How To Use A Barlow Lens and What A Barlow Lens Does

What is that 2x Barlow lens that came with my telescope? Is it worth using? Why not just use a few favorite eyepieces? Let’s look at the benefits as well as how to use a Barlow eyepiece.

What is a Barlow lens? A Barlow lens is a special lens set that is positioned between the objective lens and the eyepiece of a telescope to amplify the magnifying power of the telescope. It does this by increasing the effective focal length of the telescope.

The Barlow Lens was invented by the English mathematician and physicist Peter Barlow (13 October 1776 – 1 March 1862).

Barlow lenses increase magnification and virtually extend your eyepiece collection to provide higher power. So, for example, if your telescope comes with two eyepieces and a Barlow lens, you have the flexibility of using higher magnification as the Barlow lens will increase the magnification of the two provided eyepieces. It gives you the benefit of four eyepieces in effect.

What Difference Does A Barlow Lens Make?

The difference that a Barlow lens makes is that it is a cost-effective way of improving the magnification power of your telescope – for seeing more details of the moon or planets, for example – without having to purchase a more expensive telescope or extra eyepieces for higher magnification.

How do Barlow lenses work?

Barlow lenses are telenegative amplifiers, meaning they produce a diverging light cone that amplifies the image.

What is a telenegative Barlow lens? To explain further…

Telenegative means a Barlow is a diverging lens, which in effect increases the telescope’s focal length by how much it spreads the incoming light (the amount of the Barlow’s divergence) and so increases magnification.

How a Barlow affects telescope magnification

Magnification equals the telescope’s focal length divided by the eyepiece’s focal length.

Example: 900 mm telescope focal length ÷ 10 mm eyepiece focal length = 90x magnification.

For more on this: See our guide to choosing the best telescope eyepieces

Thus, to increase magnification you have these options:

  • Option One – increase the focal length of telescope (TFL)
  • Option Two – decrease the focal length of the eyepiece (EFL) or
  • Both

Example ‘Option One’: 1200 mm TFL ÷ 10 mm = 120x

Example ‘Option Two’: 900 mm TFL ÷ 5 mm = 120x

Example ‘Both’: 1200 mm TFL ÷ 5 mm EFL = 240x

A Barlow lens works to achieve Option One. It increases the telescope focal length.

Why use a Barlows lens – in a nutshell

The advantage of using a Barlow lens is that it increases the power of your telescope. Higher magnification is achieved either by increasing the telescope focal length or decreasing the eyepiece focal length, or both. A Barlow lens achieves the first option. It increases the telescope focal length by producing a diverging cone of light that is more acute, producing the same effect as that of a telescope with greater focal length.

Other benefits / disadvantages of using Barlow lens

A Barlow lens can also increase the eye relief of a given eyepiece, which may be an advantage, especially for eyeglass wearers, with 10-12 mm eyepieces. However, a bit of Barlow lens advice…the increased eye relief effect from using a Barlow lens can be problematic when using the longer focal length eyepieces, which generally have a decent eye relief already.

As an example, where a 32 mm eyepiece has an eye relief of 16, adding the Barlow lens can result in an eye relief of 32. This can mean your eye is so far away from the lens, making it difficult to see the entire field of view. This adverse result may be worse in a shorty Barlow lens.

How Do You put on A Barlow Lens?

On a telescope, you fit the Barlow lens into the eyepiece holder immediately before the eyepiece. If you are using an extension you place it between the Barlow lens and the eyepiece.

how to use a barlow lens in a telescope, where to place Barlow lens
Diagram showing placement of a Barlow lens on a telescope

Where do I place a Barlow lens if using a star diagonal?

You can place the star diagonal either ahead or behind the Barlow lens.

Where the Barlow lens is placed behind the star diagonal (between the star diagonal the eyepiece), i.e., placing the star diagonal ahead of the Barlow lens, you’ll achieve the expected focal length or amplified magnification using the Barlow lens.

However, in placing the star diagonal behind the Barlow lens (between the Barlow lens and the eyepiece) you will get the best effect when using a refractor telescope.

How to Pick a Barlow lens

How do you choose a Barlow lens? When buying Barlow lenses, look for quality in the lenses if you want good image quality in terms of sharpness; make sure you have the right barrel size, magnification, and the type of Barlow that suits your telescope. The following looks at these features in more depth.

Correct barrel size

But first up, it is critical to choose a Barlow lens that has the correct barrel size for the eyepieces you intend using, that is, either 1.25 or 2 inches, or if you have the more expensive eyepieces, check the size, e.g. it could be 0.965 inches.

Barlow lens magnification

For magnification, it depends on the eyepieces you already own whether you opt for a 2x, a 3x, or 5x Barlow lens, as well as the magnification limits of your telescope.

The most common Barlow lens included when you purchase a telescope for beginners is the 2x.

What Is A 2x Barlow lens?

A 2x Barlow lens doubles the magnification of your eyepieces.

So, if you have three eyepieces, say with focal lengths of 10, 16, and 25 mm, by adding a 2x Barlow lens you get the experience of six, i.e. 5, 8, 12 mm as well as 10, 16 and 25 mm.

This should answer your question on what is a 2x Barlow lens.

Similarly, a 3x Barlow triples the magnification, a 4x quadruples it, and so on.

Adjustable Barlows are also available where power is increased by adding an extension tube between the Barlow and the eyepiece.

Remember the smaller the focal length the higher the magnification.

Limits of magnification

You don’t want to double up on what you already have nor expand to unusable magnification.

For example, if you own 10 mm and 5 mm eyepieces, getting a 2x Barlow won’t benefit you as a 10 mm with a 2x Barlow will give you a 5 mm effect – which you already have. But, getting a 3x Barlow may work depending on the power limit of your telescope. You’ll find more helpful info regarding these specs in our article on how to get the best eyepiece collection.

Quality images

For the best quality images, look for a Barlow with fully multi-coated lenses to minimize stray and reflective light and maximize the light coming through to your eye.

A Barlow lens with three or more lens elements in its optics system will typically give you better quality images. You might find mention of achromatic or apochromatic lenses.

Lens elements

Barlow lenses may have optic systems with 2-, 3-, and 4-element lenses.

Achromatic lenses are generally a 2-element lens fused together but can involve three elements, as noted by Edmund Optics. No matter the number, achromatic lenses correct for two wavelengths of visible light (red and blue) to improve aberration effects. Apochromatic lenses are an advancement of the achromatic and involve at least three lenses fused together in the optic system.

Shorty vs standard

Types of Barlow lens to choose from include the shorty or the standard long version.

The shorty suits reflectors and the longer Barlow versions are for refractors, in general speak.

What Is A Shorty Barlow lens?

A shorty Barlow is a compact lens; hence its name. It has a short barrel of 1.25-inch diameter and 1.6-inches in length.

End to end, its length is 3-inches, which is about half that of the standard Barlow lens.

Best Barlow lens by popular sales at Amazon

The following are the top Barlow lens according to sales at Amazon…

Last update on 2021-04-16 / Affiliate links / Images from Amazon Product Advertising API

Why Use A Barlow Lens?

Here are four common reasons to use a Barlow lens…

  • Increased magnifications. This gets around the aberrations of having to use short focal length eyepieces that have curved optical surfaces.
  • Increased focal ratio. This improves the focus. Great in cases where eyepieces with short focal-ratio objectives do not work well.
  • A more acute light cone. A higher effective focal ratio of the eyepiece objective means less demand on the eyepiece quality.
    • Cone peripheral rays are more paraxial and less subject to aberration, and
    • There’s a smaller area of the field lens.
  • Better eye relief for eyeglass wearers, since the eye relief of eyepieces typically relates to eyepiece focal length (in some cases this can be a disadvantage – see notes above). 

The one main disadvantage of using a Barlow lens is slight loss (around 3%) of light.

Video: How to use a Barlow lens

For Barlow lens advice in visual format…


Can I Use 2 Barlow Lens Together?

You can pair them up. But of course, you need to ensure what you are looking for before venturing deep.

In summation

Eyepieces may come and go, but it is worth getting a good quality Barlow lens to last the test of time and to extend your collection of eyepieces to get a better experience without outlaying a lot of money.

Info sources

Scientific American: The Amateur Astronomer | The Astronomical Unit: Barlow Lens | Edmund Optic: Anatomy of Achromatic Lens

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