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What Does the Diaphragm Do on a Microscope? Pros, Cons, Types, & FAQ

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white microscope

The microscope is made up of many interworking parts. For the whole microscope to work properly, there must be enough light, and that light must be magnified. The microscope diaphragm is what controls how much light hits the specimen and what shape it takes. Without the diaphragm, you won’t be able to see the specimen under the microscope.

There’s a lot more to understand about what the diaphragm does on a microscope and why it’s important. Keep reading for a complete look at how diaphragms aid the microscope and when you need to adjust the diaphragm.

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Purpose of the Diaphragm

The diaphragm is responsible for changing the angular aperture of the light cone after the light goes through the condenser. It ensures that the optical numerical aperture and light cone size are the same. Whenever they are the same, you will have a crisp image. If they are not the same, the image quality will be poor.

In simpler terms, the diaphragm controls how much light gets to the specimen and what shape the light hits the specimen. As such, the diaphragm allows the microscope to produce a clear image for the viewer.

microscope diaphragm up close
Image Credit: Dani Kristiani, Shutterstock

How Does It Work?

Even though the purpose of the diaphragm may sound fancy, how the part works is actually very simple. The diaphragm itself is normally a strip of metal or plastic within the microscope. It will then have one hole that you can adjust the size or multiple holes that you can flip through.

This piece will be connected to a dial or control on the outside of the microscope so that you can change the hole size. Whenever you change the hole size or select another hole, more or less light will be able to get through the metal or plastic piece.

For example, some holes have a large diameter. As such, more light will get through the hole. In contrast, less light will get through holes with a smaller diameter. Because of the various diameter sizes of the holes, you can control how much light accesses your specimen.

What Are the Different Types of Diaphragms?

To better understand how diaphragms work, it’s best to look at the different types of diaphragms. There are three main types of diaphragms, and they all work slightly differently.

Disc Diaphragm

The most basic type of diaphragm is the disc diaphragm. This diaphragm is a circular metal or plastic piece. It has multiple different holes of varying diameters. You will spin the wheel to select which hole you want to use. The smaller holes will constrict the light, whereas the larger holes will allow more light to shine through.

Aperture Iris Diaphragm

Microscope Adjustable Aperture

One of the most advanced diaphragms is the aperture iris diaphragm. It allows you to control the exact size and shape. It is designed to function like your iris. You slide the diaphragm so that the hole expands or constricts to the exact size you need.

Field Diaphragm

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The field diaphragm is located right by the microscope’s light source. It is a single piece with only one hole, much like the iris diaphragm. Unlike the iris diaphragm, there are only set settings, which means you don’t have complete control over the exact diameter.

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Where Is It Used?

Diaphragms are used in almost all microscopes. Without the diaphragm, you would not be able to control the image quality of the specimen you are looking at. As a result, diaphragms are an essential part of all microscopes. They allow you to control the light so you can control the image quality.

Advantages of Diaphragms

All diaphragms have the same major advantage: they allow you to control light amount and shape so you can increase the image quality. Without diaphragms, some microscopes would be almost useless since the quality would be so grainy.

From there, the advantage of the diaphragm depends on the diaphragm type you selected. For example, a disc diaphragm is beneficial because it is simple to use and affordable. The aperture iris diaphragm, in comparison, offers maximum control so you get the optimal photo image quality. The field diaphragm is beneficial because it combines the affordability of the disc with the convenience of the aperture.

Disadvantages of Diaphragms

There is no one big disadvantage of the diaphragm. Because it is an essential part of the microscope, it’s not like you can choose to purchase a microscope without this part. On the contrary, the diaphragm is a necessary part that is outright advantageous.

That being said, certain diaphragm types have disadvantages. For example, the disc and field diaphragm does not allow you complete control over the size of the hole. As such, you might not get the exact image quality you want since you cannot control the size of the hole. The hole sizes are already present.

The aperture iris diaphragm does not have a disadvantage in terms of functionality, but it is much more expensive. Individuals who want a beginner microscope will likely not be able to afford an aperture iris diaphragm as a result.

diaphragm of a microscope
Image Credit: TheBlueHydrangea, Shutterstock

microscope 2 divider Frequently Asked Questions (FAQs)

What is the function of the diaphragm on a microscope?

The diaphragm controls how much light reaches the microscope’s condenser. In other words, it controls the width of the light that reaches the specimen so you can improve the image quality.

Do microscopes need diaphragms?

Yes. Microscopes need diaphragms. Without a diaphragm, you would not be able to control the light that reaches the specimen, which may result in poor image quality.

How do diaphragms work?

Diaphragms work by constricting how much light reaches the specimen. There are various hole sizes. The larger the hole, the more light will reach the specimen. In contrast, smaller holes will constrict the light.

Where is the diaphragm on a microscope?

Where the diaphragm is located depends on the diaphragm type. Field diaphragms are located closer to the source of light, whereas the iris diaphragm is closer to the condenser.

When to Change the Diaphragm in a Microscope

Knowing when to adjust the diaphragm on your microscope will take some skill and practice. When you first begin using a microscope, it can be difficult to know what you are looking at and when you should change the diaphragm. In short, you want to change the diaphragm if you don’t have a clear picture.

Let’s say that the picture is blurry. In that case, you likely need to select a larger diameter for the diaphragm and allow more light in. Adjust the diaphragm so a larger hole allows more light to access the specimen. In contrast, you may find that the picture is too pixelated or too bright. In that case, you need to do the opposite. Select a smaller hole so less light accesses the condenser.

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Final Thoughts

Once again, the diaphragm is an essential part of any microscope. It allows you to control how much light reaches the condenser.

Diaphragms essentially work by constricting how much light reaches the condenser. The exact method by which the diaphragm works depends on the diaphragm type you select.

Nevertheless, all diaphragms serve the same purpose and proved to be an essential part of all microscopes. If you do not have a diaphragm, the image quality would likely be poor.

Featured Image Credit: Misael Moreno, Unsplash

About the Author Robert Sparks

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.