When reading, how much more will a patient need to converge with a new RX that has a large increase in plus power?

When a patient receives a new prescription that features a significant increase in plus power, the amount of convergence required while reading will increase. This is primarily due to the optical effects of a higher plus lens power, which causes the visual image to appear further away than it actually is.

As plus lenses magnify images and make them appear larger, the visual system compensates by requiring more convergence—essentially the inward movement of the eyes toward each other—to maintain single, clear vision at a closer working distance, such as that required for reading. As the added plus power increases the effective optical distance, the eyes need to converge more to keep the target in focus.

It’s important to understand that while prescribing plus lenses can help with near vision, the resultant increase in convergence requirements must be assessed, especially in patients with existing convergence insufficiency or similar conditions. Thus, a significant increase in plus power directly correlates with an increased demand for convergence when reading.