Fig 10. When both horizontal and vertical prismatic power are required in a single lens, a Fresnel membrane prism can be applied at an oblique axis to provide the resultant prism. In the lens on the left, a 7Δ prism has been applied with its base at 45° down and out. This application provides a 5Δ base down and a 5Δ base out. The lens on the right has a 5Δ prism applied with its base out.