Measures the spherical power, astigmatism of prisms, plane parallel plates, reflector assemblies, etc. or to set the diopter scale reading in various optical systems to a resolution of 1 millidiopter. Range of ±200 millidiopters and 20x magnification. Successful repeating of sharp focus setting is within 0.5 milliopter when viewing a 7.5 second National Bureau of standards resolution target, at a distance of 40 feet. Field of view: 2°. Weight: 3¼ lbs.