Fig 21. Bifocal wearers can have the power of their reading segment modified in the office by applying a low power plus Fresnel lens as shown on the left. This additional +0.50 D serves the patient while he awaits his new bifocal glasses and afterward as a spare pair of updated bifocals. A variation of this procedure is to apply the Fresnel lens so that it covers the existing bifocal and extends above it to create an intermediate focal area shown on the right.