How To Find and Use Field of View (Like the Best of ‘Em!)

Today in science, we will calculate the field of view in a microscope

We use light microscopes in biology class so that is what I’m going to talk about today.

The eye-scope itself, called the ocular scope, is a magnification of 10x and the scopes below the eye-piece, called objectives, is 4x, 10x, 40x, and 100x. So to get the right kind of magnification, you just simply times the ocular magnification by the objective magnification that you’re using.

Meaning: 10 x 40 = 400

Or ocular times objective = total magnification

Keep in mind, when I look into the microscope and the object is usually at 400x not at 40x. You have to start out at 40x and work your way to 400x or if you want to look at bacteria, 1,000x.

Now you’re ready to be science-y 🙂

Microscopes are fun, but if you never used one before you should know that through a microscope everything is opposite. In biology class, we looked at the letter “E.” Under the microscope, it was reverse and upside down.

Calculating how large (or in this case how small) something is can be fun. First, you must measure the diameter of what you can see through the microscope. Usually the diameter is 4.5 mm (millimeters) through a compound light microscope at 40x.

Let’s use that.

Aw, way cool.

If you want to look at a cute little squiggly specimen you might have changed the objectives around. If you did, there is a way to find how big or small your little guy is. It doesn’t matter what objective your microscope is in. Say you’re at a total magnification of 400x. The first thing you want to do is find the diameter to get the field of view at this magnification.

Through your microscope at 40x, say you already took the diameter of the field of view, and that was 4.5 millimeters. The magical equation to finding the field of view through a microscope is:

(FOVd1)(Magnification1) = (FOVd2) (Magnification2)


(4.5 mm)(40x) = (FOVd2)(400x)

You solve for Field of View, which is the diameter for 400x.

(4.5 mm)(40x) = (x)(400x)

x = 0.45 mm

Field of View at 100x

OK, now we can calculate the field of view at 100x.

The equation is (4.5 mm)(40x) = (x)(100x), so all you have to do is multiply 4.5 and 40, which gives you 180. You then divide 180 by 100, and you get 1.8. So the field of view is 1.8 mm through 100x.

How to get the area, diameter, and radius at 40x


You take the FOV (or Field of View) diameter that you measured, which is 4.5 mm and you plug-in the number in this magical equation:

radius = d/2

r = 4.5/2

equals to….(drum roll, please) 2.25 mm.


The total area at 4ox is:

area = radius² x π

2.25² = 5.0625 x π = 15.90

Usually we science-y people like to use a total magnification of 100x or 400x.

 Wow that was fun! So how big is it?

That was fun, but you know what’s even better?

Finding how big something is just from the field of view.

Here are my specimens.


I took a gander at these at 400x.

To get how big these bad boys were I estimated how many I could fit across the diameter. After that, I divided the field of view by how many specimens I could fit across.

Like this:

FOV divided by the number of specimens = 0.45/1.5 = 0.3 mm.

Then we change that into micrometers.

0.3 x 1,000 = 300 micrometers.

So that first guy was about 300 micrometers long.

Wow that was pretty awesome-sauce! Once you know how to science, it get’s really fun!