Distance which is determined along the optical axis within the column from (1) the lower end of the final optical element between the source and the specimen stage; to (2) the point where the beam is focused.

Working distance is a property of the optical system and as such independent of the position of the specimen. During regular operation, e.g. of SEM & STEM, the here mentioned point of focus will be in direct vicinity of the specimen surface.Typically, the unit used for expressing the working distance is millimetre.The term is frequently used in scanning electron microscopy & scanning ion microscopy.In modern SEM and focused ion beam microscopes working distance is displayed in the user interface as a calibrated value based on the final lens excitation. For practical reasons it commonly refers to the lower end of the column instead of the lens as this gives the user an indication how much space there is between the column and the sample surface when the latter is in focus. In general, a short working distance improves resolution and signal collection for in-column detectors.