Large aperture (23mm) rotating prisms permit more flexibility than a Phoropter. This version is +/- 20 PD and has a convenient handle. The prism is controlled using a knurled rotating knob. The accurately calibrated dial shows prism diopter and rotates through 20 PD right-exo to 0 PD to 20 PD left-eso. A removable Red Maddox Filter is available to fit this prism. See product SGR-968-MF.