WORKING DISTANCE is space between the bottom of objective the lens and the surface of the slide.
The distance between front edge of the objective and the lens and the specimen surface ("with the surface of the cover glass in case of the cover glass objective lens") when the specimen is also focused.
The working distance (W.D.) which is determined by the linear measurement of the objective and front lens to the focal plane. In the general, the objective of the working distance decreases by the magnification and the numerical aperture of both increase. The working distance is the distance and from the front of the lens to be the focal plane
