American Board of Opticianry (ABO) Practice Test

If the pupillary distance is ground too wide for a plus prescription, what type of prism base is likely to result?

1. Base out and base out

2. Out and In

3. Up and Down

4. In and Out

The correct answer is: Base out and base out

When the pupillary distance (PD) is ground too wide for a plus prescription, the lenses will not properly align with the patient's visual axis. In this situation, the optical center of each lens ends up further away from the line of sight, creating a need for compensation through prism. For a plus prescription, if the PD is too wide, the resulting deviation will lead to a base-out prism effect in both lenses. This is because the eye will have to turn outward to compensate for the misalignment caused by the excessively wide PD. In practical terms, this means that the patient may experience double vision or discomfort because the lenses are not directing light properly into the eyes' visual center. Thus, the correct answer reflects that both lenses would create a base-out prism when the PD is ground too wide for a plus prescription, as they require outward adjustment to compensate for the error.