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

• A. A strengthening effect
• B. A weakening effect
• C. No effect
• D. Depends on frame size

Answer: (A)

Understanding how vertex distance affects the power of a minus lens is key. When a minus (diverging) lens is moved closer to the eye, its effective power seen by the eye increases in magnitude, meaning the correction becomes stronger. This happens because the lens’s diverging effect is applied over a shorter distance to the eye, pushing the eye’s focal point toward the retina more than when the lens sits farther away. In practical terms, a minus lens with a shorter vertex distance behaves as if it’s a stronger prescription. That’s why moving the lens closer strengthens the effect.