Barrel diameter is not focal length
“Eyepiece size” can mean two different things, and mixing them up causes most of the confusion.
- Barrel diameter is the width of the tube that slides into the focuser, measured in inches. It decides whether the eyepiece physically fits.
- Focal length is the number in millimeters printed on the eyepiece, such as 10mm or 25mm. It decides magnification.
This article is mainly about barrel diameter, with a short section on focal length at the end.
The standard barrel sizes
| Barrel size | Approximate metric size | Where you find it |
|---|---|---|
| 0.965 inch | about 24.5 mm | Older and very small, inexpensive telescopes |
| 1.25 inch | about 31.75 mm | The most common size on amateur telescopes |
| 2 inch | about 50.8 mm | Larger telescopes and wide-field eyepieces |
Some 2 inch eyepieces are built for 2 inch focusers only, while many 1.25 inch eyepieces can also be used in a 2 inch focuser through an adapter, which is a common way to use both sizes with one telescope.
1.25 inch versus 2 inch
The barrel size affects the largest field of view the eyepiece can offer. The tube forms a physical limit, called the field stop, so a 1.25 inch eyepiece cannot show quite as wide a field as a 2 inch one at the same focal length. That is why very long focal length, wide-field eyepieces come in 2 inch barrels.
Practical differences:
- 2 inch eyepieces are usually larger, heavier and more expensive.
- Heavier eyepieces can unbalance a small telescope or mount.
- 1.25 inch eyepieces have the widest choice and price range.
- 2 inch eyepieces mostly help with low-power, wide views of large objects.
Will my eyepieces fit?
Check your focuser. Many telescopes have a 2 inch focuser with a removable 1.25 inch adapter, while others accept only 1.25 inch. If your focuser is 1.25 inch only, you cannot use 2 inch eyepieces. Small telescopes with a 0.965 inch focuser often limit your options; some can be upgraded with a 1.25 inch focuser or an adapter, but this depends on the model.
Also confirm that your telescope can reach focus with a given eyepiece. Some combinations need more focuser travel than a particular telescope provides.
Eyepiece focal length: quick answers
What is the focal length of an eyepiece? It is the number in millimeters on the barrel. Divide your telescope’s focal length by it to get magnification.
Is a 10mm or 20mm eyepiece better? Neither is better overall. The 10mm gives twice the magnification of the 20mm, and the 20mm gives a wider, brighter view. Many observers use both.
Is an eyepiece always 10x? No. Magnification depends on the telescope and the eyepiece together, so an eyepiece has no fixed power on its own. For example, a 1000mm focal length telescope with a 10mm eyepiece gives 100x, while a 2000mm telescope gives 200x with the same eyepiece. See the magnification chart for more combinations.
What are the different sizes of telescopes? Telescopes are usually described by aperture (the diameter of the main lens or mirror) and focal length, for example a 150mm aperture with 750mm focal length. This is separate from eyepiece barrel diameter.
Choosing barrel size
For most beginners, 1.25 inch eyepieces are the practical choice. If your telescope has a 2 inch focuser, a 2 inch eyepiece is something to consider later for wide fields of large star clusters and nebulae.
For the wider picture of eyepiece specs and designs, return to the telescope eyepieces guide.