Which combination of factors tends to produce a higher PRA?

Which combination of factors tends to produce a higher PRA?

• A. High AC/A and Low BI
• B. Low AC/A and Low BI
• C. High AC/A and High BI
• D. Low AC/A and High BI

Answer: (D)

PRA reflects how much accommodation you can add while keeping the eyes aligned, and it hinges on two opposing parts of your vergence system: how strongly accommodation drives convergence (AC/A ratio) and how much divergent (base-in) fusional vergence you have to offset that convergence. If the AC/A ratio is low, increasing accommodation (which you do when minus lenses are added) doesn’t pull the eyes inward very much. That means less convergence is generated as you accommodate more, so there’s less pressure on the divergent system to compensate. If the divergent reserve is high (strong base-in fusional vergence), you can diverge more to maintain single vision as accommodation increases. Together, low AC/A and high BI allow you to tolerate more minus power before fusion breaks, yielding a higher PRA. In contrast, a high AC/A would cause more inward pull with accommodation, demanding greater divergence that may quickly exceed the BI reserve, lowering PRA. Similarly, a low BI limits how much you can diverge to offset the accommodation-driven convergence, also reducing PRA. So, combining low AC/A with high BI best supports a higher PRA.

PRA reflects how much accommodation you can add while keeping the eyes aligned, and it hinges on two opposing parts of your vergence system: how strongly accommodation drives convergence (AC/A ratio) and how much divergent (base-in) fusional vergence you have to offset that convergence.

If the AC/A ratio is low, increasing accommodation (which you do when minus lenses are added) doesn’t pull the eyes inward very much. That means less convergence is generated as you accommodate more, so there’s less pressure on the divergent system to compensate. If the divergent reserve is high (strong base-in fusional vergence), you can diverge more to maintain single vision as accommodation increases. Together, low AC/A and high BI allow you to tolerate more minus power before fusion breaks, yielding a higher PRA.

In contrast, a high AC/A would cause more inward pull with accommodation, demanding greater divergence that may quickly exceed the BI reserve, lowering PRA. Similarly, a low BI limits how much you can diverge to offset the accommodation-driven convergence, also reducing PRA.

So, combining low AC/A with high BI best supports a higher PRA.