The whole exercise is arithmetic, and doing it on paper first is what separates a useful case from an expensive one.
Write down your telescope focal length
It is on the tube or in the manual. Everything below divides by an eyepiece focal length to give a magnification, and that magnification is the only number worth comparing.
Work out what you already have
Divide for every eyepiece that came with the telescope. Two results close together mean you effectively own one eyepiece, which is common with supplied pairs.
Choose the low power first
The eyepiece used to find things and to frame large objects. It is the one that gets the most use, it benefits most from a wide apparent field, and it is where a 2 inch barrel earns its keep if the focuser has one.
Then the high power, with eye relief in mind
Rather than buying a very short eyepiece of simple design, consider a longer one behind a Barlow: the magnification is the same and the eye relief survives, which decides whether it gets used.
Fill the middle last
Once the two ends are settled, the gap between them is obvious and one eyepiece usually closes it. Buying in this order means every purchase answers a question the previous one raised.
The questions that keep arriving
How many do I actually need?
Three that give genuinely different magnifications, and a Barlow if the gaps are wide. Beyond that, additions should fill a measured gap rather than complete a set.
Should I buy a set or singles?
A set is cheaper than the same range bought singly and nearly always contains a duplicate or a focal length your aperture cannot support. Work out the magnifications the set would give you before deciding.
Last reviewed 17 September 2026