American Board of Ophthalmology (ABO) Practice Exam

When calculating vertical imbalance, the primary concern is determining how the different lens powers create deviations in line of sight, particularly when there are differences in lens prescription between the two eyes. Vertical imbalance primarily arises from differential prism effects created by the combined effect of sphere power, cylinder power, and the segment heights of the lenses.

In this context, the sphere power in the 90-degree meridian is crucial because it affects how light is refracted through the lens. Additionally, the segment height can influence the position of the optical center, thus impacting prism effect. While prism power directly contributes to vertical imbalance, the cylinder power in the 90-degree meridian does not create vertical imbalance on its own. Its effect is considered in terms of astigmatism rather than vertical deviation.

Therefore, when assessing vertical imbalance, the cylinder power in the 90-degree meridian is not necessary to factor in for the calculation, as it does not contribute to the vertical displacement of images as sphere power and prism do.