What is a Barlow Lens?

The Barlow lens is one of the most useful and cost-effective tools in the accessory kit of an amateur astronomer!

The Barlow Lens

Invented in the early 1800s by British mathematics professor Peter Barlow, this is a simple concave (negative) lens. When placed between the objective or mirror of a telescope and the telescope’s eyepiece, it amplifies the magnifying power of the instrument.

For example, if your telescope’s standard eyepiece provides a magnification of 50x, a 2x Barlow lens will increase this magnification to 100x (50 x 2). Similarly, if your telescope’s high-power eyepiece offers a magnification of 150x, inserting a 2x Barlow lens in front of the eyepiece will boost the magnification to 300x (150 x 2).

How Does a Barlow Lens Work?

A Barlow lens works by reducing the convergence of the light cone heading toward the telescope’s eyepiece. In doing so, it effectively increases the focal length of the telescope. Since magnification is determined by dividing the telescope’s focal length by the eyepiece’s focal length, doubling the telescope’s focal length with a Barlow lens doubles the magnification for a given eyepiece.

Benefits of a Barlow Lens

A Barlow lens is useful in several ways. First, it allows you to achieve higher maximum power. However, this isn’t always advantageous, as the maximum useful power is often limited not by the telescope or eyepiece but by the atmosphere.

Yes, that’s right! Observing the Moon or Saturn’s rings with a Barlow lens won’t help if the atmospheric conditions are unfavorable. If atmospheric turbulence causes the image to appear wavy and blurry at 150x, doubling the power to 300x will only worsen the image quality.

Ideal for Short Focal Length Telescopes

Barlow lenses are particularly beneficial for short focal length telescopes. These telescopes often struggle to achieve high magnifications, even with short focal length eyepieces. A Barlow lens can increase the magnification, allowing the telescope to reach its maximum usable power.

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Doubling Your Eyepiece Collection

The greatest advantage of a Barlow lens is that it doubles the number of magnifications available, effectively doubling the utility of your telescope’s eyepieces. For instance, if you own eyepieces of 26mm, 18mm, and 10mm, adding a 2x Barlow lens will allow them to function as 13mm, 9mm, and 5mm eyepieces. It’s like getting three additional eyepieces for the price of one Barlow lens (which is often cheaper than a single eyepiece).

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Enhanced Viewing Comfort

Another less obvious but highly appreciated benefit of Barlow lenses is that they can make telescope viewing more comfortable. High-power eyepieces (short focal lengths) often have very little eye relief, requiring you to place your eye very close to the lens to see the image clearly. A Barlow lens allows you to achieve the same magnification with a lower-power eyepiece, which typically offers greater eye relief. This is especially advantageous for eyeglass wearers, enabling them to see the entire field of view at higher magnifications.

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Improved Optical Performance

A well-made Barlow lens with anti-reflective coatings can actually enhance the performance of your eyepiece, delivering sharper images and reducing optical aberrations at the edges of the field. This comes at the cost of only a 2-3% reduction in image brightness. In fact, many high-end, high-power eyepieces incorporate built-in Barlow lens elements to achieve their magnifications.

Choosing a Barlow Lens

When selecting a Barlow lens, the first thing to consider is the barrel size of the telescope eyepieces you plan to use with it. (The barrel size refers to the diameter of the eyepiece tube that fits into the focuser. The most common size is 1.25″, though some larger telescopes and refractors may use 2″ eyepieces, and some budget telescopes still use smaller 0.965″ eyepieces.) Whatever the size of your telescope’s eyepieces, you’ll need to choose a Barlow lens of the same diameter.

Barlow lenses are also available in different magnifications. The most common is 2x, which doubles the power of the eyepiece it is used with. Other options include 3x, 5x, and even intermediate magnifications like 1.5x and 2.5x.

A 2x Barlow lens provides 2x magnification when placed between the telescope’s diagonal and the eyepiece.

Quality Matters

Be sure to invest in a high-quality Barlow lens with fully multi-coated anti-reflective coatings. Many cheap Barlow lenses included with low-end telescopes from department stores are virtually useless due to their lack of coatings and poor performance. A prime example is the x2 Barlow lens that comes with the excellent Skywatcher 130/900 telescope on an EQ2 equatorial mount.

How to Use a Barlow Lens

Using a Barlow lens is very simple: insert it into the telescope’s focuser in place of the eyepiece, then insert the eyepiece into the Barlow lens itself. Once the Barlow is in the optical path, you’ll need to refocus, often requiring a significant adjustment. To minimize excessive refocusing, it’s best to start with the eyepieces without the Barlow, then, if the object and conditions justify higher power, insert the Barlow and work through the eyepieces again, starting with the longest focal length (lowest power).

Pro Tips for Observing

As with any visual observation, avoid increasing the magnification to the point where the image becomes blurry. Once the image is blurred, no additional details can be seen, and excessive magnification only makes it harder to discern celestial details.

When planning your eyepiece collection, consider the role of a Barlow lens. If you’re just starting out, you’ll find that most telescopes come with a single low-power eyepiece, often with a focal length of around 25mm. A good strategy is to purchase a 2x Barlow lens and a second eyepiece with a focal length about 1/3 the length of your first eyepiece. (For example, if your first eyepiece is 25mm, your second eyepiece should be around 8-10mm.) These two eyepieces and a single 2x Barlow lens will provide you with a range of four different magnifications, from low to relatively high power—a great way to start exploring the skies beyond the pages of astronomy books!