prismatic effects
prismatic effect with the prism base pointed away from the optical center. Conversely, a plus lens, which is thicker at the center and thinner at the edge, produces a prismatic effect with the prism base pointed toward the optical center (Figure 3). Figure 3. A minus lens, which is thicker at the edge, produces a prismatic effect with the base pointed away from the optical center, whereas a plus lens, which is thicker at the center, produces a prismatic effect with the base pointed toward the optical center of the lens. In order to bring rays of light to a focus, an optical lens must deviate these rays toward the focus of the lens in much the same way. that a prism deviates light.