Which of the following decreases spectacle magnification in a plus lens?

• A. Increasing vertex distance
• B. Increasing central thickness of lens
• C. Increasing base curve
• D. Increasing index of refraction

Answer: (D)

Spectacle magnification for a plus lens comes from how the eye-lens system alters the angular size of objects as seen by the eye. A key part of that effect is where the principal planes lie relative to the eye; thicker lenses and steeper surfaces push those planes farther from the eye, increasing the magnifying effect.

If you raise the index of refraction, the same plus power can be produced with a thinner lens and with less curvature, so the lens material becomes more refractive without needing extra thickness. That keeps the principal planes closer to the eye and reduces the angular magnification produced by the lens. In other words, higher index reduces spectacle magnification.

The other factors would tend to increase magnification or have less impact on reducing it: increasing vertex distance often lowers magnification by moving the lens away from the eye; increasing central thickness or base curve tends to push optical planes farther from the eye and can increase magnification.