The Microscope’s Iris Diaphragm: What It Does And How It Works

Microscopes are designed to be able to see things the naked eye never could, and so to do this they require attachments and additions to help us clearly see the cells and structures that make up an organism’s body.

The Microscope’s Iris Diaphragm: What It Does And How It Works

This requires microscopes to be incredibly focused but also adjustable in many ways in order to illuminate the cell or tissue being observed.

This is where the iris diaphragm comes into play, allowing for a very small amount of light to pass through while blocking out most of the rest, and granting a clear insight into the scope itself, however also holds other features that allow a microscope to be as intrinsic as possible when in use.

What Does An Iris Diaphragm Do?

The iris diaphragm is an incredibly crucial component for any microscope to be of effective use as it works to directly influence the amount of illumination, focus and contrast of the chosen magnified specimen image.

It can be used to adjust these aspects of the microscope by altering its aperture size, meaning that it can also be conveniently adjusted to provide either more or less illumination, better or worse focus and sharper or softer images, which can make a huge difference depending on the specimen and the environment you are surrounded by. 

Iris diaphragms have been used since the early days of microscopy to improve the quality of images taken, as they were originally developed to help reduce the effects of chromatic aberration which is the color fringing caused by multiple wavelengths of light passing through different parts of the lens which would make it unclear.

Now, however, they are a main component of modern microscopes as they are often used to help eliminate unwanted reflections from surfaces such as glass slides or water.

They can also be used to increase the depth of field, giving a range within which objects appear sharp rather than blurred, and to control the amount of light entering the objective lens. 

How Does An Iris Diaphragm Work?

An iris diaphragm works by using a series of blades that are placed between two lenses. The first lens is known as the objective, and the second one is called the ocular lens.

The blades are made of thin metal plates which are attached to each side of the objective lens and then cut at various lengths to create varying sized openings.

These openings are then arranged in a pattern that allows for the maximum amount of light to enter the objective lens while still making sure to prevent too much light from reaching the ocular lens so as to not distort the image. 

This allows the user to fine tune the amount of light coming through the objective lens, and therefore the amount of light hitting the sample under observation.

This is done by adjusting the length of the blades, and the distance between the blades and the objective lens, and thus the amount of light allowed to pass through.

How To Use An Iris Diaphragm In Your Microscope

To use an iris diaphragm, simply place it in front of the objective lens before focusing your microscope, then adjust the settings accordingly.

Once you’ve done this, turn the knob until you feel comfortable with the setting, and then remove the iris diaphragm from the objective lens.

You will notice that some people prefer using iris diaphragms with a hand held microscope, while others prefer using them with a stand mounted microscope, however if you’re unsure about which one to choose, try starting with a stand mounted microscope since it gives you better control over the settings.

If you do decide to go for a handheld microscope, make sure you get a good quality one, you’ll be able to achieve much greater results when using a high quality microscope than when using a cheaper one.

Why Is It Important To Have An Iris Diaphragm In A Microscope?

Nowadays if you want the most clear and efficient microscope that you can get results from, an iris diaphragm is a must.

Having iris diaphragms in your microscope will greatly enhance the overall experience of observing specimens, especially if you are working with low power magnification (1x-10x).

The reason for this is because having an iris diaphragm means that you can choose exactly where and when you want to take the best picture possible.

You can even choose to switch off the iris diaphragm completely if you wish, so you don’t need to worry about damaging your specimen by over exposing it.

Another reason iris diaphragms can be great is that they allow you to change the focal plane, allowing you to view different areas of a specimen without moving it which is particularly useful if you are trying to observe something that is very close to the surface of a slide, or you want to see what’s happening inside cells.

Finally, an iris diaphragm can be used to adjust the amount of light entering into the objective lens, allowing you to adjust the brightness of your image.

For instance, if you are viewing a specimen on a dark background, you may find that you need to brighten up the image to see all the details clearly, and so it is vital for getting all the necessary data you need out of your microscope.

Types Of Iris Diaphragms

Types Of Iris Diaphragms

There are many types of iris diaphragms available, but the most common ones are listed below:

Variable Iris Diaphragm

These are the common iris diaphragm we see today and, as the name would suggest, they are known for their ability to vary the size of the opening depending on the settings selected.

The blade sizes can also be adjusted to give more control over the amount of light being let through to help the quality of the specimen you are observing. 

Open Iris Diaphragms

An open iris diaphragm has no blades at all, just holes cut in the objective lens.

These are usually found in lower end microscopes, and are often referred to as “open” because they have no blades to stop the light passing through them. 

Slit Diaphragms

Slit diaphragms are similar to open diaphragms, except that there are blades instead of holes.

These are generally found in higher end microscopes, and usually come equipped with additional features such as variable aperture stops, focus knobs and much more.

Tunable Iris Diaphragm

These are actually the newest type of iris diaphragm, and are designed to allow you to quickly and easily alter the characteristics of the diaphragm.

They are adjustable, and can be set to automatically adjust themselves depending on the conditions around you, making them very smart and almost like an AI companion observing along with you. 


Iris diaphragms may seem easy to use and while they certainly can be, they are still one of the most crucial components of a microscope, allowing the observer to analyze their specimen without a huge worry of light interference.

So if you plan on buying any new microscope, make sure it comes with an iris diaphragmatic and if it doesn’t, then either get one from somewhere else or go ahead and order one online to ensure you get the results you need. 

Jennifer Dawkins