What Is Microscope Field Of View And How To Calculate It?

What Is Microscope Field Of View?

The microscope field of view is the maximum area visible when looking through the microscope eyepiece and is usually given as a diameter measurement. A 20x lens with a field count of 18 would actually have a field of view of 0.9mm.

The field of view for a microscope is the extent of the observable area in units of distance. The optics provide a clear and undistorted view in a field around the optical axis from which the field of view is selected. The rays that produce the image in this view are generally aberration-free and do not produce a significant drop in image intensity.

The Definition of the Microscope Field of View Put simply, this is the area you see under the microscope for a given magnification. Suppose you are looking at a cell or sample under an optical microscope. The diameter of the circle you see is the microscope’s field of view.

The field of view of the microscope depends on the magnification of the objective, more specifically the combined magnification of the objective and eyepiece.

As you increase the magnification, the field of view decreases. Depending on the lens system, this can vary. A rough way to measure the field of view is to use a ruler under the microscope for a specific magnification.

Microscope Field Of View

What Determines Microscope Field Of View?

As mentioned above, the field of view is determined by the diameter of the diaphragm and the magnification of the lenses. You can usually find these numbers printed on the side of the eyepiece of the microscope.

The first number ending with an X is the magnification while the second number is the diameter. This is called the field of view number, or simply field number, and is expressed in millimeters.

The maximum field diameter typically falls in the range of 18 to 28 millimeters (or more) depending on how advanced the objective lens is, such as whether the lens is a special type of flat field lens.

How to Calculate Microscope Field of View?

To calculate the field of view of the microscope, you must first examine the microscope itself. The eyepiece of the microscope should be marked with a sequence of numbers, e.g. 10X/22 or 30X/18. These numbers are the eyepiece magnification or field number, respective.

Also note the magnification of your lens on the underside of the microscope, if necessary usually 4X, 10X, 40X or 100X.

After noting the magnification of the eyepiece, the field number, and the objective magnification number, if applicable, you can calculate the field of view of your microscope by dividing the field number by the magnification number.

Field of View Formula:  Field of View = (Field Number (FN))/(Objective Magnification)

Here’s how to find the field of view if your microscope only uses one eyepiece. For example, if the microscope eyepiece reads 30x/18, then 18 ÷ 30 = 0.6, or a field of view diameter of 0.6 millimeters.

If your microscope uses both an eyepiece and an objective lens, you need to do one step before finding the field of view. You must multiply the eyepiece magnification by the objective magnification to get the total magnification before dividing the field number.

For example, if your eyepiece reads 10X/18 and your objective magnification is 10. First, multiply 10 and 10 to get 100. Then, divide 18 by 100 to get a field of view diameter of 0 .18 millimeters.

If you change microscopes or change eyepieces or objectives, remember to repeat the field of view calculations with the new field number and magnifications. When dealing with objects viewed at higher magnifications, it may be helpful to convert your measurements from millimeters to micrometers. To do this, multiply the diameter of the field of view in millimeters by 1000 to convert the diameter to micrometers.

Field of View at Different magnification:

Eyepiece MagnificationField NumberObjective lens MagnificationTotal MagnificationField Of View
10X1810X100X0.18 mm
 28  0.28 mm
10X1040X400X0.045 mm or (45 µm)
 28  0.07 mm or (70 µm)
10X10100X1000X0.018 mm or (18 µm)
 28  0.028 mm or (28 µm)

How Does Magnification Affect Microscope’s Field Of View?

Microscope field of view changes as magnification changes. In short, as magnification increases, the field of view decreases. When looking through a high-power compound microscope it can be difficult to determine what you will see through the eyepieces at different magnifications.

From the example above we can conclude that the magnification of the microscope in turn affects its field of view – the more the sample is magnified, the less you can see of it. Or at least less in terms of area size and more in terms of detail.

To give you an idea, a 5X eyepiece or eyepiece usually has a 20mm field number (5X/20) while a 10X eyepiece has a slightly smaller field number of 16-18mm (10X/16 or 10X/18) has. The difference becomes noticeable as the magnification increases to a hundred or more.

Why Is The Field Of View Important?

The microscope field of view determines how much of the sample we can see. Understanding how it works can help you determine your expectations of what you can see under the microscope when manipulating the power of the magnification.

To help you visualize, let’s assume you’re looking at an insect specimen. If you want to see the entire insect under the microscope, you need to use a low-magnification lens.

However, if you want to “zoom in” on a specific part like the wing, you need to increase the magnification and move the slider slightly until the desired part is centered in the field of view.

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