On which of the following lenses would you perform a bicentric grind on a plastic lens to correct for vertical imbalance at the reading level?

The bicentric grind is a specific modification applied to lens surfaces, primarily used to address vertical imbalance in the reading area of the lenses. When one eye requires a significantly different power than the other eye, this can lead to discomfort or difficulties during near tasks, such as reading. The goal of a bicentric grind is to equalize the effective optical center heights of both lenses.

In this context, the mention of "O.S." (the left eye) suggests that the reading prescription for the left eye has a different base or requires an adjustment to align correctly with the right eye (O.D.). Therefore, performing a bicentric grind on the left eye lens enables the wearer to experience more comfortable vision at the reading level by ensuring that both lenses have their optical centers aligned appropriately.

Choosing to only grind the lens for one eye helps to avoid unnecessary adjustments to the other lens unless it is warranted, as can happen in cases where the imbalance is split or favors one eye's need significantly more than the other. A bicentric grind is precisely that—the solution to enable better functionality for reading by adjusting the lens in a targeted manner.