The amount of prism used for prism thinning a progressive lens should equal what fraction of the add power?

When considering prism thinning in the context of progressive lenses, the goal is primarily to manage the effects of the added power on the lens design, specifically minimizing unwanted prismatic effects. In this case, the fraction of the add power used for prism thinning is crucial in achieving the desired optical effect while maintaining the functionality of the progressive lens.

The correct choice indicates that the amount of prism should be 2/3 of the add power. This is based on the principle that the additional power introduced in progressive lenses can induce unwanted prism displacement, particularly in the lower regions of the lens where the add power is most significant. By employing 2/3 of the add power as the prism amount, the lens can help in aligning the optical axis more closely with the line of sight, thus providing a clearer vision for the wearer.

Using this ratio serves to effectively balance the lens prescription and mitigate any prismatic effects that could arise from the added curvature and power in the progressive zones. It ensures that the wearer does not experience excessive prismatic effects while transitioning between different focal areas of the lens. This careful calibration is essential for creating a comfortable and visually effective lens design.

In summary, the use of 2/3 of the add power for prism thinning is justified as