By changing the distance between the lens and the object being viewed , its focus will change as well. This distance depends on the focal length of the lens which is mounted in the microscope.
The most commonly used lens in dentistry is 250mm lens (rarely 200mm or less 300mm).
For 250mm lens the seen  picture will be sharp only when the lens of the microscope will be at a distance of 250mm from the treatment area (similarly for the other lenses).

Ways of the focus setting:

• Change the height of the microscopes’ head.
  By observing the image through the microscope oculars one should raise or lower the arm until the image becomes sharpest.
   
• Change the height of the unit with the patient.
  By observing the image through the microscope oculars one  should raise or lower the unit with the patient until the sharpest image occurs.
   
• Working with the mirror.
  Pulling up or pushing down the mirror form the watching object changes the distance between the lens and the object, and thus the image sharpness.

You should set the height of the head of the microscope, dental unit and operator chair so the  body silhouette was fully straight. Just proper selection of these settings enable ergonomic work (see ergonomics).

If you already know how to influence the image sharpness, you can go to other microscope settings: stereoscopic view setting.