For precise measurement of objects you will need to calibrate your eyepiece scale for each objective lens to know the value of the minor divisions of the scale at each magnification. Using a stage scale and checking each objective lens does this. The stage scale is usually a glass slide with 1 mm engraved on it showing 100 minor divisions. Each of these 100 minor divisions will be equal to 10 micrometers, or 0.010 mm

1) Place the stage scale on the microscope stage and focus onto it using the objective lens that you wish to calibrate.

2) Align a convenient major marker to one end of your eyepiece scale and look across the two scales for another point where the division points are aligned.

3) In the example above 40 divisions (400 micrometers) of the stage scale are equal to 70 minor divisions of the eyepiece scale.

4) 400 divided by 70 equals 5.71 so that each minor division of the eyepiece scale, while using this objective lens, is equal to 5.7 microns.

Repeat these steps for each objective lens that you wish to use for measuring.