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Field of view, or FOV in short, is a term many people understand as a fundamental aspect of optics and visualizing a subject. We use this information through many applications such as cameras and binoculars to see what’s around us.

FOV plays a vital role in microscopes as well, so we’re going to explain the specifics of what it is and how it works. **In short, the Field of View is the area you can see through the eyepiece of your microscope.**

Generally speaking, the field of view is the amount of visual area you can see when viewing through an optical instrument, namely the eyepiece or viewfinder. A wide field of view means you can see a lot of stuff around you, whereas a narrow field of view constricts you to a smaller area. On that note, it also affects the perceived size of a subject.

With microscopes, the field of view varies depending on the eyepiece diameter as well as the objective lens diameter. FOV can change how much you can see on a sample, especially when using different magnifications. For example, a 4X objective lens will provide a much wider FOV compared to a 20X objective lens. The latter will let you see less area, but it also shows a perspective that enables one to see at an even smaller scale.

Calculating the FOV of a microscope takes a bit of math and patience. There is a simple formula to find the field of view on a microscope. However, we need to discuss a few other terms before we show you that.

Firstly, you’ll need to know the field number (FN). The FN should be inscribed on the eyepiece/ocular lens, but you can measure it by finding the diameter of the area that you can visibly see out of. This is done by taking a measuring tape and placing it where you would have the sample. Count the number of all lines that can be observed on the measuring tape to find the FN. Keep in mind that the measurements are recorded in millimeters.

After that, you’ll want to seek the objective lens magnification (MO). You should be able to see this inscribed on the outside of whichever objective lens you are using. For example, it may be 4X, 10X, 20X, or even 100X.

With this information in hand, there’s only one step left. Take the field number and divide it by the objective lens magnification. Your results will be equal to the field of view measured in millimeters.

To see this in action, let’s take a common example to demonstrate how it works. Let’s say you have a field number of 15 and your objective lens has a 4X magnification. If we use the FOV solution formula, the solution will come to ~3.75 mm.

15 ÷ 4 = ~3.75

Oh, and don’t forget to always remember to use an approximation symbol when jotting down the solution. Even though our example came out to 3.75, the actual measurement taken with the field number isn’t absolute.

Field of view is used as a means of finding how much observable space there is in the eyepiece. Furthermore, this can be an important way for us to make adjustments for micrography; or photography through a microscope.

There are a couple of big reasons why finding the field of view of a microscope is beneficial. Solving the FOV formula enables observers to spend more time examining new subjects. If you can pre-calculate your FOVs for each objective lens, you’ll be able to quickly switch from different magnifications while viewing the same subject because you can make the adjustments effortlessly. Finding the FOV is also crucial for micrography, as we mentioned earlier.

In any case, it’s good to know this stuff as it shows that you have confidence and knowledge in the lab. It can also be a positive if you want to learn more about optics in general, as these equations can help you become familiar with other aspects of the subject.

Finding the FOV isn’t always easy. The process of getting the field of view number takes some time upfront and you may need to do this for every new objective lens you use. Anyway, there are many observers who don’t care to learn these parameters as you can still enjoy viewing samples and focus properly without this information.

There isn’t exactly a limit to how large the FOV of a microscope can get. It all depends on which one you’ve got and potentially what camera sensor you’re using to photograph your subject(s).

We won’t go too far into the details, but depth of field (DOF) is a term we use to describe the focusing area on a viewable plane. A shallow DOF in micrography will have lots of background blur and focuses more on a specific area. It also means more light will be captured. A deep DOF will show an image or observation that illustrates more areas of the sample in focus (no blurry areas).

Nope. An increase in magnification decreases a lens’s FOV. A 4X magnification will always have a larger FOV than a 20X magnification; so long as all other parameters are the same. You’ll end up seeing a smaller and smaller area if there is more zooming.

Find a cup or an object that has a hole in it that you can easily see through. Bring it close to your eye, and then slowly move it farther away. You probably noticed that the amount of stuff that you could see out the other end became less and less. This is essentially what happens with microscope magnification too!

There are quite a lot of terms and things you should know to better understand the processes of a microscope and its observation abilities. Field of view is a fundamental term for pretty much anything that has optical capability—including our own eyes! With microscopes, the basic formula of the FN divided by the OM gives us the total field of view of our microscope, which is something all microscope lovers should learn. Either way, this article should definitely set you in the right direction so that you can take a closer look at a whole world of subjects!

Featured Image Credit: Matej Kastelic, Shutterstock

Robert’s obsession with all things optical started early in life, when his optician father would bring home prototypes for Robert to play with. Nowadays, Robert is dedicated to helping others find the right optics for their needs. His hobbies include astronomy, astrophysics, and model building. Originally from Newark, NJ, he resides in Santa Fe, New Mexico, where the nighttime skies are filled with glittering stars.

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