Best Barlow Lens -

Whether it’s visual astronomy or astrophotography, you may be looking for a cheap and easy way to extend the focal length of your optics.

The easiest way to do this is using a focal extender, commonly known as a Barlow lens.

In this post, I will talk about the best Barlow lenses and when you may be better off using a good eyepiece instead.

But before we dive into that, what is a Barlow lens exactly?

Table of Contents

What is a Barlow lens?

A Barlow lens is essentially an additional optical element that’s added in between the telescope optics and either an eyepiece or a camera sensor.

The purpose of the Barlow lens is to increase the magnification and extend the focal length of the telescope. You can get 2x Barlows, 2.5x Barlows, all the way out to 5x Barlow lenses.

The Barlow fits into the eyepiece receiver of your telescope, and then the eyepiece or camera adapter is inserted into the receiving end of the Barlow.

Advantages of Barlow lenses

They are a cheap way to effectively double the number of eyepieces that you have (in a sense)
They are often cheaper than buying a new set of eyepieces
They can be easily added or removed from the optical train
They can be purchased in a wide array of different magnification options

Disadvantages of Barlow Lenses

They will make the object you are looking at dimmer
They will never improve (and often hinder) the quality of the image
It’s easy to push past the maximum useful magnification when you are using a Barlow
In the realm of astrophotography, they can only be used for planetary imaging
They make it much more challenging to stay on your target due to the increased focal length
They do not correct pupil size

Best Barlow Lenses

If you are just beginning visual astronomy, the best Barlow lens is the one that you already have.

Sometimes eyepiece kits will come with cheap Barlows, and chances are you won’t notice a significant difference in image quality right away.

I used to work for a guy that was majorly into wine. I explained to him that I’ve always been just fine with a $10 bottle of Bogle and that I didn’t notice a big difference between it and the couple of sips I had of really “nice” wine.

His answer was basically, “good, just keep drinking the Bogle then.”

So essentially, the lesson was once you acquire a taste for good wine, it ruins the cheaper stuff that you were perfectly happy with.

Stick with the cheaper options when you first start, but if you would like something a little nicer, take a look at the Orion 2x Shorty Barlow.

Orion Shorty 2x Barlow

This little guy is about half the length of most other 2x Barlows, coming in at just 3 inches long.

The inside housing is baffled to prevent undesired reflections and boost contrast, and the doublet lens system is multi-coated for good transmission.

If you want something better than a cheap kit Barlow, this one should do the trick without breaking the bank. The current price at the time of writing this is $40.

If you want to jump straight to what is typically considered the gold standard for Barlow lenses, take a look at the Televue 2x Barlow.

Televue 2x Barlow

Like I mentioned before, adding another element such as a Barlow to your optical train never improves the image quality vs. an eyepiece alone.

The best that you can hope for is that the Barlow is basically invisible and maintains the image quality that you would expect from a good eyepiece.

Televue has been around for over 30 years, and their eyepieces and Barlows are considered among the best out there.

The goal with their Barlow lenses is to lay to rest the idea that Barlows will degrade your images. Most will argue that they have succeeded.

Their 2x Barlow will set you back about $140, so you certainly pay for the quality that you are getting.

I personally would use the Orion Shorty Barlow if your goal is visual astronomy only. However, if your goal is planetary astrophotography, I would consider a Televue Barlow.

2x or 3x Barlow Lens

Deciding which Barlow to get will depend on your telescope and what you are trying to accomplish.

For example, if you are only using it for visual, you should consider the telescope’s aperture and maximum useful magnification.

If you use it for planetary astrophotography using the “lucky imaging” technique, you should also consider your camera’s pixel size.

The basic rule of thumb is that you want your focal ratio to be roughly 5 times your camera’s pixel size.

Let’s use a classic 8 in Dobsonian as an example with a 1200mm focal length and a focal ratio of about f/6. Let’s also assume that we are using a Canon T3i with a pixel size of 4.30µm.

In this case, you want to use a Barlow lens that puts the telescope’s focal ratio as close to 21.5 as you can.

If you used a 3x Barlow, the focal length would increase to 3600mm. 3600mm divided by the telescopes 203mm aperture would give a focal ratio of 17.7

A 4x Barlow would give you a focal ratio of 23.6. If you can find a 3.5x Barlow, that would yield a focal ratio of 20.6.

In all practicality, any of these would likely work just fine. However, planetary astrophotography will probably be the only thing you use it for anyway, and I will explain why.

For visual astronomy, there are some good reasons to consider not using a Barlow at all, and investing in better eyepieces instead.

Barlow Lens Advice

As mentioned earlier, if you add more glass, such as a Barlow lens, the view will typically deteriorate to some degree.

The degree of which will really depend on the optical quality of the Barlow lens that you choose to use.

Instead of utilizing a Barlow, you’re almost always better off using an eyepiece that provides the magnification you require. However, if you have good eyepieces and a good Barlow, you won’t notice much difference in the view as a beginner.

When using a Barlow you should always consider the maximum magnification that can be used. You can only magnify an image so far before it literally disintegrates due to light diffraction principles.

There are several essential factors to consider when determining how much usable magnification a scope can provide.

The first is, of course, the optical quality. You won’t obtain good images if your optics aren’t up to par.

According to the common rule of thumb, the most useful magnification you can acquire from a scope is roughly 50X magnification per inch (or 2x per millimeter) of aperture.

So, under ideal sky circumstances and assuming the optics of the telescope and eyepieces are all of reasonable quality, an 8″ telescope should be good up to roughly 400X.

Magnification is computed by dividing the telescope’s focal length by the eyepiece’s focal length. So, to get 400X magnification, you’ll need an eyepiece with a focal length of around 3 mm.

You could theoretically magnify as much as you wanted. However, as the view becomes less clear and the field of view shrinks, it will become increasingly difficult to maintain the object you’re looking at in the field of view.

As you stretch the light out in your area of view, it will make items appear dimmer and dimmer.

Now, keep in mind that anything larger than a 2X Barlow won’t make any sense for viewing planets.

With a 2x Barlow, you could take a nice 6mm planetary eyepiece and push it to 3mm. This would be dead on the telescope’s maximum useful magnification.

A 5X Barlow sounds good on paper, but it would take that same 6mm eyepiece and yield a magnification of 1000x.

This is well beyond the scope’s maximum useful magnification, and the resulting image will resemble a shapeless blob of light.

Another disadvantage of Barlows is that they do not correct pupil size. This can result in substantial vignetting with some eyepieces, especially with high-magnification Barlows (3x and more) and those with shorter physical length.

Conclusion

If this is your first foray into visual astronomy, a cheap Barlow lens will suit you just fine.

If you want better optical quality, consider Barlows such as the Orion Shorty. If your budget allows for it, Televue Barlows are top of the line.

Barlow lenses are necessary for planetary astrophotography, and shouldnt be used for any other type of astrophotography.

For visual purposes, a 2x Barlow often makes the most sense. But, even then, it’s typically advised to invest in some decent eyepieces vs using a Barlow lens in the first place.