What Is a Parfocal Objective
To understand what a parfocal objective is, it's best to take the two words and define them individually first. An objective lens is actually a combination of lenses that directly observe the specimen Parfocal is a little more complicated; it's all of the eyepieces and objectives that focus their images on the same plane so they can be changed without readjusting the focus. When put together, a parfocal objective means you can change the objective without having to readjust the focus. That means it holds the focus with only minor or no adjustments after you've changed an objective lens in your microscope.
How to Check if Your Microscope is a Parfocal Microscope
If you're not sure if your microscope is parfocal, then you can do this simple test to check. Select a slide and start with your lowest power objective. Focus on the slide. Then switch to the next most powerful objective, all the way up to the most powerful objective you have. If you can still view the slide with little or no refocusing throughout the process, then you have a parfocal microscope. It should be noted that most modern, brightfield microscopes are parfocal.
Parfocal lenses don't just appear in microscopes. These lenses are particularly useful in cinematography, as they hold the focus of a subject as the camera operator zooms in or out. Typically, these are referred to as zoom lenses, but they're also parfocal. You'll also find parfocal lenses in use in photography and telescopes.
What Is the Advantage of Parfocal Objectives
If you're wondering why it is desirable that microscope objectives be parfocal, the main advantage of parfocal lenses is the ability to maintain focus while changing the focal length. This makes the microscope easier to use. It's also more efficient to work without constantly having to refocus or make major adjustments to get a clear view every time the magnification is adjusted.
Luckily, most microscopes made today are parfocal, but there are some brands that do not have this feature, and they tend to be significantly less expensive. Microscopes that are not parfocal often fall into the unfortunate category of you get what you paid for. There is a chance you can make adjustments to the objectives in a non-parfocal microscope through the use of shims to create appropriate spacing, but this is very difficult, and you might need a professional, which can make the cost of hiring a professional a wash when you consider what it would have cost to purchase a parfocal microscope in the first place.
Parfocal Lenses in Other Applications
As mentioned above, parfocal lenses aren't just useful in microscopy. In photography, a parfocal lens lets you accurately focus at the maximum focal length, then, if necessary, you can zoom in to a shorter focal length easily and quickly without having to refocus. This can save a shot, especially in live-action photography.
In the world of videography or cinematography, parfocal zoom lenses are particularly useful, and they're very common. Using a zoom lens holds the focus as the focal point changes, which changes the focal length and the magnification. For the camera operator, this provides smooth movement and effortless focus as your subject moves or as the camera is moved. It can also be used to create dramatic effects when the camera is zooming in (or out), and the camera itself is moving in a dolly or other motion.
Finally, in the world of telescopes, the major benefit remains the same, parfocal lenses keep the focus when the magnification is changed. This saves time, which is not only efficient but can prevent the user from missing key moments or images while they're messing with the focus.
What is the Opposite of a Parfocal Lens
While most microscopes made today are parfocal, not all of them are, and sometimes that's a good thing. The opposite of a parfocal microscope is a varifocal microscope, sometimes called a varifocal zoom. A varifocal lens has a variable focal length which changes the focus as the focal length is changed. These lenses are more popular in photography and video applications as auto-focus capabilities account for the change in focus and quickly adjust. They're not often used in the field of microscopy but can be used in situations where the operator wants multi-depth microscope imaging.
Buying a Parfocal Microscope
If you're interested in learning more about buying a parfocal microscope or purchasing a parfocal objective, contact our team at New York Microscope Company. Our trained experts are happy to help you find the microscope equipment that fits your specific needs and lands within your budget parameters.
Further Reading
You can learn more about microscopy below: