The working distance (W.D.) is determined by the linear measurement of the objective front lens to the focal plane. In general, the objective working distance decreases as the magnification and numerical aperture both increase.
The working distance is the distance from the front of the lens to the focal plane. The objectives are designed that this is also where zero OPD occurs. Because of this, the terms W.D., focus, and zero OPD are often used interchangeably.