There are two working distances for microscopes: objective and stage. Together they define the space you have to work with objects and still focus with your microscope.
Objective working distance is the vertical distance from the objective’s front lens to the closest surface of the specimen when the specimen is sharply focused. It’s the space you have to get a specimen in under the lens and still get a focus. Some objectives take into account the thickness of the slide coverslip in working distance measurements.
Working distance can range from 50 mm for dissecting or stereo microscope objectives to less than 0.1 mm for high-power oil immersion objectives. It decreases as magnification, resolution and numerical aperture increase. Microscope objectives are usually designed with short working distances.
The stage working distance is the vertical distance that is available for focusing by moving the stage, arm, or tube, either separately or together. When combined with the objective’s working distance, it determines how big or thick a specimen can be and still be viewable. Larger working distances are best for looking at larger specimens like whole rocks and entire stamps. Low-power microscopes will have more generous stage working distances than high-power microscopes. Low-power microscopes will also be able to see more of the surface of the specimen, since the larger working distance and low power give them the ability to view more of the specimen at once.