What is a Barlow Lens for a Telescope? 2023 Easy Guide

By Chris Klein

What is a Barlow Lens for a Telescope

Ever wonder what is a Barlow lens for a telescope, or how they work? Did you know they are separate elements from your telescope eyepieces and increase your telescope’s focal length? The good news is I’ll explain all that and more in this article.

Invented by English mathematician and physicist Peter Barlow, the Barlow Lens is a visual tool that installs between the primary mirror or objective lens and the eyepiece. Barlow lenses are a cost-effective way to get eye relief by giving your telescope higher magnification. They are often used for planetary astrophotography and are a simple accessory that can double or triple the magnification of your telescope.

In this article, you get

Answers to the critical question “how does a Barlow lens work?”

A diagram of the optical path through lens elements to illustrate the effective focal length of the telescope

A list of the good reasons for getting high-quality Barlows to see deep sky objects

A look at the main disadvantages of cheaper Barlows

By the end of this article, you’ll understand Barlow lenses and why it makes sense for your to consider them.

Let’s dive right in.


The two most common types are the 1.25″ model and the 2″ model.

Both are available in different magnifications, including 2x, 3x, and 5x magnification factors.

For example, a 2x Barlow converts a 250mm f/5 telescope to a 250mm f/10 scope.

Most astronomers use 1.25″ models. These models can be stacked, which allows you to squeeze more detail from bright objects.

To reduce the length of the Barlow, you can choose a threaded design.

While these Barlows are lighter, they require you to unscrew them regularly to ensure they are correctly mounted.

You can also purchase a T2 adaptor, which can attach to your focuser. For example, you can mount a 2″ focuser onto a 1.25″ telescope, allowing you to get up to a maximum of 4x magnification.

If you want to do more, you can buy Powermate focal extenders.


A Barlow lens is an essential piece of equipment.

Typically, they go in the eyepieces of a telescope, but a few types go within the telescope.

If you’re new to the world of telescopes, read my articles What is a Newtonian Telescope?Dobsonian Telescope vs NewtonianRefractor Telescope vs Reflector, and Keplerian vs Galilean Telescope.


A Barlow lens increases the magnification of a telescope, while an eyepiece views the image produced by the objective.


A Barlow lens is an optical device that increases the effective focal length of a telescope.

It goes between the eyepiece and the objective lens. It works by bending the light rays coming from the objective lens in such a way as to increase the effective focal length of the system.

The result is an increased image magnification, making it easier to see small details.


On the other hand, an eyepiece of a telescope is a lens or number of lenses placed at the eyepiece end of the telescope. It views the image produced by the objective.

There are different types of eyepieces, such as Kellner, Plössl, and Orthoscopic, each with its design and characteristics.


Barlow lenses are a popular and inexpensive way for an amateur astronomer to multiply the focal length of the eyepiece. However, they are also helpful in reducing magnification.

They are commonly packaged with telescopes and used in planetary astrophotography.

For example, a two-element Barlow provides a 2 x magnification factor.

It’s also possible to find models that offer up to 3x magnification. If you’re looking to double the effective focal length of your eyepiece, a three-element Barlow will do the trick.

Nevertheless, the best Barlow lenses are multi-element, all-glass optical systems. This eyepiece collection will minimize light loss and ensure the maximum light transition.

However, it should not be a single lens element, as this will introduce spherical aberration.


A Barlow usually goes between an eyepiece and the focuser.

Moving the focal point closer to the objective makes the eyepiece more potent. When using the Barlow, it is essential to ensure no stray light affects the image.

There are many different types of Barlows. Good quality Barlows will provide more magnification without introducing optical aberrations.

To determine the best Barlow for your needs, you’ll need to understand what the various features and functions do.


You can use Barlow lenses in a telescope to increase the eyepiece’s magnification. These can be a single-lens element or a multi-lens optical system.

The Barlow must be a high-quality, all-glass optical system.

The high quality will minimize the amount of light loss and internal reflections.

Additionally, the barrel size must be the right size for the eyepiece. Otherwise, the Barlow lens can be pushed out of place, causing a blurry image.

The distance between the Barlow and the eyepiece is called the Barlow focal length. For example, a 10 mm eyepiece on a 1000 mm focal length will give you high magnification at 50x.

The Barlow focal length is a function of the depth in the focuser and position of the focal plane of the eyepiece. Therefore, increasing the Barlow and eyepiece separation will improve the amplification factor.
If the Barlow stacks between the camera and the diagonal, it can provide up to five times the amplification.



An achromatic Barlow lens can double or triple the magnification of any eyepiece. This is often helpful for low-power wide-view usage, such as photographing the moon.


An apochromatic Barlow lens is a multi-coated optic that produces a sharp image. It is an excellent option for beginners and is especially helpful when working with a telescope with a narrow focal length.

Apochromatic Barlow lenses provide high power with an increased level of optical quality while minimizing the effects of optical aberrations.

The lens is usually of two elements, but some of the best apochromatic Barlow lenses use three or four parts.


As illustrated in the diagram above, Barlow Lenses are used to increase the eyepiece’s focal length. They are also helpful in astrophotography, typically mounted in a short tube and fit inside a telescope’s focuser.

The Barlow Lens has two main elements.

  • First, a positive lens element ensures the rays exit the field parallel to the optical axis.
  • Second, a negative lens element prevents the rays from converging. This enables them to spread outward.

A Barlow Lens can be stacked in series to magnify the image multiple times. However, the magnification factor is affected by the position of the sensor.

For example, suppose a DSLR camera mounts behind the Barlow. In that case, the Barlow might not be able to reach its total magnifying capacity.

The Barlow can be adjusted for visual observation to fill a circle of light on a paper. For example, everyday use is to view objects near the horizon. When this is done, the image is inverted, but the up-down and left-right directions are restored.

Barlows are very useful for astronomy photography, as they can produce different factors than nominal ones.

But be careful when using them. Vignetting and other spherical aberrations can occur.

In addition, they can cause image quality to suffer, hindering your viewing experience.


The Barlow is not a miracle worker but an excellent addition to any telescope.

If you want to spot a planet, for example, or to get more detailed views of the moon, a Barlow may be your best bet.

Some people even use Barlows to see more solar system planets and nebulae in the night sky. If you’re interested in seeing planets, read my articles on seeing SaturnMarsJupiterNeptune, and Uranus.

But, can you use a telescope to see the flag on the moon?


Barlow lenses serve a variety of purposes. As mentioned earlier, they are handy for astrophotography.

However, they also fit a variety of other applications. Among other things, you can use them to increase the magnification of an eyepiece.

Besides providing good image quality, they can also save space.

The most apparent advantage of a Barlow lens is the increased magnification. With a good Barlow, you can at least double the magnification of your eyepiece.

If you are using DSLR cameras, you’ll need to insert your Barlow lens into the focuser slot. You’ll then be able to adjust the focuser to bring the image into focus.

However, it’s important to remember that the magnification factor may vary from camera to camera. For example, a DSLR camera might have a different magnification factor than an astronomy camera.

Another advantage of Barlow lenses is that they are straightforward to use.

However, unlike focal extenders, which you can stack in series, a Barlow requires an appropriate eyepiece.


There are a few downsides to Barlows.

For example, the optical quality could be better. This is most apparent in the lateral color near the edge of the field. Also, adding additional elements to the optical chain increases aberrations.

There are also better options for traveling than a Barlow.

While they save space in a telescope kit, they may need to be more practical for portable telescopes. In addition, a Barlow will only work with a scope with adequate focuser travel. Many focusers need more inward travel.

As with other optical accessories, using a Barlow can reduce the light to the eye. However, while the light loss is minimal, it can make the image slightly darker.

As illustrated in the diagram above, please remember that a Barlow lens stretches the focal length of your telescope’s objective. As a result, the ray of light enters the eyepiece at a less steep angle.

Because of the longer focal length, the field of view is smaller, and you’ll have to focus closer when you use a Barlow extension tube.


A Barlow lens is an optical device that is used in telescopes to increase the effective focal length of the primary lens or objective.

This results in a higher image magnification, allowing you to see small details.

Barlow lenses are an essential accessory for any telescope, as they provide a convenient and cost-effective way to increase the magnification of the image without the need to purchase additional eyepieces.

By using a Barlow lens, amateur astronomers can easily explore the wonders of the night sky and observe a wide range of celestial objects in greater detail.



There are several reasons why you should use a Barlow lens. You can use a Barlow lens to increase the eyepiece’s magnification, allowing you to see more minor details. Barlow lenses also improve image quality by decreasing the number of aberrations in the eyepiece. You can switch between different magnifications quickly by using a Barlow lens. Barlow lenses are a cost-effective way to increase the telescope’s magnification without purchasing additional eyepieces.


No, you don’t need a Barlow lens for a telescope, but it can be helpful in certain situations. A Barlow lens can be a valuable accessory for any amateur astronomer who wants to increase the telescope’s magnification or improve the image quality.

About the Author

Chris Klein

Chris Klein is an amateur astronomy advisor, astrophotographer, and entrepreneur. Go here to read his incredible story "From $50,000 in Debt to Award-Winning Photographer Living in Switzerland". If you want to send Chris a quick message, then visit his contact page here.

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