Plus lenses indirectly test which vergence function?

• A. PFV
• B. NFV
• C. Accommodation
• D. Proximal vergence

Answer: (A)

Plus lenses reduce the need for accommodation when viewing a near target, which in turn lowers accommodative convergence. With less accommodation-driven convergence, maintaining single vision relies more on fusional vergence that can increase convergence without changing accommodation. That ability is positive fusional vergence. So the plus lenses indirectly probe how well the eyes can converge using fusional vergence to keep the image single. Negative fusional vergence would come into play if divergence were required, which isn’t what plus lenses primarily induce. Accommodation is a focusing process, not a vergence function, and proximal vergence is driven by awareness of near distance rather than the lens-induced shift in accommodation.