How to Determine Total Magnification

A microscope magnifies small items so that we can see them more clearly.

The level of magnification needed to examine something with a microscope depends on the quality and size of the specimen. Most microscopes have several elements used to vary the level of magnification. Those elements are referred to as "objectives." Microscopes also have an ocular lens as the eye piece that further magnifies the specimen. All levels of magnification are clearly marked on the microscope. By calculating the total magnification on the microscope, you can tell how big items actually are. This is helpful when trying to make comparative measurements of very small objects.

Things You'll Need

• A microscope.

Instructions

1. Determine the viewing power of the eyepiece of the microscope. It is printed on the eyepiece. In most cases, the viewing power is 10X.

2. Determine the magnification level of the objective. There are usually three or four objectives on the microscope. Each objective has the magnification written on the objective. The scanning objective is 4X, the low power objective is 10X, the high power objective is 40X or 43X and the oil immersion objective is 100X.

3. Multiply the viewing power by the magnification level of the objective to get total magnification. For example, if the viewing power is 10X and you use the high power objective of 40X, then the magnification level is 400X.

4. Find the size of the sample you see under the microscope. Many microscopes have a microscopic scale built into the eyepiece called at "reticle" or "eyepiece micrometer." When looking at the object under the microscope, measure the number of "ocular divisions" it spans on the eyepiece micrometer. The length of the ocular divisions varies depending on your total magnification. To calculate the length of the ocular divisions, you must compare the ocular divisions with a microscopic ruler called a stage micrometer. For example, at 40X, an ocular division on the eyepiece micrometer might be 25 micrometers, which means that at 400X, an ocular division would actually be one tenth that distance (40 divided by 400), or 2.5 micrometers.

   Once you know how long an ocular division is at each magnification, you can use the eyepiece micrometer to measure the objects you are viewing.

References

• SUNY Clinton Community College: Microscopy
• An Introduction to the Laboratory
• Measuring with the microscope