How To Use A Barlow Lens – What It 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.

What does a Barlow lens do?

A Barlow lens increases the power of your telescope by increasing the telescope focal length as it creates a more acute diverging cone of light.

It can help get you better views of the details of planets.

For eyeglass wearers: A Barlow lens can increase the eye relief of a given eyepiece. This can aid eyeglass wearers looking through a telescope with 10–12 mm eyepieces. Note: The longer focal length eyepieces generally have a decent eye relief already.

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.

Tip: Keep fingers away from the lens part when fitting. Fingers leave oil and grease on the lens and this will require cleaning. I explain how best to remove these deposits in my article covering the dos and don’ts of cleaning telescope eyepieces.

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- to 5-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.

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

What Is A Shorty Barlow lens?

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

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

Why Use A Barlow Lens?

Four common reasons for using 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 can be 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 think about what you’re wanting to look at 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