For a plus lens, moving the lens closer to the eye (reducing the vertex distance) reduces the amount of convergence that reaches the retina, so the effective dioptric power at the cornea is weaker. In practical terms, the power you see at the eye is less when the plus lens sits nearer the cornea, and it increases if the lens sits farther away. Therefore, plus lenses effectively become weaker as they approach the cornea.

Vertex distance changes how much power a lens actually provides at the cornea. For a plus lens, moving the lens closer to the eye (reducing the vertex distance) reduces the amount of convergence that reaches the retina, so the effective dioptric power at the cornea is weaker. In practical terms, the power you see at the eye is less when the plus lens sits nearer the cornea, and it increases if the lens sits farther away. Therefore, plus lenses effectively become weaker as they approach the cornea.