What is a Barlow Lens

A Barlow lens holds lenses that can increase or decrease magnification to your viewing experience. The Barlow lens was invented in the early 1800s by Peter Barlow, a British mathematician and physicist.

Barlow lenses are also known as supplemental lens and auxiliary lens. They are used in stereo microscopes and can be found in magnifications from no magnification (as protection glass) to 2x magnification. For example, if your microscope has a magnification of 100x, then using a 2x Barlow lens will boost the magnification to 200x. Barlow lenses are frequently used in conjunction with telescopes and microscopes as they work equally well in both situations. On microscopes, these lenses are screwed onto the bottom of a stereo microscopes.

What Does a Barlow Lens Do

To boost magnification, a Barlow lens is inserted into the light path of a scope and thereby extends the focal length of the scope. Simply put, increasing the focal length also increases the magnification. Additionally, by diverging the ray of light and magnifying the subject, the exit pupil of the eyepiece moves further outward, which increases eye relief and can make viewing more comfortable. In the end, you can count on a Barlow lens to increase the magnification of your subject while also making viewing more comfortable.

Another side benefit is that a Barlow lens doubles your lens collection. Consider the home astronomer who has three eyepieces of different magnifications in her kit. A Barlow lens can be used on all lenses to extend the focal length of each, effectively giving each lens its original magnification and the enhanced Barlow magnification power.

How to Use a Barlow Lens

Using a Barlow lens is easy to do. Insert the lens in place of the scope eyepiece in the focuser, and the scope eyepiece is then inserted into the Barlow. This will require refocusing. If you’re using a telescope with a refractor, you’ll also typically have a diagonal, and that will give you the option of putting the diagonal into the Barlow and the Barlow into the focuser. Once the Barlow lens is installed, you can use your scope as you normally would.

Using a Barlow Lens for Microscopes

In Microscopy, Barlow lenses are used to change the field of view and the working distance. Many different applications may require a broader view of the specimen and/or more space in which to work. In these situations, using a reducing Barlow lens will lower the magnification and increase the field of view and focal distance, this gives you more space in which to work, and you can see more of the sample.

Much like using a 2x Barlow lens would increase your 100x magnification, using a 0.5x Barlow lens (a reduction lens) will cut your 100x magnification in half to 50x, but it will also double the working distance and double the field view.

Barlow lenses and auxiliary lens are most often used on stereo microscopes as they have lower magnifications, typically between 5x and 50x. These microscopes are often used to look at larger objects and specimens which may also require frequent switching of subjects or room to work on a subject. In these situations, a Barlow lens can be a great benefit.

Benefits of a Barlow Lens

A Barlow lens is typically a great tool to keep nearby for situations where it’s needed. It’s not necessary and not needed for all applications. As with all equipment purchases, knowing what your application will be is critical. Some benefits that certain users of Barlow lenses may enjoy include:

Increased magnification: This is probably the most popular reason people use a Barlow is they want additional magnification.
More workspace: In microscopy, especially soldering or working on computer chips, having additional space to work is key and a reducing Barlow is beneficial.
Doubles eyepiece collection: In essence, adding a Barlow lens to the toolkit is like adding a Barlow lens to every eyepiece.
Provide some eye relief: Not in all situations but in some, the addition of a Barlow lens gives greater eye relief to the user.

Downfalls of a Barlow Lens

It must be noted that a Barlow lens is not without its downfalls, it’s also not the right tool in all situations. Some considerations include:

Dimmer views: A Barlow lens can produce a dimmer view of a subject because the light has to go through more layers of lenses.
More magnification isn’t always better: Again, it’s important not to confuse magnification with resolution. Seeing an object at greater magnification will not make it clearer, and if there is any turbulence in telescope viewing, a Barlow can make your view even blurrier.
Vignettes: One common issue with changing optical systems is creating a vignette effect or viewing the image in the center clearly but then seeing an increase in blurriness as you move to the outside of the viewing area.

Selecting the Right Barlow Lens

There are a wide variety of companies offering Barlow lenses, so you’ll have a lot of options to choose from. The first determination when buying a Barlow lens is how it will be used. Go beyond microscope, telescope, or photography and consider what the viewer hopes to do with their scope and how the Barlow lens may aid them.

The next thing to do is determine the barrel size of the eyepiece of your scope. This is the diameter of the eyepiece tube that drops into a focuser. You’ll need to select a Barlow lens with the same diameter to ensure the two pair together.

After having this number handy, you’ll then want to consider the magnification number. Barlow lens have different magnification, some adding magnification and some reducing it.

Finally, there are some added benefits that cost more but are worth it. One thing to look for is an antireflection coating, this is very important and will give you greater performance.

If you’re interested in learning more about Barlow lenses or purchasing a Barlow lens, contact our team at New York Microscope Company. Our trained experts are happy to help you find a lens that fits your specific needs and one that lands within your budget parameters.