Most eyepieces conform to one of three standard barrel sizes:
- 0.965 inches (obsolete, no longer made except for very cheap telescopes)
- 1.25 inches
- 2 inches
The modern standard sizes are 1.25" and 2".
It should be noted that 2" barrels are not inherently an upgrade to 1.25" barrels the way say, a faster CPU or a camera with more megapixels might be. Different sized barrels are more akin to different sized paint brushes. Sometimes you need a larger paint brush, sometimes you don't.
2" vs 1.25" Barrels
The reason for the existence of 2" barrels is typically to provide a wider field stop, which allows for a wider field of view at lower magnification. For example, a 40mm eyepiece in a 1.25" barrel can only have an apparent field of view of about 42 degrees, but in a 2" barrel it can have an apparent field of view of about 70 degrees.
But since not all eyepieces have such long focal lengths, it is not always necessary to house their optics in a larger 2" barrel. For example, the 13mm Tele Vue Ethos has an extremely wide 100 degree apparent field of view, but only requires a 1.25" barrel to achieve it. Conversely, the 17mm Ethos requires a 2" barrel in order to provide that apparent field of view at that focal length.
If you have a 2" focuser, don't select eyepieces based on their barrel size..The barrel size is merely a consequence of their optical design and characteristics. Instead, choose eyepieces for their focal length and apparent field of view, and don't worry about what the barrel size is.
Common Maximum Apparent Fields of View By Barrel Size and Focal Length
- 40mm ~42 degrees
- 32mm ~52 degrees
- 24mm ~68 degrees
- 16mm ~82 degrees
- 13mm ~100 degrees
- 40mm ~70 degrees
- 32mm ~82 degrees
- 25mm ~100 degrees
- 9mm ~120 degrees (technically could be a 1.25" barrel, but the size and weight requires a 2" barrel to hold securely)
- 30mm ~100 degrees
Dual Barrel Eyepieces
It's common for some 1.25" eyepieces to also have an outer 2" barrel. The reason for this is so that they can be used in 2" focusers without the need for an adapter. It has no bearing on the eyepiece's optical performance, it simply is a matter of convenience.
It should be noted however, that depending on the design of the barrel, using these dual barrel eyepieces in 2" mode might result in damage to a prism or mirror diagonal if it's too shallow to accept the eyepiece.
In some cases, the focuser might not have enough outward travel for the eyepiece to come to focus in 2" mode, since 2" mode puts the eyepiece's focal plane much deeper in the focuser than the 1.25" mode does.
The only mass produced 3" eyepiece on the market right now is the Explore Scientific 30mm 100 degree eyepiece. Generally speaking, 3" eyepieces are not common as there is not a strong need for them. The only reason to have a 3" barrel is to achieve 100+ degree apparent fields of view in focal lengths longer than 25mm. However, the sheer size and weight of such eyepieces means they are only suitable for large Dobsonians or strongly mounted refractors that have large focusers. However, large Dobsonians tend to have short focal ratios, and 30mm eyepieces may result in exit pupils that are too large to be usable.