American Board of Ophthalmology (ABO) Practice Exam

When looking below center of a minus lens, what type of prism results?

1. Base up

2. Base down

3. Base in

4. Base out

The correct answer is: Base down

When looking below the center of a minus lens, the optical effect created is similar to the action of a prism that has its base pointed downward. This is because the minus lens diverges light rays, which alters the apparent position of objects in relation to the visual axis. Specifically, when a person looks below the optical center of a minus lens, the visual axis is shifted downward relative to the lens center. The resulting perception is that objects viewed through the lens appear lower than they actually are, mimicking the effect of a prism that has its base directed downwards. This effect can lead an observer to perceive a movement of the visual field in a downward direction, which is characteristic of base down prism. In contrast, looking above the optical center of a minus lens would shift the visual perception upward and relate more to a base up prism. Similarly, lateral movements (base in or base out) traverse horizontal changes in perception, but these are not applicable when the focus is strictly on vertical deviations that arise when looking below the center of the lens.