What is a Barlow Lens?

The Barlow Lens is an extremely useful object in the world of astronomy. Essentially an optical device allows changes in the magnification and field of view. Not only this, but its various uses and benefits are realized when a single Barlow Lens is able to do the job of multiple eyepieces.

What is a Barlow Lens?

Invented in 1934 by Peter Barlow, an English mathematician, and physicist, the Barlow Lens is an optical device that is placed between the objective lens or mirror and the eyepiece. As it is basically a concave lens, it is also termed as a diverging lens.

To put it simply, the Barlow Lens is a simple solution to enhance the magnification of eyepieces without burning a hole through the wallet. Consisting of multiple lenses with lens coatings, the Barlow Lens has increased light transition capacity, enhancing the viewing experience.

An important point to remember is that this is a negative lens which indicates that the light rays are diverged after reflecting off the surface of the lens.

There is a range of optical instruments that employ the Barlow Lens. These include cameras, telescopes, and microscopes, etc. However, the type of Barlow Lens used in each of these optical systems often varies.

Moreover, this lens has two main types: the Standard or Long Barlow Lens with an average length of 5 to 6 inches and the Short Barlow Lens which is usually 2.5 to 3 inches long.

How do Barlow Lenses Work?

The main objective of a Barlow Lens is to increase the magnification of the eyepiece. Therefore, it is important to understand how the magnification is calculated before delving into the specifics of how a Barlow Lens increases it.

The magnification of a telescope is given by:

Focal Length of the Objective / Focal Length of the Eyepiece

As a result, one can either increase the focal length of the objective or decrease the focal length of the eyepiece to achieve increased magnification. Barlow Lens, however, increases the focal length of the objective. Therefore, when a Barlow Lens is referred to as a 1.5X Barlow it means that it increases the focal length by a factor of 1.5.

For instance, consider a 30mm eyepiece with a focal length of the objective as 1500mm. The magnification would be 50x. However, by using a 2X Barlow, the focal length of the objective would be increased to 3000mm, doubling the magnification to 100x.

Lastly, the question arises as to how exactly does a Barlow Lens increase the focal length of the objective?

The lens reduces the convergence of the light rays coming towards the telescope eyepiece.

Structure of a Barlow

Even though it might not be true in rare exceptions, a Barlow Lens is made up of multiple glasses instead of one. This is done to reduce the risk of chromatic aberrations which refers to the failure of a lens to focus all colors at a single point.

How do Barlows Differ from Zoom Lenses?

It might be confusing to differentiate between a Barlow and a zoom lens because both of them are used to change the focal length of the objective and result in the magnification of an image. However, there are certain prominent differences between the two.

  • Zoom lenses often present the issue of a limited range. For example, 1.25 inches zoom eyepieces usually have a range of 8 to 24 mm which is quite limited.
  • With a zoom lens, the hassle of swapping eyepieces that are experienced with the Barlow Lens is eliminated.
  • Therefore, it is also quicker to observe an image or sight at different magnifications with the use of a zoom lens.
  • A Barlow Lens is usually less expensive than a zoom lens which makes the former a great option for those with a tight budget.
  • The field of view in a zoom eyepiece is often smaller than that in a Barlow Lens.
  • By increasing the magnification of the eyepieces, a Barlow Lens is able to essentially provide more options and a greater range with one purchase only, as opposed to a zoom lens. This is because with two magnifications, with and without the Barlow Lens, the range of magnifications is also increased.

That being said, when it comes to the final choice between a Barlow and a zoom lens, it is important to make the decision according to the field of view one wants to maintain and the flexibility of the budget. Moreover, it is necessary to consider the size of the Barlow or zoom lens for carrying and storing it.

Barlow Lens Pros and Cons

Here are a few reasons due to which Barlow Lens has been widely used over the years by astronomers to enhance their viewing experiences.

Barlow Lens Pros

  • The Barlow Lens is an extremely cost-effective solution to increase the magnification of the telescope. It allows the magnification to be doubled or even tripled without having to buy more expensive lenses or equipment. Therefore, it essentially increases the number of eyepieces available. Moreover, the cost of a Barlow Lens, in most cases, is less than the cost of an eyepiece.
  • A few Barlow Lenses come with the option of variable magnification, for instance, from 2X to 3X. This is enabled by different focal lengths provided by the lens.
  • With this lens, the experience of high-power viewing through a telescope is made much more comfortable and gives greater eye relief which is the distance of the exit pupil from eye lens. This is particularly useful for those who wear glasses. This eliminates the need to place the eye very close to the lens in order to see the image clearly.
  • Barlow Lenses can be stacked which means that instead of getting an expensive 4X Barlow, two 2X Barlows can be stacked together to give the same power.
  • The Barlow Lens is very advantageous when it comes to telescopes with short focal lengths. In such telescopes, it is often nearly impossible to reach high powers despite short focal length eyepieces. However, this lens effectively solves this problem and provides high power.
  • Interestingly, the performance of an eyepiece can also be enhanced with this lens provided it has antireflection coatings. These enable viewing sharper images. This is why many high-power eyepieces come with built-in Barlow Lens.

Barlow Lens Cons

While the Barlow Lens is popular for its benefits, it comes with its own set of cons. A few of them are explained in detail below.

  • There is a small decrease in the light throughput due to there being the additional glass in the path of the light.
  • Often times, even though the required magnification of the image is achieved, the image is not very sharp and appears to be dimmer.
  • The handling time might also increase depending upon how many times the ‘barlowed’ and ‘non-barlowed’ eyepieces are switched. This can be quite a hassle for a beginner who has just started using the Barlow Lens.
  • There is increased strain on the focuser due to the equipment being pushed out slightly more than normal.
  • In most cases, the dust on the glass can be seen in the field of view as well, particularly if the Barlow Lens is not of very high quality.
  • When it comes to viewing deep-sky objects using a telescope, the Barlow Lens has limited use since increasing the magnification leads to a dimmer view.
  • The use of the Barlow Lens often leads to narrow views. This is partially due to the fact that the eyepiece seems to stick out of the tube.

Despite these, it is considered that the pros of the Barlow Lens far outweigh its cons, which is why the Barlow Lens is an ideal choice of lens to increase the magnification at a low cost. Moreover, it must be mentioned that many of the aforementioned ‘cons’ are only observed in low-quality Barlow Lenses and be easily avoided by using the lens of a finer quality.

How to Choose a Barlow?

Before getting a Barlow Lens, it is important to consider certain factors that will narrow down the options and help make the correct final choice.


The right size of the lens to mount on the microscope or use with a telescope is incredibly important. This means that if the lens is not the right fit, it should not be forcibly inserted or placed at a particular position for a number of reasons. For instance, it poses the risk of the lens getting broken and even if it doesn’t, it might not perform its intended function properly.

The right size will also ensure that when the time to replace it comes – the Barlow Lens often need to be changed after a certain time – it can be easily removed.

Furthermore, this lens comes in various sizes, for example, 1.5X Barlow, 2X Barlow, 2.5X Barlow, and 3X Barlow. Moreover, it should be ensured that a Barlow Lens is selected whose barrel size would fit into eyepieces meant to be used with it.


Contrary to popular belief, the Barlow Lens does not only provide increased magnification. Instead, it offers two main functions – amplifying and reducing the magnification. In fact, the most common type of lens is a reducing Barlow which decreases the magnification while increasing the field of view.

Therefore, while a 3X Barlow might triple the magnification, a 0.5X Barlow would reduce the magnification by half. This is why it is important to know what function a certain lens provides to ensure that the right lens is being bought.


This is a major factor when considering various different Barlow Lenses. While it is true that most high-quality Barlow Lenses are more expensive than those with lower quality, there are still certain pointers that need to be kept in mind regarding quality, apart from cost.

  • An all-glass lens performs better than the plastic ones which are often quite lightweight.
  • Barlow Lens bought from a highly reputed brand will usually give great results.

Type of System

Knowing the difference between achromatic and apochromatic systems – two types of a Barlow Lens – can help in choosing the required lens. An achromatic system consists of two lenses while an apochromatic system includes three lenses. It goes without saying that the apochromatic lenses work better.

Apart from these, there are several other factors such as the intended purpose of the lens or the field of view to be maintained which need to be considered before getting a Barlow.

How to Use a Barlow?

Using a Barlow Lens is quite simple. The eyepiece needs to be removed from the instrument, such as a telescope. Afterward, the Barlow Lens is to be inserted. Lastly, the eyepiece is to be inserted into the Barlow Lens.

However, there are certain Barlow Lenses that can be used in more than one way. These allow the eyepiece to be placed inside the lens or the element can be removed and attached to the eyepiece. Interestingly, the magnification achieved using these two different methods would be different. When the eyepiece is inserted into the Barlow Lens, it usually gives greater magnification than screwing the element onto the eyepiece.

When to Use a Barlow?

While a Barlow Lens can be used as required, there are certain situations in which using this lens can prove to be quite beneficial.

  • If an eyepiece for the required magnification is not available, a Barlow Lens can be used instead to achieve the right magnification.
  • If there is a limited budget, a single Barlow Lens can provide more than one magnification that would otherwise require more than one eyepiece.
  • It is also useful when longer eye relief is to be retained using a low power eyepiece.


A concave lens diverging light rays, the Barlow Lens is used to increase or decrease the magnification by an increase or decrease in the focal length of the objective. This gives improved range and is equivalent to doubling the number of eyepieces available. The minor cons encountered can be catered by getting a high-quality Barlow Lens.

Apart from its technical benefits, this is an excellent choice of lens for those who need to stretch their budget. Before buying this, however, it is important to give some thought to factors such as its size, intended function, and quality, etc. Lastly, the best part is that it is extremely easy to use.