How to Choose a Telescope Eyepiece
Choosing the best telescope eyepiece can seem daunting for beginners, but it is easy if you remember a few basic considerations before you buy a new eyepiece for your telescope:
Eyepiece Focal Length
All eyepieces are marked with their focal length in millimeters (mm). The focal length of the eyepiece, along with the focal length of your telescope, determines the magnification of the image.
To figure out the magnification of any given eyepiece, just divide the focal length of your telescope in mm by the focal length of the eyepiece, also in mm. For instance, a 10 mm eyepiece used in a telescope with a 1,000 mm focal length will yield 100X magnification.
Choose a selection of eyepieces that will cover a variety of magnifications and fields of view (see below) for the types of objects you like to view best, and don’t forget to count the eyepiece you received with your telescope. Most astronomers have from 3 to 5 eyepieces and a Barlow lens in their collection. Read our Telescope Eyepiece Guide for a lot more info on focal length and magnification!
Eyepiece Maximum Magnification
It would be nice if we could magnify an object enough to see, say, the US flag on the Moon, but Earth-based telescopes run into a little barrier when attempting to achieve clear views at extreme magnifications, and that little barrier is our atmosphere. The Earth’s atmosphere allows us to breathe, which is a good thing, but it also causes distortion. The level of distortion, or seeing conditions, varies all the time depending on the current weather, air currents and the geographical location you are observing from. In typical seeing conditions, therefore, the highest magnification eyepiece in your viewing arsenal should produce no more than 40X to 50X per inch of your telescope’s aperture, up to about 300X. That means if you are looking for a high power planetary eyepiece for a telescope with 4.5” (114 mm) of aperture, the eyepiece should not produce magnifications higher than 225X if you calculate using the guideline of 50X per inch of aperture. At 40X per inch, that number would be 180X magnification. Learn more about Seeing Conditions!
Eyepiece Field of View
The eyepiece field of view is often listed on the eyepiece itself, but not always. Still, you can easily find it in any eyepiece’s specifications. This is the measurement most eyepiece manufacturers use when listing an eyepiece field of view. It describes, in degrees, the calculated angular diameter of light you can see through the eyepiece before it is inserted into the telescope. The simplest designs available today, such as Plossl eyepieces, have a narrower field of view that is well suited for use with the planets or the Moon. Eyepieces with a wider field, like many Celestron eyepieces, Tele Vue eyepieces, or Explore Scientific eyepieces, are perfect for large deep-sky objects such as galaxies or nebulae.
Eyepiece Barrel Size
An eyepiece barrel is the chrome portion of the eyepiece that slides into the telescope focuser or diagonal. When you see a reference to a 1.25” eyepiece, it is referring to the barrel size. The standard, and most popular size eyepiece, has a 1.25” diameter barrel. These eyepieces fit in 1.25” focusers and 1.25” diagonals, or in larger focusers or 2” diagonals when used with a 1.25 inch adapter.
2” telescope eyepieces work with 2” focusers and 2” diagonals. Eyepieces with 2” barrels are generally designed for low magnification views of faint deep-sky objects. Their larger barrel and low magnification allow the most light possible to reach your eye; a good thing if you’re trying to see a galaxy or other celestial object at the far reaches of our universe.
Lastly, there are telescope eyepieces available that have a hybrid 1.25 and 2 inch barrel, which means they will fit into either size focuser or diagonal. You may also run across .965” eyepieces for sale from time to time, but they are old-school for the most part and will not fit into the majority of current telescopes unless you have the proper adapter.
Decide which eyepiece barrel size you need before you begin shopping for a new eyepiece. If you are not sure, you can find and read the specifications in your telescope instruction manual or online. It is also easy to do a quick measurement of the eyepiece that came standard with your scope by turning it upside down and pulling a tape measure across the diameter of the opening in the barrel. Odds are, it will be 1.25”.
Eyepiece Eye Relief
Eye relief refers to how far your eye needs to be from the eyepiece in order for an image to come to focus. Eye relief is measured in millimeters (mm). Long eye relief eyepieces make observing more comfortable to all viewers but is especially important for those who wear glasses. We recommend an eye relief of at least 10 mm for all viewers and at least 15-20 mm for those who want to, or need to, wear glasses. People who have an astigmatism should keep their glasses on while viewing, but other folks can decide which works best for them…on or off.
Different Types of Telescope Eyepieces
There are many different telescope eyepieces available on the market today, so it's important to review the different types in order to make an informed decision about which type(s) will best meet your specific needs:
The Plossl is a simple, four-element eyepiece design that is relatively inexpensive to produce. Most telescopes come with one or two Plossl eyepieces as standard equipment. The apparent field of view for the Plossl eyepiece design is right around 50 degrees.
When it comes to Plossl eyepieces, the longer the focal length the better the eye relief. For those who wear glasses and are shopping for the best telescope eyepiece for viewing planets or other objects that benefit from higher magnification, we recommend spending a bit more on a shorter focal length eyepiece with decent eye relief, such as the Celestron X-Cel LX Eyepiece or a Televue Delite.
Wide Angle Eyepieces
A wide angle eyepiece is defined by its apparent field of view, but what, exactly, is considered “wide”? Traditionally, a wide angle eyepiece had at least an 80-degree field of view, the most famous example being the Televue Nagler, which was designed back in 1979 with a then astounding 82 degree AFOV.
Today, you can find a huge variety of eyepieces dubbed as wide angle, ranging in an apparent field of view from 68 degrees to as high as 120 degrees! While a wide angle view is very pleasing to the eye, especially when viewing huge deep-sky objects like galaxies and nebulae, it is not absolutely necessary in any case. Wide angle really is in the eye of the beholder.
Telescope Zoom Eyepieces
A telescope eyepiece with a variable focal length is called a zoom eyepiece. The most common focal length range is 8 to 24; good examples include the popular Celestron zoom eyepiece or Baader zoom eyepiece. Sometimes, however, you get a little more range, like with this Apertura 9-27 Zoom Eyepiece.
An important thing to remember about zoom eyepieces is their field of view and eye relief will become smaller as you zoom up from the lowest power. Also, while a zoom eyepiece is very convenient, it will most likely not cover the entire range of magnification your telescope can support. So take a few minutes to calculate the magnifications a zoom eyepiece will produce in your scope and read the specifications on the field of view range and eye relief, especially if you wear glasses.