Barlow Lens: What, How To and The Best

First I will explain the benefits and uses of the Barlow Lens. Later in the article, I will provide a list of best Barlow lenses based on price points.

Note that there are other devices called focal extenders that provide a similar function but their optics are somewhat different. However, from a practical point of view, they serve a similar purpose, as a magnification multiplier, so I will be including them in the recommendation section.

What's a Barlow Lens?

A Barlow lens is an intermediate optical device that goes between the eyepiece and the objective lens or primary mirror. It is named after Peter Barlow, so you will often see it capitalized.

barlow lens from celestronThe photo shows a typical Barlow lens. The eyepiece is slipped into the top and the Barlow lens is slipped into the focuser or diagonal, where the eyepiece would normally go.

We insert a Barlow lens in the optical path to increase the magnification provided by any given eyepiece. In this way, a single Barlow lens can increase the effective number of magnification choices you have in your eyepiece set. It does so economically and can simplify your eyepiece needs.

While we often associate the Barlow Lens with the eyepiece, it is really more correctly associated with the optical tube. A Barlow lens is a divergent lens which means that it moves the focal point out, effectively giving the telescope a longer focal length. So, if your telescope has a 400 mm focal length, a 2X Barlow inserted between the eyepiece and the objective lens or primary mirror, will effectively make it behave like it is in a telescope with an 800 mm focal length. In this way, it gives each eyepiece two magnifications, one with and one without the Barlow.

In practice, we more often talk about Barlow lenses in the context of the eyepiece as it seems more convenient to think of it that way. You will read that a 2X Barlow will make a 10 mm eyepiece deliver the magnification of a 5 mm eyepiece. The net effect is the same so it really doesn’t matter, unless you are really interested in how the optics work.

For visual use, Barlow lenses from 1.5X to 3X are common. Generally, Barlow lenses of greater than 3X are considered applicable to astrophotography though there is nothing to prevent you from using one with your eyepiece.

Barlow lenses come in a variety of designs. The typical design is shown above where you place the eyepiece into the Barlow and then place the Barlow into the focuser or the diagonal.

However, there are also Barlow lenses that are screwed onto the eyepiece.eyepiece and barlow element These are sometimes referred to as Barlow elements. An example is shown in the photo. The top shows an eyepiece and the Barlow element separately. The bottom shows the same eyepiece with the Barlow element screwed on like a filter.

Some Barlows can be used both ways. You would place the eyepiece into the device, or you can remove the element and screw it directly onto the eyepiece. When used this way you will get two different magnification factors. The combined Barlow unit might provide 2X while the element alone on the eyepiece might provide 1.5X.

As mentioned they are usually labeled with a multiplication factor, such as 2X. If your eyepiece produces 100X and you put it in a 2X Barlow it will deliver 200X.

Barlow lenses can be based on a single lens element, two lens elements, three lens elements, and a few incorporate four lens elements. Some incorporate exotic glass in the design.

And, like eyepieces, there can be lens element edge blackening. And there can be various levels of coatings on the lenses, with “fully multi-coated” being the best. The coatings and blackening are there to minimize light loss and internal reflections.

Without going into a deep optical evaluation, the 2 element Barlow lens is more or less the industry standard. I would not recommend buying a single lens element Barlow. These are usually packaged with low-end telescopes. While they work, they often introduce optical aberrations. If you have one of these, use it until you can afford to replace it with something better.

A Barlow should disappear in use. That is to say, it should not introduce optical aberrations. It should not cause a loss of field of view, cutting off the edges of a comparable eyepiece at the implied focal length. If you took a 10 mm eyepiece and a 5 mm eyepiece of the same design when putting the 10 mm in a 2X Barlow you should get an image that is very close to or equivalent to the image produced by the 5 mm eyepiece. If you don’t, the Barlow is of poor design and quality.

Naturally the more glass elements you add in the light path the more light that is lost as it passes through each element. Some people object to the use of a Barlow lens, for this reason. In the past, this may have been a valid argument. However, modern design and optical coatings can reduce this to the point that any light losses in a quality Barlow are minimal and likely not noticeable in the eyepiece. 

Eye Relief

One reason to use a Barlow lens is to take advantage of the longer eye relief typically found in longer focal length, lower power eyepieces. Eye relief defines the distance you have to place your eyeball from the top lens in order to see the full field of view.

Short eye relief can make eyepieces uncomfortable to use. In many eyepiece designs, as the focal length of the eyepiece gets shorter the eye relief gets smaller. And people who wear eyeglasses while observing may not be able to get their eye close enough to the lens to see the image fully. So longer eye relief can be helpful to them.

To illustrate, a 30 mm Plossl eyepiece might have 22 mm of eye relief. Anything less than about 18 mm may be no good for glasses wearers. Anything less than 10 mm may be uncomfortable for some people even if they don’t wear glasses.

If you wanted a 10 mm Plossl to give you the magnification you wanted, you would have to deal with an eyepiece with about 7 mm of eye relief. That would be no good for the eyeglasses wearer and others would have their eye almost on the lens.

But place that 30 mm Plossl in a 3X Barlow and you would have the equivalent magnification of a 10 mm Plossl but would retain the 22 mm of eye relief of the 30 mm eyepiece.

How to Use a Barlow?

The use of a Barlow lens is quite simple. You place the Barlow in the focuser or the diagonal and insert the eyepiece into the Barlow.

For refractor, SCTs and MCTs, which typically have a diagonal in the path, you do have the option of putting the diagonal in the Barlow and putting the Barlow into the focuser. Then you would place the eyepiece in the diagonal as usual.

This will result in a different magnification factor than what is shown on the Barlow. In some cases, this might create an awkward set-up with the diagonal, but it is a valid way of using a Barlow.

Extending Your Eyepiece Set

Typically each eyepiece delivers one magnification. If you want more choices you need more eyepieces. But this can become expensive and require a larger storage case.

If you buy your eyepieces with the use of a Barlow in mind, a Barlow can greatly enhance your choices at a minimal cost. For example, say you have eyepieces of 30, 12, 8 and 5 mm focal lengths. That would give you four magnification choices. But if you add a 2X Barlow into the mix you would be adding the equivalent of having 16 mm, 6 mm, 4 mm and 2.5 mm eyepieces giving you 8 magnification choices. And a single Barlow would typically be much less expensive than four eyepieces.

Or use the Barlow for those magnifications that you don’t think you will be able to use very often. Many nights I can’t get above 200X on with my scope due to atmospheric conditions. An economic plan would be to buy eyepieces for 200x and below and to use a Barlow to provide those magnifications above 200x that I may not be able to use very often. If I find that 250X can be used often then I might add an eyepiece for that later, when I know it is justified. Or I can continue to use the Barlow.

Most people will have one Barlow that is well suited to their eyepiece set and their telescope. I like to experiment so I have three Barlows at 2X, 2.5X, and 3X. My 2X Barlow has a removable element that provides 1.5X so for practical purposes I have four Barlow magnification options.

These allow me to use the same eyepieces in all 5 of my scopes which have focal lengths of 400 mm to 1900 mm. The Barlows let me optimize those eyepieces according to the focal length of each scope. I don’t use them all the time, but they are there to help me extend my reach when I need it, or to fill in a gap when conditions make that advantageous.

Best Barlow Lens For 2" Focuser - My Recommendations

If you have a 2” focuser and a mix of 2” and 1.25” eyepieces, the following recommendations can be great Barlow lens choices. However, note that a 2” Barlow with a 2” eyepiece can be quite heavy and may cause some telescopes to have balance problems. Consider this when you make your selection.

1. GSO 2” 1.5X/2X

GSO 2' 2x ED Barlow Lens # GS22B

Price – $65.95

This Barlow Lens is a real bargain. It will accept 2” and 1.25” eyepieces allowing you to add a second magnification option for both size eyepieces. It includes a 2 lens design with multi-coated ED glass elements which can help maintain image quality. It also has a removable Barlow element that can be screwed directly onto a 2” eyepiece for 1.5X giving you an additional magnification option. If you want a low cost 2” Barlow to extend your 2” and 1.25” eyepieces, this is the one to get.

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2. Celestron Luminous 2” 2.5X Barlow Lens

Celestron 93436 Luminous 2-Inch 2.5x Barlow Lens (Silver)

Price – $119.99

The Celestron features a fully multi-coated 4 element design for better image and color correction for 1.25” and 2” eyepieces. The 2.5X multiplication allows you to space your magnifications a little differently than a 2X or a 3X. For some, 2.5X might be the better choice. The Celestron name is well known in the consumer optics industry for service and support.

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3. Explore Scientific 2” 2X Focal Extender

2X Explore Scientific Focal Extender; 2-inch Barrel; 4 Elements

Price – $229.99

A focal extender is based on a slightly different design from a true Barlow lens but serves the same purpose, adding multiplication options for your eyepieces. Explore Scientific has the reputation of offering a higher quality build and delivering a better experience but at a higher price. If you are going to go for higher priced eyepieces, this might be a better choice. Their service and support are excellent.

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4. Tele Vue 2X 2” Powermate

Televue PMT2200 2X Powermate 2 inch

Price – $327.00

Tele Vue is one of the top names in astronomy optics. The Powermate is optically a little different as it is based on a telecentric optical design where a Barlow lens is based on a divergent ray design.  That being said, they are used in the same way.  If you must have the highest quality image to support your top of the line eyepieces, this is the one to get.

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Best Barlow Lens For 1.25” Focuser - My Recommendations

Even if you have 2” eyepieces, those larger 2” eyepieces are typically dedicated to low power wide view use. Many people do not use them with a 2” Barlow lens due to the weight. Most people will only use a Barlow lens with their 1.25” eyepieces.

1. GSO 1.5X/2X Shorty Barlow Lens

GSO 1.25' 2X Shorty Achromatic Barlow Lens # GS2BL

Price – $35.00

If you are looking for a low-cost Barlow to get you started, and a 2X Barlow is a good fit for your eyepiece set, this is the one I would recommend. This is a two-lens element design that features full multi-coating. Just like their 2” Barlow, this one has a removable 1.5X element which effectively gives you two additional magnifications for each of your eyepieces.

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2. Celestron X-Cel LX 1.25-Inch 3x Barlow Lens

Celestron 93428 X-Cel LX 1.25-Inch 3x Barlow Lens (Black)

Price – $89.95

The X-Cel series Barlow of Celestron features a three-element design and full multi-coating. The eyepiece is held in with a compression ring so that the locking screw does not mar the eyepiece. The three lens design provides enhanced image correction from a name that is well known and trusted in the astronomy hobby. This is their 3X Barlow but is also available in 2X.

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3. Tele Vue 3X Barlow

Televue 3x Barlow 1.25 inch (1-1/4 in.)

Price – $128.00

Tele Vue brings its quality and reputation to a 3X Barlow. It is also available as a 2X model. This is of a longer design than many of the Barlow designs. Some prefer this longer design. While only a 2 element design, Tele Vue adds a level of polish and quality control that assures the best performance and the lowest light loss.

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4. Tele Vue 2.5X Powermate

Televue 2.5x Powermate 1.25 inch (1-1/4 in.)

Price – $218.00

Powermate incorporates a telecentric optical design, compared to a Barlow lens’ divergent ray design, but it is used in the same way. For advanced users this can offer more options.  The Powermate series is preferred by high-performance eyepiece users for the way it parallelizes the light rays. If you want the top of the line performance, the Powermate series is where you will find it.

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In Summary

Where can a Barlow lens be helpful?

  • When you don’t have an eyepiece at the magnification you want to use.
  • When you want to retain the long eye relief of a low power eyepiece at high power.
  • When you want to stretch your eyepiece budget, effectively doubling your eyepiece set.

I usually recommend that new people on a tight budget incorporate a Barlow into their eyepiece strategy to provide maximum flexibility at the lowest cost. You can always add more eyepieces later, but if you have a Barlow, each of those eyepieces will provide two magnifications.

4 thoughts on “Barlow Lens: What, How To and The Best”

  1. The goal of the article is not to do a competitive comparison between barlows.

    As far as 1.25″ vs. 2″, the main consideration is whether you have 2″ eyepieces and whether you want to barlow them. Most people don’t barlow 2″ eyepieces.

    I have a 2″ 2X GSO barlow that I like very much, but I don’t use it as much as I thought I would. When I didn’t have wide angle 1.25″ eyepieces I used it for a while, but as I filled in my eyepiece set with 82 degree 1.25″ eyepieces I stopped using the 2″ barlow. But, for a while, I used it a lot.

    Most people have one or two 2″ eyepieces and the rest are 1.25″. The exception may be those people who have very long focal length scopes, say over 3000 mm, who would like to leverage their 2″ eyepieces and their wide apparent field of views. 2″ eyepieces tend to be quite expensive so being able to barlow them can be a very reasonable approach if your eyepiece budget is limited.

    One of the scopes I use is a 14″ Meade LX200 with a 3500 mm FL. I have two 2″ eyepieces. I could use my 2″ barlow with this one, but find I tend to go to my 1.25″ 82 degree eyepieces rather than barlow the 2″ eyepieces.

    Reply
  2. Ed, Thanks. I do understand that the point of the article was to compare barlows, sorry I didn’t mean to go off on a tangent. It’s just that I see lots of reviews on different equipment and barlows is one I have not seen.

    Regardless, going back to your comment regarding 1.2″ vs. 2″. I assume that while there may be limited use for a 2″ barlow that as long as you have a 1.25″ adaptor there is no harm in a 2″?

    Reply

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