A plus lens moved downward from the primary position results in a:

When a plus lens is moved downward from the primary position, it creates a prismatic effect that is interpreted as a base down effect. This phenomenon occurs because of the optics involved with lenses.

In a plus lens, when the optical center is moved downward, the light rays passing through that point are bent more towards the base of the lens. This deflection directs the image perceived by the observer upward. The upward shift in the perceived image corresponds to a base down prism effect, meaning that the light appears to be deviated in such a way that the base of the prism is positioned downward.

Understanding this concept helps in practical applications, such as when prescribing lenses or assessing visual alignment, ensuring that the effects of lens positioning on visual perception are appropriately managed.