What is the magnification of a specimen if the eyepiece power is 15X and the objective power is 40x?

Updated December 08, 2020

By Karen G Blaettler

Microscopes magnify the tiniest inhabitants of this world. From the minute details of cells to the delicate cilia of paramecium to the intricate workings of Daphnia, microscopes reveal many miniscule secrets. Calculating total magnification uses simple observation and basic multiplication.

Microscopes use lenses to magnify objects. A simple microscope uses only one lens; a magnifying glass could be called a simple microscope. The magnification of a simple microscope doesn't need any calculation because the single lens is usually labeled. A hand-lens, for example, might be labeled with 10x, meaning the lens magnifies the object to look ten times larger than the actual size.

Compound microscopes use two or more lenses to magnify the specimen. The standard school microscope combines two lenses, the ocular and one objective lens, to magnify the object. The ocular or eyepiece is found at the top of the body tube. The objective lens points down toward the object to be magnified. Most microscopes have three or four objective lenses mounted on a rotating nosepiece. Rotating the nosepiece lets the viewer change the magnification. Different objective lenses provide different magnification options.

Finding the magnification of each lens requires examining the casing of each lens. On the side of the casing is a series of numbers that includes a number followed by x, as 10x. This 10x shows that the lens magnifies an object to appear ten times larger than reality. Depending on the manufacturer, this magnification number may appear at the beginning or at the end of the number sequence. To calculate total magnification, find the magnification of both the eyepiece and the objective lenses. The common ocular magnifies ten times, marked as 10x. The standard objective lenses magnify 4x, 10x and 40x. If the microscope has a fourth objective lens, the magnification will most likely be 100x.

Once the magnification of each individual lens is known, calculating total magnification is simple math. Multiply the magnification of the lenses together. For example, if the eyepiece magnification is 10x and the objective lens in use has a magnification of 4x, the total magnification is:

10\times 4 = 40

The total magnification of 40 means that the object appears forty times larger than the actual object. If the viewer changes to the 10x objective lens, the total magnification will be the ocular's 10x magnification multiplied by the new objective lens's 10x magnification, calculated as:

10\times 10 = 100

Note that calculating magnification in telescopes uses a different equation than calculating magnifiction in microscopes. For telescopes, one magnification calculation uses the focal lengths of the telescope and the eyepiece. That calculation is:

\text{magnification}=\frac{\text{focal length of telescope}}{\text{focal length of eyepiece}}

Like the microscope, these numbers usually can be found on the telescope.

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Magnification:

Figuring Total Magnification

Magnifying Objects/ Focusing Image:

When viewing a slide through the microscope make sure that the stage is all the way down and the 4X scanning objective is locked into place. Place the slide that you want to view over the aperture and gently move the stage clips over top of the slide to hold it into place. Beginning with the 4X objective, looking through the eyepiece making sure to keep both eyes open (if you have trouble cover one eye with your hand) slowly move the stage upward using the coarse adjustment knob until the image becomes clear. This is the only time in the process that you will need to use the coarse adjustment knob. The microscopes that you will be using are parfocal, meaning that the image does not need to be radically focused when changing the magnification. To magnify the image to the next level rotate the nosepiece to the 10X objective. While looking through the eyepiece focus the image into view using only the fine adjustment knob, this should only take a slight turn of the fine adjustment knob to complete this task. To magnify the image to the next level rotate the nosepiece to the 40X objective. While looking through the eyepiece focus the image into view using only the fine adjustment knob, this should only take a slight turn of the fine adjustment knob to complete this task.

Total Magnification:

To figure the total magnification of an image that you are viewing through the microscope is really quite simple. To get the total magnification take the power of the objective (4X, 10X, 40x) and multiply by the power of the eyepiece, usually 10X.

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This entry was posted on March 16, 2020 by Accu-Scope.

Most compound microscopes come with interchangeable lenses known as objective lenses. Objective lenses come in various magnification powers, with the most common being 4x, 10x, 40x, and 100x, also known as scanning, low power, high power, and (typically) oil immersion objectives, respectively. Let’s take a closer look at each of the different magnifications of objective lenses and when you would use them.

Scanning Objective Lens (4x)

A scanning objective lens provides the lowest magnification power of all objective lenses. 4x is a common magnification for scanning objectives and, when combined with the magnification power of a 10x eyepiece lens, a 4x scanning objective lens gives a total magnification of 40x. The name “scanning” objective lens comes from the fact that they provide observers with about enough magnification for a good overview of the slide, essentially a “scan” of the slide. Some objectives with even lower power are discussed in Specialty Objectives below.

Low Power Objective (10x)

The low power objective lens has more magnification power than the scanning objective lens, and it is one of the most helpful lenses when it comes to observing and analyzing glass slide samples. The total magnification of a low power objective lens combined with a 10x eyepiece lens is 100x magnification, giving you a closer view of the slide than a scanning objective lens without getting too close for general viewing purposes.

High Power Objective Lens (40x)

The high-powered objective lens (also called “high dry” lens) is ideal for observing fine details within a specimen sample. The total magnification of a high-power objective lens combined with a 10x eyepiece is equal to 400x magnification, giving you a very detailed picture of the specimen in your slide.

Oil Immersion Objective Lens (100x)

The oil immersion objective lens provides the most powerful magnification, with a whopping magnification total of 1000x when combined with a 10x eyepiece. But the refractive index of air and your glass slide are slightly different, so a special immersion oil must be used to help bridge the gap. Without adding a drop of immersion oil, the oil immersion objective lens will not function correctly, the specimen will appear blurry, and you will not achieve an ideal magnification or resolution. Oil immersion lenses are also available from some manufacturers in lower magnifications, and provide higher resolution than their "high dry" counterparts.

Specialty Objective Lenses (2x, 50x Oil, 60x and 100x Dry)

There are several other objective lens magnifications available with utility for particular applications. The 2x objective, widely used in pathology, has only ½ the magnification of a 4x scanning lens, thus providing a better overview of the sample on the slide. The 50x oil immersion objective, often used in place of the 40x objective, is used as a gold standard for observing blood smears. The 60x objective, often available in either dry or oil immersion, provides 50% greater magnification than a 40x lens. The 60x dry is sometimes chosen over a 100x oil immersion lens for higher magnification without the need to use immersion oil.  Finally the 100x dry objective doesn’t need immersion oil to deliver high magnification (still 1000x when combined with 10x eyepieces). However, the numerical aperture (an indication of resolving power of an objective) of a 100x dry objective is much lower than that of a 100x oil immersion objective and, as a result, the ability of the lens to resolve fine details in the specimen is much lower, too.

It is important to always use the correct immersion media (e.g. air, water, oil, etc.) that is specified by your objective lens.

• The image produced by the wrong immersion media will be blurry. In general, objectives are engineered to "look" through an immersion medium with a particular refractive index (a topic for another article). For example, air has a refractive index of close to 1.0, whereas standard immersion oil has a refractive index of ~1.51.
• You can damage the objective if you use the wrong immersion oil.

If you are interested in buying various types of objective lenses for your microscope in the classroom, laboratory, research facility, or any other purpose, ACCU-SCOPE can provide the products you are looking for. Contact us today to learn more about our objective lenses and other microscope accessories.