Eyepieces, in addition to having the magnification inscribed, are marked 20, or 22, or 26.5. What do these numbers mean?
The designation is the field number of the eyepiece. The higher the field number of the eyepiece being used with a particular objective, the more specimen area will appear in the field of view. The diameter of the field of view, in millimeters, is calculated by dividing the field number of the eyepiece by the magnification of the objective. (For example, with a 10x objective and an eyepiece with a field number of 22, the diameter of the field of view would be 2.2 millimeters) For many microscopists, e.g. hematologists, it saves time to be able to see more of the specimen at a given time. Eyepieces with a field number of 26.5 are called super-wide eyepieces.
