What is a Barlow lens and why do astronomers use them?

We are interested in learning new technologies that allow us to see star-gazing without having our pockets buried!

The Barlow lens was invented by mathematician Peter Barlow (1776-1862).

The Barlow lens was an exceptional example of that kind of tool. It is essential equipment that astronomers need, and the article below explains everything decent Barlow lens you need to know.

what is a barlow
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How does a Barlow lens work?

A Barlow lens is a diverging lens that raises the effective focal length, and width of an optical system, increases the magnification, and diverges the light passing.

It’s not the last element of the optical train, and it extends more outward the cone of light from the part of the optical train before it to allow what follows, typically the eyepiece of the camera, to view a larger image.

The Barlow lens magnifies the image due to changes in focal length.

It is important to consider that the Barlow lens should have the same barrel size as the connected eyepiece.

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Pros and cons of Barlow lens

Even though the Barlow lens is an impressive technology that enables it to open new opportunities for human eyepieces, it has several advantages and disadvantages.

Barlow lens pros

Increased magnification

  • Barlow lenses play a crucial role in increasing the magnification of telescopes, enabling astronomers to deep sky astrophotography.


  • These accessories can be seamlessly integrated into various types of telescopes and eyepieces, making them an indispensable tool in an astronomer’s arsenal.


  • Compared to investing in multiple high-power eyepieces, a Barlow lens provides an economical way to achieve higher magnification.

Viewing comfort

  • Using a Barlow lens can maintain or even improve the eye relief of the original eyepiece, providing a comfortable viewing experience due to the multiple coatings that are responsible for decreasing light reflections.

Image quality

  • The high-quality Barlow lens maintains good image sharpness and clarity, especially that made with multiple lens elements.


  • Barlow lenses are valuable for astrophotography as they allow for higher image scales and can be used with cameras to capture close-up shots of celestial objects. They offer different magnifications such as 2x and/or 3x.
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Barlow lenses’ cons

Reduced field of view

  • Reduced field of view can be a common drawback when using Barlow lenses, as they tend to narrow the scope of observation

Increased optical aberrations

  • Optical aberrations are another concern that may arise with cheaper or poorly designed Barlow lenses, impacting the overall image quality.

Additional weight and length

  • The addition of a Barlow lens to the optical setup can lead to an increase in weight and length, often due to the large barrel size.

Limited use with smaller telescopes

  • For a smaller telescope with a very short focal length, using a Barlow lens may exceed the practical maximum magnification. So powerful Barlows deliver dim, leading to diminished image quality.

Higher magnification challenges

  • At extremely high magnifications, atmospheric turbulence and other external factors can degrade image sharpness, impacting the viewing experience, since they do not pull in much light path.

Compatibility issues

  • Not all eyepieces are compatible with such zoom lenses, and some may cause vignetting, blackened edges, or other optical issues when used together. Sometimes, it is challenging to find out an effective focal length that is compatible with lenses.
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How do Barlow lenses work?

What is a Barlow lens?

You can see that the magnification power of an eyepiece in a telescope depends on the distance between the telescope’s objective lens, and the focal lengths of an eyepiece. Therefore, for a telescope increases higher overall magnification, we can either use a Barlow lens (concave lens) or both.

It shows a successful increase in telescope focusing distance – its point is further from the objective divided by the ‘diverging eye’ than the points of the gray light spheres.

The longer Barlow lens increases its focal length due to a Barlow lens impact. Changing focal length makes a significant difference in the fixed magnification level achieved through a telescope.

When a glass element from a Barlow lens is introduced into the optical system, it increases the effective focal length of the telescope. This extension allows for higher magnification when observing distant celestial objects.

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How to use a Barlow lens for astrophotography

Using a Barlow lens is possible with DSLR cameras, but only for lunar and solar views. When it comes to deep-space objects, your setup may become too slow. For use with a digital camera, you can first place your Barlow lens in the vertical or focus ring.

Effective focal length: what benefits Barlow has?

Barlow lenses are easy to use and cost-effective. In special cases where the camera uses a Barlow lens rather than an eyepiece projection to view the astrophotography images, the lens is necessary. The resulting images can be magnified with a single focal length.

What is a Barlow variable? 

Barlow’s common zoom lens offers two, three, or more magnification than four Megapixels. Usually, Barlows with high magnification – referred to as variable Barlows – have an extended nosepiece: unlike eyepieces, you may adjust the magnification by sliding or dropping the Barlow. Meades – 3-6x variables best Barlow zoom lenses here.

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Things to know when buying a Barlow lens

When you buy a Barlow lens, you have to keep in mind that the lens must be inserted into the eyeglass’s tube. Most optical tube sizes are available in two sizes: 1.75 or 2 inches, so finding a suitable piece should not be a problem. You just have to find it.

Barlow lenses have varying sizes of magnifications. Most commonly are 2x; however, offering 3 or 5x is also available. We recommend 2x for many users, however, please see our complete review for 3x Barlows here.

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Recommended Barlow lenses

These are one of the best Barlow lenses, including 2x Barlows and 3x Barlows.

Tele Vue 2x Barlow lenses

The 2x Barlow lens is 98 cm long with 94 cm wide, extending just 41 mm out from the focuser when installed.

The lower, 2x Barlow lens and housing has threaded threading for standard filters, and it does not release all screws that hold an eyepiece so that it will never fall and be lost in darkness.

Our favorite part is that the brass compression ring keeps the same eyepiece firmly held together, unlike screws they do not affect the eyes or their barrels.

Altair Lightwave Premium 3x flat-field tele extender

The Altair Lightwave Premium 3x Barlow features four-layer lenses. The 165 g lenses are lightweight, but the body is elegant with a silver undercut barrel and chunky rubber grips.

Celestron X-CEL LX 3x Barlow

Designed for Celestron, this eyepiece features an all-black silk textured finish in a black satin finish and a rubber grip. The multilayer, 3-element design is the second lighter within this group, 142 grams, and the lens is the second smallest in length, but it has an impressive, well-built feeling.

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Barlow lens vs. small camera sensor: Which is better?

In addition, cropped sensor cameras with zoom lenses produce bigger pictures than full-frame ones, as long as the resulting focal lengths are the same as those in the camera.

This effect is referred to as crop factor and is not an actual zoom but is equivalent to cropping the whole-screen picture.

But what is a decent Barlow lens and how do they differ from zoom lenses? Barlow provides in the end a real magnification with increased optical resolution without compromising its color quality.

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Overall, the Barlow lens is a tool for astronomers seeking to expand their telescope’s magnification range and for astrophotographers looking to capture detailed images of celestial objects.

However, like any optical accessory, they have some limitations that users should be aware of and consider when choosing to use one.

Opting for a high-quality Barlow lens from a reputable brand can help mitigate many of the potential drawbacks associated with lower-quality lenses along with their use.

Frequently asked questions

How does a Barlow work?

Barlow lenses are diverged lenses that vary in focal length and therefore in the field of view between optical elements of zoom lenses.

What is the purpose of a Barlow lens?

Barlow lenses increase magnification and allow for shorter focal time to achieve maximum use power.

Probably the biggest benefits of a 2x Barlow lens, are the additional number available for your eyeglasses, which effectively doubles the magnification multiplier and the amount of the eyepiece.

Does a Barlow increase focal length?

The Barlow lens has a negative focal length, resulting in it reducing the convergence of the light cone from the primary/another telescope to the large scope objective lens and mirrors and thus increasing the focusing area of the photographic lens in a camera scope.

What is the difference between an eyepiece and Barlow?

The 2x Barlow effectively tripled the focal ratio scopes and eyepieces were primarily based on an F10 light cone rather than an F5.

Ida Stewart

I have had the incredible opportunity to work as a tour guide at the planetarium for over 5 years. Ever since I was a child, astronomy has held a special place in my heart, and I have nurtured a deep passion for exploring the wonders of the universe. Among all the celestial bodies, Mars has always fascinated me the most, captivating my imagination with its mysterious allure.

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