What Is A Barlow Lens And Why Every Astronomer Needs One

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Astronomy is one of life’s greatest, most beautiful mysteries. We look up to the sky and we are looking back in time. It’s such an amazing thing for the mind to try and comprehend.

We look up to a sky full of possibility and wonder. Some of the greatest minds of all time have stared up at the sky and tried to figure out some of the biggest questions in life.

“Why are we here”?, “Are we the only planet with life?” “What is a black hole?”

These are all such incredible, mind-bending questions, and some believe they are the only real questions worth asking. If we can’t ask the big questions then what is the point in asking the small questions.

We need these thinkers in life, the thinkers that push the limits of the human brain, breaking boundaries and discovering the undiscoverable.

In this article, we will be talking about what a Barlow lens is, and why every astronomer needs one.

By the end of this article you’ll be out the door to get one, so you’ll be looking up at the wonderful night sky with more clarity than ever before.

What Is A Barlow Lens And Why Every Astronomer Needs One


What is a Barlow Lens?

Steve Richards, BBC’s Sky at Night magazines Scope master tells us, “A Barlow lens is an optical tube containing lens elements that diverge the light passing through them.

Inserting a Barlow lens into the light path of any type of telescope increases the effective focal length, usually by doubling it, although some designs have an even greater effect.

You can calculate the magnification of a telescope by dividing its focal length by the focal length of the eyepiece. The cool thing about a Barlow lens is it essentially doubles the number of lenses you have in your collection!”

The Barlow lens was invented in the 19th century by Peter Barlow, A mathematician, optician, astronomer, a member of the Royal Society of London from Norfolk, England.

The Barlow lens is a concave lens that is placed between your telescope’s objective lens or mirror and eyepiece. (Actually, they contain more than one glass element to reduce chromatic aberration, read more about that here).

The purpose of this lens is to increase the magnification of any eyepiece used with them, usually 2 or 3 times. As you’d expect, a 2x Barlow doubles your eyepiece magnification, whilst a 3x triples it.

Here’s an example:

If you are using a 20mm eyepiece on a telescope with a 1000mm focal length gives 50x magnification (1000mm / 20mm). Attach a 2x Barlow lens to that same eyepiece and you double its magnification to 100x!

How does a Barlow lens work?

To put it simply, the magnification of a telescope is the focal length of the objective divided by the focal length of the eyepiece.

Therefore, to increase the overall magnification, we can do either (or both) of the following:

  • Increase the focal length of the objective
  • Decrease the focal length of the eyepiece

Barlow lenses do the first of these, they increase the effective focal length of the telescope allowing you to see a more magnified image.

Things to know when buying a Barlow lens

Something to remember while buying a Barlow lens is it needs to fit the tube of your eyepiece. Most tubes come in one of two standard sizes, either 1.25 or 2 inches, so finding the one that fits shouldn’t be an issue.

You just need to make sure to get the right one for you. Barlow lenses come in different magnifications. The most common are 2x, but lenses that offer 3x or 5x are also available. We recommend going with 2x for most users however you can read our full recommendations on the best barlow lenses here.

What Is A Barlow Lens And Why Every Astronomer Needs One
Alchevsk Ukrain – 18 March 2021: Barlow lens, telescope eyepiece on white background, emitting.

When you’ve picked one you’ll be pleased to find they are very intuitive. Instead of inserting an eyepiece into the focuser, you place it in the Barlow lens. Your eyepiece is then inserted into the Barlow.

It’s that simple to enjoy the vast expanse of beauty above us. Enjoy the beautiful stars with more magnification for a more immersive experience.

Why are Barlow lenses different from zoom lenses?

Barlow can be easily confused with zoom lenses, which is understandable, they pretty much do the same thing. But, there are some differences. Zoom lenses have a variable amount of magnification, whereas Barlow lenses have a fixed magnification level, for example, 2x.

Also, Barlow lenses have a smaller field of view than zoom eyepieces. The lenses used in SLR and DSLR cameras to vary the focal length (and hence zoom) are zoom lenses.

With zoom lenses, the changing focal length makes it harder to correct for aberrations, so zoom lenses have a combination of at least 4 glass elements. This makes them bigger and heavier than Barlow lenses.

The convenience of a zoom lens variable focal length comes with a compromise on image quality. It’s safe to say you’ll be better off with a decent Barlow lens for the most part, especially if you are a novice stargazer when keeping it simple and affordable is important.

Barlow Lens Pros and Cons

A Barlow lens is an almost must-have instrument to have in your collection. We think they are an essential piece of equipment for every backyard astronomer!

However, there are a few things you need to be aware of such as cheaper Barlows having fewer and lower quality lenses with cheaper glass compromising on image quality.

This can show small astigmatism and imperfect colour at the field edges with fast (smaller than f/6) focal ratio scopes. But, since a Barlow is normally used to look at objects at high power in the centre of the field (where the image is unaffected), this isn’t the end of the world.

We think the 2x ratio is the way to go because more powerful Barlows don’t work well with smaller telescopes. Small scopes don’t pull in much light, so powerful Barlows deliver dim, unsatisfactory images.

That being said, if you have a large scope, say an Celestron Nexstar 8se, then a 3x Barlow might be a good investment for you, especially if you are a more experienced stargazer.

What specifications should you look for in a Barlow lens?

There are a few specifications you need to keep an eye on when it comes to Barlow lenses to differentiate the better ones from the rest.

Here’s what the specifications mean.

  • Barrel size – This can be 1.25″ or 2″. You will need to select the one that fits your telescope. If you don’t know which one you have, your eyepieces probably have it on their label.
  • Power – This is the magnification multiplier you will get from using the Barlow. It is generally in the name and it can be 1.25x, 2x, 3x, 5x, and more.
  • Multi-Coating – Just like in glasses or any other type of lenses, multiple coatings reduce light reflections.
  • Blackened edges – This will darken the edges around the image, increasing the contrast.
  • Achromatic – Designed to reduce the effects of colour aberrations/filters. This means your image will not look coloured. This is usually not listed in the specifications as it should be a given, but some Barlows are better at it than others.


With all things considered, a Barlow lens is an amazing piece of equipment for a keen stargazer, novice or expert. It doubles the size of your collection, and for one piece of equipment that’s pretty impressive.

This is the most cost-effective way to increase the magnification power for your telescope. There are differences between the cheaper and more expensive Barlow lenses, and quality loss can be seen as discussed above.

If you are starting and want a greater magnification, I’d say the Barlow lens is an essential item for your astronomy kit. It’s also very useful for the more experienced astronomers as it offers a lot more versatility to your lens collection.

In short, this is a must-have item for all the star loving fanatics out there to help you get lost in a magical world, enjoying enhanced magnification and beautiful clarity.

Head out to your local store and pick yourself up one of these amazing pieces of equipment. You won’t be disappointed. And may your stargazing career become evermore wondrous. Happy gazing!

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