To correct vertical imbalance at near, a bicentric lens would correct the power at?

The reason why the choice of 090 degrees is the correct answer relates to the specific measurement of vertical imbalance in the context of binocular vision and lens design. A bicentric lens is designed to balance the optical power in both eyes at a specified viewing distance, typically used to correct for differences in the effective lens power that can cause vertical imbalance when looking at near objects.

When correcting vertical imbalance, the primary concern is aligning the optical centers of the lenses to ensure that light entering through them is focused correctly on the retina during near tasks, which generally involves reading or similar activities. The geometric orientation of 090 degrees corresponds to the vertical meridian, which directly influences how light is managed vertically through the lenses.

Using a bicentric lens at this orientation allows for alignment adjustments that can effectively reduce or eliminate this imbalance. Thus, the lens power at 090 degrees is critical in making fine adjustments that contribute to the overall visual comfort and clarity for tasks performed at near distances, especially for those who may have varying prescription strengths in each eye.