What Is a Barlow Lens? The Device That Doubles Magnification (and Your Eyepiece Collection)

A Barlow lens is a type of lens that can increase the magnification of your telescope. The most common type is a 2x Barlow, which doubles the magnification, but you can also find them in other magnifications, such as 3x and 5x.

As I explain in my article on calculating magnification, each eyepiece can give a different magnification based on its focal length. By adding a Barlow lens into the mix, you’ll have two magnification options for each eyepiece, effectively doubling your eyepiece collection.

How to use a Barlow lens

To use a Barlow lens, insert it into your telescope’s focuser, then place an eyepiece into the Barlow and tighten the thumb screws to keep everything secure. That’s it!

Using a Barlow lens with a diagonal

If you have a refractor, Schmidt-Cassegrain, or Maksutov, you’re probably using a diagonal between the focuser and eyepiece. In that case, you may have two options:

  • Place the Barlow lens between the diagonal and the eyepiece. This option may only work with shorter Barlows, as longer ones may not fit and may even hit the diagonal’s mirror or prism.
  • Place the Barlow lens between the focuser and the diagonal. This option will result in a higher magnification; a 2x Barlow will often act like a 3x Barlow when in this position. It could also cause problems with heavier eyepieces since the Barlow may allow the diagonal to rotate.

How a Barlow lens works

A Barlow is a diverging lens, which means it spreads the rays of light outward. As you can see in the diagram below, this causes the focal point to move farther away from the objective lens. The result is that the Barlow effectively increases the focal length of the telescope, which in turn boosts the magnification.

Let’s say we’re using a 2x Barlow with a telescope that has a 1,200mm focal length. The Barlow lens will cause the telescope to behave like it has a 2,400mm focal length. This will double the magnification.

You can also think of the Barlow as decreasing the eyepiece’s focal length (rather than increasing the telescope’s). A 2x Barlow will cause your 20mm eyepiece to act like a 10mm eyepiece. Again, this doubles the magnification.

Examples of Barlow lenses

Orion Shorty 2x Barlow

The Orion Shorty is a popular, affordable option. It uses a basic doublet lens design, and it’s short enough to work with almost any telescope, even if you use a diagonal.

Tele Vue 2x Barlow

I’ve been using this Tele Vue Barlow for many years. Due to the high-quality glass and coatings, it is very transparent—it does not noticeably degrade the optics at all, as far as I can tell. However, it is more expensive.

Orion 2-inch 2x Barlow

If your telescope has a 2-inch focuser, you can buy a 2-inch Barlow. Although this is not required (you could use a 1.25-inch Barlow with an adapter), it will allow you to use your Barlow with 2-inch eyepieces. The Orion 2-inch 2x Barlow comes with an adapter that lets you use it with both 2-inch and 1.25-inch eyepieces. Keep in mind that this Barlow will not fit 1.25-inch focusers.

What to look for in a Barlow lens

  • Barrel diameter: Your telescope’s focuser is probably either 1.25 or 2 inches in diameter, so you’ll need to buy a Barlow lens that fits your focuser. However, if your telescope has a 2-inch focuser, you may have an adapter that will let you use either size.
  • Magnification: I recommend starting with a 2x Barlow. Don’t get a 5x Barlow unless you already have experience with Barlows and know exactly what you’re getting into.
  • Optical quality: Look for a Barlow with fully multicoated lenses with blackened edges (which reduces internal reflections). But keep in mind, quality can’t be boiled down to a list of features, and some cheap brands may stretch the truth in their claims.
  • Barlow length: If you use your telescope with a diagonal, you may need to buy a short Barlow that will fit between the diagonal and eyepiece. If you buy a longer Barlow, you may be able to place it between the focuser and diagonal.

A Barlow lens doubles your eyepiece collection

One of the biggest benefits of buying a Barlow lens is that it essentially doubles your eyepiece collection. Let’s say you own the following eyepieces:

  • 32mm
  • 25mm
  • 17mm
  • 9mm

With a Barlow lens, you’ll now have twice as many focal lengths to choose from:

  • 32mm
  • 25mm
  • 17mm
  • 16mm (32mm + Barlow)
  • 12.5mm (25mm + Barlow)
  • 9mm
  • 8.5mm (17mm + Barlow)
  • 4.5mm (9mm + Barlow)

So with the Barlow, you now have eight different options—and it’s certainly cheaper than buying eight eyepieces.

OK, sometimes it doesn’t *exactly* double your eyepiece collection

Let’s say you own a 20mm eyepiece and a 10mm eyepiece. If you buy a 2x Barlow, the available focal lengths will be:

  • 20mm
  • 10mm
  • 10mm (20mm + Barlow)
  • 5mm (10mm + Barlow)

Notice something? Yep, you effectively have two 10mm eyepieces. So the Barlow didn’t double the number of focal lengths that you can use.

But wait! That doesn’t mean there’s no reason to use the 20mm + Barlow. There are a couple of benefits:

  • Eyepieces with longer focal lengths tend to have longer eye relief. That means you can keep your eye slightly farther away from the eyepiece, which can help with comfort and also makes things easier if you wear glasses.
  • Depending on your eyepiece collection, perhaps you have invested in a nicer 20mm eyepiece. Maybe the apparent field of view is wider, the coatings are better, and the eyepiece is just a joy to use. With a Barlow lens, you can get all of those benefits at the 10mm focal length as well.

So you shouldn’t just think of a Barlow as a way to get more focal lengths. It’s also a way to get even more use out of your favorite eyepieces. 

Don’t get greedy! Get a 2x Barlow.

Perhaps you’ve been disappointed by how small the planets look in your eyepiece, and you’d like to dramatically zoom in with a 5x Barlow. You’ve crunched the numbers and determined that a 5x Barlow would boost your telescope and eyepiece to a whopping 750x.

Don’t do it—trust me on this. That 750x is only going to increase your disappointment. Sure, the planets will look larger, but they will also get much blurrier.

Due to the Earth’s atmosphere and the limits of your telescope’s resolution, huge magnifications will almost always look bad. So it’s better to start with a more modest 2x Barlow.

Advantages of Barlow lenses

Let’s take a look at some of the pros and cons of Barlow lenses. As you may have already noticed, they have some big advantages:

  • A Barlow can increase magnification and make planets look larger. They can also be useful for observing planetary nebulas, double stars, and the moon.
  • It can essentially double your eyepiece collection, and lets you get more use out of your favorite eyepieces.
  • It preserves the eye relief of your lower-magnification eyepieces while increasing magnification. So it allows you to view higher magnifications more comfortably.

Disadvantages of Barlow lenses

Barlow lenses are not without their disadvantages. For example:

  • Because you’re adding more glass to the light path, there is a risk of degrading the image. Many Barlows will add some chromatic aberration (color fringing) or other optical flaws. Avoid buying a cheap Barlow that will be the “weak link” in your setup.
  • If you use a Barlow lens with a low-focal-length eyepiece, the resulting magnification may be too high. Depending on the atmospheric conditions and the aperture of your telescope, a high magnification may look blurry and disappointing.
  • Since it increases magnification, it makes objects dimmer. However, this happens any time you increase the magnification (for example with a higher-magnification eyepiece), so it’s not a problem unique to Barlows.
  • The Barlow may reduce the apparent field of view of your eyepiece. However, this usually isn’t a problem since you’ll be using your Barlow to look at smaller objects like planets.
  • Buying a Barlow lens means spending money that could otherwise be spent on an eyepiece. I recommend buying at least one good eyepiece before buying a Barlow lens—and then you’ll be able to get the most benefit from your Barlow (more on that below).

Should you buy a Barlow lens?

At this point, it may seem like a no-brainer to buy a Barlow. But depending on where you are in your astronomy journey, you may not want to buy one (yet).

If you just bought a telescope that came with one or two eyepieces, I strongly recommend buying at least one good eyepiece before getting a Barlow. Most telescopes come with mediocre eyepieces, so investing in a good eyepiece is going to give you much more benefit than a Barlow.

Keep in mind, a Barlow lens won’t improve the optical quality of your setup, but a better eyepiece will.

If you already have a quality eyepiece that you love using, go for the Barlow! A Barlow lens will let you get twice as much use from that eyepiece—and any other eyepieces you buy in the future.