Calculate what you already own
The basic relationship is magnification = telescope focal length ÷ eyepiece focal length. Exit pupil is approximately telescope aperture ÷ magnification. Before buying, list your telescope aperture, focal length, focal ratio, and each eyepiece you already own. Do not duplicate a focal length simply because one brand is on sale.
Worked example: a 1,200 mm focal-length telescope with a 25 mm eyepiece gives 48×; a 12 mm gives 100×; a 6 mm gives 200×. With a 200 mm aperture, those combinations provide roughly 4.2 mm, 2.0 mm and 1.0 mm exit pupils respectively. This is a planning illustration, not a recommendation to use 200× every night: seeing, optical quality and collimation determine whether that power is useful.
Build an intentionally spaced three-eyepiece set
- Wide / finder: favor a comfortable low-power view for locating targets, extended objects and star hopping.
- Everyday medium: use a medium-power eyepiece for the Moon, smaller nebulae, open clusters and many general targets.
- Occasional high power: reserve a shorter focal length for steady nights and compact targets. If the atmosphere rarely supports it, the third slot can wait.
Compare the actual magnifications produced in your telescope. A 10 mm and 11 mm eyepiece may be effectively redundant, while a 10 mm and a wider-field 10 mm might still provide a distinct comfort or framing advantage.
Apparent field is not true field
Apparent field describes the eyepiece's apparent viewing angle. True field describes the slice of sky seen through the complete telescope. Apparent field ÷ magnification provides only a rough true-field estimate; exact field-stop dimensions and optical distortion matter. Eye relief and eyeglass comfort can be more valuable than a few extra degrees on the label.
Check fit and conditions before buying
- Check the focuser barrel size, adapter needs, back focus and any Barlow you plan to use.
- Confirm weight and balance, especially with smaller Dobsonians and lightweight mounts.
- For used eyepieces, request photos of glass under angled light, coatings, barrel threads and dust or fungus, and ask about return terms.
- Try the eyepiece on a clear night before deciding you need the adjacent focal length.
Tools: eyepiece calculator · eyepiece hub · used-eyepiece inspection. Reference: Sky & Telescope's telescope calculator.