How to Calibrate Magnification

by Dana Schafer

The magnification of a microscope is measured by multiplying the magnification of the ocular lens, which is usually 10X, by the objective lens on the microscope. The four objective lenses on a microscope are 4X, 10X, 40X and 100X. Every microscope may vary in magnification; therefore, it is important to calibrate your microscope before using it to obtain accurate measurements and images. Calibration of magnification requires manipulating both the ocular lens, objective lens, and the stage micrometer.

Locate the ocular micrometer inside the ocular lens. Place your eyes over the microscope eyepiece. The ocular micrometer is the visible scale shown as you look through the eyepiece. Each line represents 1 micrometer. Note that the ocular micrometer will not change in size when the 4X, 10X, 40X and 100X objective lenses are changed; therefore, each objective lens must be calibrated before using.

Locate the stage micrometer. There will be an additional scale that is shown on the stage. Each line in this scale represents 0.01 micrometer.

Start with the lowest scanning objective lens, which is the scanning lens, or 4X lens. Focus your view on the stage micrometer using the Coarse Objective Knob and Fine Adjustment Knob located on the side of the microscope. Move the ocular micrometer until it is exactly in place with the edge of the stage micrometer scale at both ends of view.

Look for the second point on the scale, which would be the point from the beginning, where the scales overlap again. This will give you the correct units. Count the number of micrometers and millimeters. Convert the micrometers to millimeters by multiplying your number times 1,000, since there are 1,000 microns in one micrometer. Divide the number of ocular units into the number of stage units that you determined.

Record your results. Repeat the procedure for each objective lens.

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About the Author

Based in Huntington Beach, Calif., Dana Schafer has been writing environmental articles and grant proposals since 2006. Schafer has written for Grace Unlimited Corporation and Youth Have Vision. Schafer is in the process of receiving a Master of Science in biology from California State University, Long Beach.

Photo Credits

  • photo_camera microscope image by Fotocie from Fotolia.com