Calibration of Microscope for Nematode Morphometrics

(Prepared by Dr. Ignacio Cid del Prado, Colegio de Postgraduados, Mexico)

 Rev 03/29/2021  



To classify an organism, it is necessary to study of the internal and external morphology, including the measurements of the morphological structures. Accuracy of the measurements is determined by the micrometric coefficient, which is determined using an ocular scale and a stage micrometer slide that is divided into micron units (1 micron = 0.001 mm).

The ocular scale is a glass disc of (usually) 2 cm in diameter with a scale 1 to 2 cm long that is dividied into 20 major divisions, each with 10 subdivisions.

The stahe micrometer slide is engraved with a scale 1 to 2 mm long that is divided into 100 parts so that each division measures 10 microns.

The micrometric coefficient is the the number of microns on the stage micrometer that is equal to one subdivision  on the ocular scale.


The micrometric coefficient differs for each objective of the microscope and must be determined separately for each objective.

1. Place the  stage micrometer slide on the stage, focus on the scale

2. Replace one of the eyepieces with the eyepiece that has the ocular scale, correct the focus of both scales so that they are very close to each other or overlapped in such a way that they coincide.

3. Starting from zero of both scales, count the number of divisions on the ocular scale that exactly coincides with a mark on the stage micrometer. The more distant the coincident lines are from the starting point, the more exact the calibration.

4. Note that in the figure below, the zero of the ocular scale is aligned with the zero of the stge micrometer slide. Then note that the 57 of the ocular scale aligns with the 0.27 of the objective slide. Since each division of the stage micrometer is 10 microns, 57 units of the ocular scale corresponds to 270 microns.

The micrometric coefficient is then 270/57 and each division of the ocular scale represents 4.73 microns.

Thus: micrometric coefficient = (Number of divisions of the stage micrometer x 10) / (Number of corresponding ocular scale units)


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