What Is a Parfocal Microscope? Pros, Cons and How It Works

Last Updated on Mar 12 2024

Most microscopes are parfocal, which is the desired objective by most users. But why is it desirable that microscope objectives be parfocal? Why are they preferred over other microscope objectives, such as low-power or scanning?

In simpler words, parfocal microscope objectives stay focused even when the magnification changes. So, if a scientist changes the microscope from a lower power objective to a higher power, their object will remain focused.

If parfocality seems like an alien term, you’re at the right place. Here’s a comprehensive guide to parfocal microscopes, how they work, their common uses, and their pros and cons. We’ve even included how to change your microscope’s lens to a parfocal objective.

How Does a Parfocal Microscope Work?

A parfocal lens maintains focus even when the user changes the magnification. The inevitable focus error that occurs is too small to be considered significant. In microscopy, most modern bright-field microscopes have parfocal objectives.

All objectives you can change with minimal or no refocusing are parfocal, making it possible to adjust your microscope’s magnification while retaining the lens’s focus. Scientists perform a procedure called “parfocalling compound microscope objectives,” allowing them to adjust each objective lens to retain its relative focus with the other objective lenses on the microscope as they switch the magnification.

Microscope objectives that aren’t parfocal require you to refocus the microscope every time you change the objective turret to a different magnification. Of course, this is inconvenient and time-consuming, resulting in eye strain and excessive microscope use. All microscope manufacturers offer different methods for parfocalling objectives. However, it’s worth noting that parfocal microscopes only retain focus on the object if the magnification increases or decreases sequentially.

Parfocality also works differently for a stereo microscope. For example, if you focus on an object at the highest magnification on a parfocal instrument, it will remain focused when you zoom out to any lower magnification. You can recognize the parfocality of a microscope if both the camera image and eyepiece image retain focus simultaneously. If your microscope isn’t focusing, ensure the eyepieces are set to zero.

Where Are Parfocal Objective Lenses Used?

Parfocal objective lenses are famously used in microscopy but also play major roles in other industries. Here are some other fields where parfocality comes in handy:

Microscopy

Of course, parfocal lenses are used most in microscopy, helping operators keep the object focused while changing the magnification. In addition, most microscopes are parfocal, so you won’t have trouble finding a perfectly parfocal objective lens. However, adding parfocality to a microscope isn’t hard and can come in handy when you want to save money by opting for a cheaper, non-parfocal microscope.

Telescopy

Parfocal objective lenses in telescopes work the same way as in microscopy. They don’t require you to do anything other than adjust the eyepieces to refocus on the object. The telescope will remain focused whether you adjust the magnification from low to high or vice versa.

It’s not hard to find a telescope with a parfocal lens since they’re preferred over other types of telescope lenses. However, they can be quite expensive. For example, the $200 Orion SkyScanner 100 Reflector is a sturdy telescope for beginners using parfocal lenses. It has enough magnification power to help you look at nebulas, brighter galaxies, planets, and the moon, as its magnification ranges from 14x–200x. The best part is that its parfocality helps keep your celestial object in focus while you increase the magnification.

Photography

Cameras have two types of lenses: varifocal and parfocal. The first type features a variable focal length, meaning the focal length changes along with the magnification and focus. These cameras allow you to focus on any subject, providing flexibility and variety.

On the other hand, parfocal lenses don’t change focus along with the magnification and focal length. Therefore, these cameras work similarly to any parfocal microscope or telescope. Parfocal lenses are important in the photography field due to their focusing capabilities.

It helps photographers keep their eye on the screen as they zoom in or out and get a better view of their shot. In addition, parfocal lenses reduce lens breathing, which occurs when a varifocal lens zooms in and the edges of the frame then shift in and out. Parfocal lenses eliminate this issue by retaining focus the entire time.

Cinematography

Parfocal lenses are one of the main reasons movies are so good. Much like any microscope, camera, or telescope, parfocality helps filmmakers keep their subject in focus during zoom shots. If the subject kept shifting in and out of focus during a zoom shot, it would make for an unpleasant movie.

In addition, parfocal lenses also come in handy during shots where the camera, subjects, and actors are all simultaneously moving. Parfocality helps filmmakers focus on moving objects with ease and prevent blurry shots.

Advantages of a Parfocal Microscope

Parfocal microscopes are easy to use and help save time, allowing you to work without constantly refocusing your objectives. However, using the coarse adjustment knobs to refocus at high magnification can be more difficult than you think.

The main advantage of parfocal lenses is that you can even turn non-parfocal microscopes into parfocal ones. So, you can opt for some lower-quality scopes that were not manufactured as parfocal to save money.

When viewing a subject through your microscope, it’s easiest to first view the slide under a low magnification for a broader view. This way, you can determine the slide’s placement and switch to a higher magnification. In such cases, scientists prefer parfocal objectives to keep the slide focused while changing the magnification.

Another advantage of parfocal microscopes is that they reduce human error. Since operators don’t have to manually adjust the focus every time the objective changes, they can prevent mistakes such as turning the focusing knob too much or too little.

Since parfocal objectives are mainly found in bright field microscopes, operators can benefit from its simplicity. For example, the bright field technique ensures that its optics don’t alter the specimen’s color. This technique also adapts to new technology and optional pieces of equipment with ease.

Disadvantages of a Parfocal Microscope

Parfocal objectives are typically found in bright field microscopes, which have certain disadvantages in their optical imaging technique. For example, greater contrast can result in distortion if you use an aperture diaphragm for contrast.

In addition, bright field parfocal microscopes can’t observe living specimens of bacteria, but they may have an optimum viewing magnification of 1,000x if the operator uses fixed specimens.

These microscopes also have incredibly low contrast, so you’ll likely need to stain most cells to see them properly, which could introduce extraneous details that shouldn’t be there. Plus, you must have extensive knowledge of staining to perform it.

Staining can cause other microscopy issues. For example, it requires a strong light source for high magnification applications, ultimately producing heat and damaging the specimen under review.

How to Parfocal a Compound Microscope

If your microscope isn’t parfocal, you can adjust the objectives with spacers to make it parfocal. Here’s how you can set parfocal the objectives on a compound microscope:

1. Remove the Objectives

First, remove the objective you need to parfocal from the microscope. After removing each objective, you’ll need to open the outer covers. If there are 4x or 10x objectives in your microscope, there is no need to remove them as they don’t have adjustable settings. Instead, you’ll use the 10x objectives as a baseline to parfocal the higher-power objectives.

2. Remove the Covers

As we mentioned, you must remove the outer cover of each objective lens. Like most threaded devices, Meiji microscopes feature objectives that unscrew with a counterclockwise screw. Luckily, the cover is easy to unscrew, so you won’t have to apply too much pressure.

Also, ensure you’re removing each objective cover one at a time, so you don’t mix them up.

3. Access the Objective Adjustment Ring

After removing the covers, you’ll be able to access the ring that allows you to adjust the parfocality of the objective lens. Try turning this ring. If you’re successful, you can skip the next step.

If not, there’s most likely a dab or drop of optical cement that keeps it from rotating. Look closely to detect this optical cement before moving to the next step.

4. Remove Optical Cement

First, try using your fingernail to remove a small amount of optical cement on the objective adjustment ring. If that doesn’t work, you can use a        Q-tip dipped in acetone to soften and remove the optical cement. Once removed, move the objective adjustment ring back and forth to remove any leftover optical cement and make the ring spin freely.

Once the ring is free, you can reinstall the objective with its cover into the microscope. But you must ensure that the objectives ascend in order. For example, you can place the 10x objective first, then follow it up with the 40x objective, the 100x objective, and so on.

5. Rotate the Objectives

Next, rotate the 10x objective lens into position over a sample and focus on a subject, such as a stage micrometer. However, you can use any object with lines on it. Then, you can move on to the next objective.

When looking through the next microscope objective lens, you can rotate the adjustment ring on the objective instead of using the focusing knobs to focus. However, this adjustment can be somewhat sensitive, so you must make sure to move it slowly.

Repeat this process for the rest of your objectives. You should be able to cycle through all objectives quickly after the whole process is complete without having to pause and refocus your microscope every time.

6. Resecure the Parfocality Ring

Finally, it’s time to re-secure the parfocality ring. To do so, you can apply a drop of optical cement to secure the objective ring. Of course, this isn’t a required step, but it can help keep your objectives parfocalled, so you don’t have to repeat the process in the future.

You can substitute it with clear nail polish if you don’t have optical cement. Moreover, you can add the optical cement while the objectives are still in the microscope nosepiece. Once the optical cement is dry, you can replace the objective covers and screw the objectives into the nosepiece of your microscope.

You must perform this process accurately since repeatedly parfocalling your microscope can wear it down.

How to Parfocal a Stereoscope

Parfocalling a stereoscope can be a complicated process. So, follow each step accurately to prevent the wear and tear of your stereoscope. Here’s what you must do to parfocal a non-parfocal stereoscope:

1. Adjust the Objectives

First, place your stereo microscope and switch to the highest magnification available before adjusting objectives to keep the eyepiece in focus. Then, ensure you’re focusing on the object on the sample perfectly.

2. Adjust the Eyepieces

Then, move your stereo microscope back to the lowest magnification available before adjusting the eyepieces for size and ensuring that the image is clear. Finally, you can connect your stereoscope to your computer and view the image on the screen throughout the process.

3. Adjust the Camera Collar

Finally, you can adjust the camera collar so that your object is focused on the screen. Now, no matter how much you increase or decrease the magnification, the object will remain focused. This method helps you avoid moving the objective up and down throughout the dynamic range.

Frequently Asked Questions About Parfocal Microscopes

Here are the answers to the most frequently asked questions about parfocal microscopes:

What Does the Parfocal Do on a Microscope?

Parfocal can be described as the microscope’s ability to stay focused as the operator switches magnification. Parfocal microscopes help the operator keep the subject focused as they switch through the different objectives. A good parfocal microscope requires less than 1/8 of a turn of the focusing knob to get the image back into focus.

Why Is It Desirable That Microscope Objectives Be Parfocal?

When viewing a subject through your microscope, it’s easiest to first view the slide under a low magnification for a broader view. This way, you can determine the slide’s placement and switch to a higher magnification. This is why it’s desirable that microscope objectives be parfocal, helping the slide stay focused while changing the magnification.

Does Parfocal Apply to the Brightfield Microscope?

Yes, most brightfield microscopes are parfocal. During imaging on a brightfield microscope, the image remains focused even when the objective changes, which means this microscope has all parfocal objectives.

How Do You Know if a Microscope Is Parfocal?

You can check the parfocality of a microscope if both the camera image and eyepiece image retain focus simultaneously. If your microscope isn’t focusing, ensure the eyepieces are set to zero. Those that don’t have parfocal microscopes can make their objectives parfocal by performing the right method mentioned above.

What Is the Parfocal Distance?

The parfocal distance is the distance between the mounting plane of the objective lens and the specimen on the sample. Most parfocal microscopes have a parfocal distance of 45 millimeters.

Summing Up

Parfocality is preferred in every microscope since it makes operations easier and quicker for users.

Now that you know everything there is to learn about parfocal microscopes, you can use yours efficiently or even turn your non-parfocal microscope to a parfocal one. But, of course, if you’re dealing with a stereoscope, it’s best to let a professional handle the job for you.

See also:

• What Is a Scanning Electron Microscope (SEM)? How Does It Work?
• What is Brightfield Microscopy Used For? The Interesting Answer!

Featured Image Credit: Konstantin Kolosov, Shutterstock