Here are arguably the most important numbers you need to know about an eyepiece:
• Focal length. Eyepieces have focal lengths of 2 mm to 56 mm. Longer focal lengths give lower magnification and larger exit pupils. You learned how to calculate these numbers earlier. Whatever magnification you choose, just make sure the exit pupil lies between 0.7 mm and 7 mm.
• Barrel diameter. Most eyepieces have a barrel diameter of 1.25”. Some low-power eyepieces use a 2” barrel rather than 1.25” to accept a wider cone of light and allow a wider field of view. You may need an adapter to accommodate these larger eyepieces in your scope.
• Apparent Field of View (AFOV). This is the angular diameter of the circle you see when you hold an eyepiece up to your eye. A wider field shows you more sky, so it's handy if you don't have a motor drive on your telescope since you have to move the telescope less frequently. A Plossl-type eyepiece has a 50-degree AFOV. A more expensive Nagler-type has an 82-degree field of view… this means you see 160% more sky than a Plossl at the same magnification.
• Eye Relief. This is the distance you hold your eye from the outer lens to see the full exit pupil. Short eye relief means you have to jam your eye up close to the lens. If your eyes suffer from astigmatism, you'll need to wear glasses when using your telescope and you'll need eyepieces with longer eye relief… at least 17-20 mm. Without glasses, 10-20 mm of eye relief is fairly comfortable.
A 9 mm Nagler eyepiece: "spacewalk views"
• Size and Weight. Some complex eyepieces weigh in at nearly 2 pounds and are the size of a hand grenade! That's too much for many small telescope mounts to support.
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