Primarily used to check the range and make the “zero” setting of focusing eyepieces for optical instruments such as binoculars, range finders, theodolites, spectrometers, telescopes, etc. It is useful in accurately determining the diopter setting of another instrument. It has a resolution of one diopeter. In use, the dioptometer is placed between the eye and the eyepiece of the instrument to be checked. The focusing sleeve of the dioptometer is then adjusted until the field is in sharp focus.