Do the arithmetic again before each purchase. It takes two minutes and prevents the commonest waste on this shelf.
List what you have, in magnifications
Divide the telescope focal length by each eyepiece. Include the Barlow combinations, because those are real magnifications you already own.
Find the largest gap
Look at the list sorted in order. The biggest jump between two neighbouring figures is where the next eyepiece belongs, and it is often not where a catalogue’s next size sits.
Check the exit pupil at the new figure
Divide the candidate eyepiece focal length by the telescope focal ratio. Very small exit pupils make an image dim and fussy to look into; very large ones waste light outside your pupil. A gap that can only be filled at an unusable exit pupil is not a gap.
Decide between glass and a Barlow
If the gap is a factor of two from something you own, a Barlow reaches it and preserves eye relief. If it is not, buy the eyepiece.
Then use it for a month before buying again
Most cases would be better with three eyepieces chosen this way than with six chosen by completing a range.
The questions that keep arriving
Should I buy the same family?
It is pleasant — matched eye relief and similar field make swapping less disruptive — and it is not a reason on its own. A gap filled by a different family is still a gap filled.
Is a premium eyepiece worth it at this stage?
At the focal length you use most, often yes, because that is where the extra comfort and edge performance are actually experienced. At a focal length used twice a year it is not.
Last reviewed 17 September 2026