This diameter is widely used in stereo microscopes, also from other brands or manufacturers. Please select the eyepiece magnification in correspondence to the objective you are using. A low objective magnification allows for a higher magnification of the eyepiece. For example, combined with 4x or 10x objectives, eyepieces with 20x or 25x magnification can be used without loss of image quality. With a 100x objective, a 10x or 12.5x eyepiece is usually the highest recommended eyepiece magnification