As you increase the magnification the working distance decreases dramatically. The shortest working distance possible is the length of the dotted line within the prisms plus the thickness of the vessel wall. Overview Of The Fesem System - Iit Of The Fesem System 1 Electron Principle Of Operation 1 By substituting smaller prisms, it may be adapted for use with a compound microscope. Shortest working distance microscope . It's called the working distance and is defined as, the distance from the front lens element of the objective to the closest surface of the coverslip when the specimen is in sharp focus. Microscope objectives are generally designed with a short free working distance, which is defined as the distance from the front lens element of the objective to the closest surface of the coverslip when the specimen is in sharp focus. Working distance is the distance between the objective lens and the specimen. This would not impose a handicap with any objective lens o...