What an eyepiece does
The telescope’s main lens or mirror gathers light and forms an image. The eyepiece is the small lens assembly that magnifies that image for your eye. Because it is easy to swap, the eyepiece is how you change magnification and field of view without changing the telescope. Most beginner telescopes come with one or two eyepieces, and many observers add a few more over time.
The specs that matter
Focal length
Eyepiece focal length is the number in millimeters printed on the barrel, such as 25mm or 10mm. Magnification is the telescope’s focal length divided by the eyepiece focal length. A shorter number gives more magnification. The magnification chart shows the numbers for common combinations.
Apparent field of view
Apparent field of view (AFOV) is how wide the view looks to your eye, measured in degrees. Typical Plossl eyepieces are often around 50 degrees, wide-field designs commonly 60 to 70 degrees, and ultra-wide designs 80 degrees or more. The real patch of sky you see, the true field, is roughly the AFOV divided by the magnification.
Eye relief
Eye relief is the distance between the eye lens and your eye where the full view is visible. Short eye relief can be uncomfortable at high magnification, and it is a particular concern for people who wear glasses. Longer eye relief is generally more forgiving, although the right amount varies from person to person.
Barrel size
Barrel diameter must match your focuser. The common sizes are 1.25 inch and 2 inch, with an older 0.965 inch size on some small telescopes. See eyepiece diameter and barrel sizes for details.
Common eyepiece designs
- Huygens and Ramsden: very simple designs found in some bundled eyepieces. They tend to have narrow fields and short eye relief.
- Kellner and modified achromats: a modest step up, often included with entry-level telescopes.
- Plossl: a popular, affordable four-element design with a sharp, comfortable view across a moderate field.
- Wide-field and ultra-wide designs: more elements, bigger fields and often more weight and cost.
- Orthoscopic: an older design valued by some planetary observers for a narrow but clean view.
- Zoom eyepieces: one eyepiece covering a range of focal lengths, convenient but usually a compromise in field width and sharpness compared to fixed designs.
Brand names and exact element counts vary, so treat these as general descriptions rather than rules.
Is a 10mm or 20mm eyepiece more powerful?
The 10mm eyepiece is more powerful. On the same telescope, it gives twice the magnification of a 20mm eyepiece but shows a smaller piece of sky and a dimmer image.
Which eyepiece for which target
- Planets and the Moon: medium to short focal lengths give higher magnification. Planets are small, so many observers use somewhere in the 6mm to 10mm range on typical telescopes, if the air is steady enough. The Moon also looks good at medium power, and a lower power is helpful for seeing the whole disk.
- Large star clusters, nebulae and galaxies: longer focal lengths give wider fields and brighter images. A wide-field eyepiece is a good choice.
- Finding objects: always start with your longest focal length eyepiece. It gives the widest view and makes targets easier to center.
On bad nights, atmospheric turbulence limits useful magnification more than the equipment does. See how to use the magnification chart for limits.
How to choose for your telescope
- Check your focuser size. Confirm 1.25 inch or 2 inch.
- Know your telescope’s focal length and aperture. These set the range of useful magnifications.
- Build a small set. A common approach is a low-power eyepiece for finding and wide views, a medium one for general viewing, and a high-power one for planets and double stars.
- Consider a Barlow lens. A 2x Barlow roughly doubles the magnification of each eyepiece, so it can stretch a small set.
- Upgrade the included eyepieces when ready. Bundled eyepieces vary in quality, and a better eyepiece can noticeably improve comfort.
Looking after eyepieces
Keep dust caps on when storing eyepieces. Avoid touching the lenses, blow off dust before wiping, and use lens cleaning supplies sparingly. Store them in a case away from damp.
Related guides
Once you have eyepieces, you also need a way to point the telescope. See the finder scope guide and how to use a finderscope.