Working distance is a term used in microscopy to describe the distance between the specimen and the lens. Microscopes usually have a short working distance.
Microscopes use a parfocal lens that stays in focus when the magnification or focal length changes. Working distance refers to the space between the front lens element, or tip of the microscope lens, and the closest cover slip surface. The cover slip is the thin protector cover placed over a specimen. In cases of specimens without cover slips, linear measurement of the front lens to the specimen surface is used to calculate working distance.
Certain substances, such as toxic or metallurgical specimens, require thick cover slips. In such cases, working distance is described as extra long (ELWD) or super long (SLWD).