Diopter Scale

A graduated scale, usually cylindrical in shape, attached to, or forming a part of, the focusing nut or knob of the eyepiece of an optical instrument. It is read in conjunction with a line or dart indicator to measure the change in refracting power of the eyepiece, in diopters, and introduces correction to compensate for the eyesight variation of the observer. It may have a knurled periphery and/or teeth on the inside diameter.