Which element is NOT critical when determining vertical imbalance?

When evaluating vertical imbalance, it is essential to understand the roles of different components in lens design and how they affect visual performance. In this context, the lens power in the 180 degrees meridian is not critical because vertical imbalance primarily arises from differences in lens powers across the vertical meridians, specifically the 90 degrees meridian.

Vertical imbalance occurs when there is an excessive difference in the refractive power of the lenses in the vertical orientation, which can lead to symptoms such as double vision or discomfort. The lens power in the 90 degrees meridian is undoubtedly critical since it directly influences the amount of vertical imbalance experienced by the wearer.

Furthermore, other factors such as lenticular design and the correct index of refraction also contribute significantly to the overall impact on visual performance but are secondary to the specific meridian measurements when it comes to determining vertical imbalance. The correct approach focuses on the vertical distribution of lens powers, primarily the 90 degrees meridian, where the power difference manifests most prominently.