A minus lens with a decreased vertex distance will have what effect?

• A. A strengthening effect
• B. A weakening effect
• C. No effect
• D. Depends on lens material

Answer: (A)

Bringing a minus lens closer to the eye increases its effective power. The distance from the back of the lens to the eye (vertex distance) modulates how strong the lens’s refraction appears at the eye. For minus lenses, shortening that distance makes the lens act more strongly (the magnitudes of the negative diopters increase), so the corrective effect becomes stronger. This happens because the divergent action of the minus lens has its impact closer to the eye, increasing the effective myopic correction. The effect is about geometry, not the lens material, so material choice doesn’t change this outcome.