Which bifocal style would have the least total prismatic effect in the reading area for the Rx -9.50 O.S. ADD +2.50?

When considering the total prismatic effect in the reading area for bifocal lenses, it's essential to understand how different bifocal designs distribute prism based on the prescription's characteristics.

The executive style bifocal is designed with a large reading segment that spans the entire width of the lens. This design allows for better stability in terms of prism as the eye moves through the lens. Since the reading area is more extensive and sits lower compared to other bifocal styles, the verticle and horizontal prismatic effects are reduced.

In contrast, styles such as round segments or ST40 have smaller reading areas, and the additional curvature at the edges can lead to higher prism effects, especially for higher prescriptions like -9.50 with an ADD of +2.50. The Ultex A style, while also offering a different bifocal design, can exhibit similar issues with prism effects if not properly aligned with the patient's visual axis due to its shape.

Therefore, the executive bifocal style, with its larger primary reading segment positioned strategically, minimizes the total prismatic effect for a patient with a significant distance correction and an add power, making it the best option among the choices.