American Board of Opticianry (ABO) Practice Test

What do we call a plus lens that, when moved from its primary position, results in a BU prism?

1. BI Prism

2. A lens that has been decentered

3. Press-On Prism

4. BU Prism

The correct answer is: A lens that has been decentered

A plus lens that, when moved from its primary position, creates a base-up (BU) prism effect is indeed described as a lens that has been decentered. When a lens is decentered, it means that it has been shifted away from its optical center. This misalignment causes light rays passing through the lens to bend in such a way that they exhibit prismatic effects—specifically, a base-up effect if the lens is a plus lens. Decentering in the context of a plus lens means that the optical center is no longer in line with the pupillary distance of the wearer. As a result, the light entering the lens does not pass through the optical center and instead creates a prism effect. In the case of a plus lens, decentering upwards will yield a base-up prism, which can be beneficial for certain conditions such as vertical imbalances or for aligning the visual axes of the eyes effectively. Other options do not appropriately describe this phenomenon. A base-in (BI) prism is the opposite of a base-up prism, and not directly related to the context of a plus lens. A press-on prism usually refers to a method of adding prism power to a lens rather than describing how the prism effect arises from lens positioning.