distance between front of microscope objective lens & upper surface of coverglass, with specimen in focus.

the distance between the cover glass or object and the tip of the objective. This governs the allowable movement of the objective in obtaining critical focus of the specimen.

The physical distance between the external, metal parts of the objective lens and the specimen's surface. This is the space available for placing certain electron, X-ray and cathodoluminescence detectors. To achieve the highest resolution, the working distance must be made as small as possible, which can lead operational complications and engineering compromises.