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Measuring and Counting with a Light Microscope

Author(s): David R. Caprette, PhD

Calibrating the Fine Focus Control

How do you suppose I found the depth of focus for each of the objective lenses on my microscope? Such practical information is hard to find in textbooks.It is tricky to measure distances in the vertical dimension, but it can be done.

The microscope I use has fine focus controls, each marked with 100 divisions. With the stage lowered, I measured the distance from top of the stage to a point on the microscope stand directly above the stage. I then brought the stage up by rotating the fine focus twenty turns and re-measured the distance. I found that with each full rotation, the stage traveled 0.2 mm, or 200 µm. That explains why the divisions on the fine control were marked from zero to 200! If I had looked in the manual, I might have found that the instrument was designed with a calibrated fine focus.