All About Barlow Lenses

Every novice telescope user has seen advertisements describing how a Barlow lens increases an eyepiece’s magnification and expands the power of a telescope. Many astronomy beginners are confused about what this piece of optical magic can do and how it works. So, let’s take a dive into Barlow lenses—and what they can (or cannot) do for you and your telescope.

First, the reason for capitalizing the word “Barlow” in “Barlow lens.” This optical device is named after the 19th-century English physicist and mathematician Peter Barlow. Although Peter Barlow is also known for his 1814 book New Mathematical Tables, his name now adorns almost every telescope box, thanks to his invention that increases the magnification of any eyepiece.

How a Barlow Works

A Barlow must always be used with an eyepiece. You cannot achieve a focused image through a telescope-mounted Barlow lens alone. A Barlow lens is typically housed in either a standard 1.25-inch or 2-inch barrel, so it is compatible with the focusers on today’s telescopes. Optically, a Barlow is a three- or more-element convex diverging lens. When placed at the telescope’s focus, typically between the focuser and the eyepiece on a Newtonian telescope, or between the star diagonal and the eyepiece on refractor or Schmidt-Cassegrain telescopes, the Barlow lens spreads out the converging light cone from the telescope objective. This causes the light cone to reach focus at an apparently greater distance from the objective than without the Barlow. 

For this reason, Barlows are sometimes referred to as “focal extenders.” A well-designed multi-element Barlow lens focuses all colors at the same point, preventing bright objects, such as stars and planets, from exhibiting red and blue fringes. When used in conjunction with photographic camera lenses, Barlows are often referred to as teleconverters. Read more about imaging with a Barlow lens.

To understand how a Barlow lens works, let’s look at the basics of telescope magnification. Magnification is determined by dividing the focal length of your telescope by the focal length of your eyepiece. For this example, we will use a Celestron NexStar 5SE with its 25mm eyepiece:

Advantages of a Barlow Lens

The most obvious advantage of a Barlow lens is increased eyepiece magnification. But, as we shall see later, there are limitations to how much useful magnification a Barlow, or any eyepiece, can provide with a given telescope.  Barlow lenses are available that multiply the telescope’s focal length by a factor of 1.8x to 5x. The most common Barlow lens is the 2x, which doubles the magnification of any given eyepiece. 

A quality Barlow can be a useful accessory if one plans their eyepiece set carefully. A 2x Barlow lens will make a 40mm eyepiece perform like a 20mm, a 32mm eyepiece perform like a 16mm, a 25mm eyepiece perform like a 12mm, and a 12mm eyepiece perform like a 7.5mm. Take care not to Barlow-duplicate the magnification of an existing eyepiece, but by carefully acquiring the proper focal length eyepieces, a 2X Barlow can effectively fill the magnification gaps between the eyepieces.

Another very useful advantage of a Barlow lens is the increased eye relief it provides. Eye relief is the distance your eye must be away from the eye lens, or the lens at the top of the eyepiece, to see the entire field of view.  Increased eye relief provides greater observing comfort and often allows eyeglass wearers to view through the telescope while wearing their glasses. This is especially welcomed by observers who need correction for astigmatism. For instance, if the eye relief of a 10mm eyepiece is too short to allow wearing glasses while observing, the added eye relief of a combined 20mm eyepiece and a 2X Barlow may enable retaining the eyeglasses while observing. 

The added eye relief provided by a Barlow can also prevent bumping or jiggling the telescope when observing the Moon or planets at high magnification. Because Barlow lenses increase eye relief, they are often incorporated into premium wide-angle eyepieces to provide enhanced eye relief.

Spotlight: Luminos Barlow Lens

If you're looking for versatility, the Celestron Luminos Barlow lens is a standout. It features a 2" barrel but includes a removable adapter that allows it to accept both 2" and 1.25" eyepieces. This dual compatibility makes it a great choice for observers who want to expand their magnification options without being limited by the size of their eyepiece. Plus, its fully multi-coated optics and machined aluminum body ensure bright, sharp views across the field.

The Disadvantages of a Barlow

A disadvantage of using a Barlow is that the eyepiece is spaced up to three or four inches farther out from the focuser or star diagonal. This means you’ll need to take caution in the dark to prevent bumping the telescope. And because most 1.25-inch Barlow lenses weigh several ounces, adding one may require rebalancing small telescopes.

Another disadvantage of a Barlow lens is that the additional apparent focal length results in a dimmer view and a smaller field of view. Using a 2x Barlow on an f/10 telescope, such as the popular Celestron NexStar 6SE or 8SE, converts the optical system to f/20, resulting in a noticeable dimming of the view due to the higher f-ratio. 

Longer effective focal lengths also reduce the field of view with a given eyepiece. This is not an issue when viewing a large target like the half-degree-wide Moon, because if any portion of the Moon is in the eyepiece, you can crater-hop to the desired lunar view. However, planetary observing with Barlow-assisted magnification is another issue. If the planet is just outside the smaller field of view, locating it to center it in the eyepiece can be more challenging. In these cases, it is best to use a well-aligned crosshair-equipped optical finder. 

Novice viewers should resist the temptation to use a Barlow lens that provides too much magnification. Every telescope has a practical limit to how much magnification it can deliver effectively. Depending on the steadiness of the atmosphere, good performance is usually achieved at 30-40x per inch of telescope aperture. Pushing beyond this limit will exceed the telescope’s optical capability, resulting in dim and blurry images.

To help determine the appropriate magnification, Celestron lists the lowest and highest useful magnifications for each telescope model in the product specifications. Staying within this range ensures you get sharp, detailed views every time.

Other Barlow Considerations

A Barlow lens does not correct any spherical aberration present in the telescope’s primary optics. However, a Barlow can improve the off-axis sharpness of an eyepiece. This effect arises from the imaging surface of most telescope objectives being inwardly curved, while the Barlow lens flattens the focal plane. Additionally, a Barlow will partially compensate for any inherent astigmatism in an eyepiece design. These positive attributes make a Barlow useful when used with poorly corrected (inexpensive) eyepieces on a fast Newtonian telescope.

Now that you understand how a Barlow’s strengths and limits play together, it can become a powerful tool for sharper views and better planetary images.