Because eyepieces are not parfocal unless they are designed to be: each brings its focus to a slightly different point behind the barrel. Within one family the difference is usually small; across families it can be a full turn.
It is a property of how eyepieces are built, not a sign that anything is wrong.
Where the focus point sits
Each eyepiece forms its image at a particular distance from its shoulder, decided by its optical design. Two designs with the same focal length can place it a few millimetres apart.
Why it is worse across families
A simple design and a wide-field design of the same focal length have very different internal layouts. Swapping between them moves the focus point most.
What a Barlow changes
It shifts the whole focus position outward, which is one reason a Barlow can rescue a camera that cannot otherwise reach focus — and one reason a telescope that focuses fine with an eyepiece may not with a Barlow in front of it.
The practical fix
Parfocal rings, or the simpler habit of touching the focuser after every change. On a tracked mount the second costs nothing; on an undriven one it costs the object drifting out of the field while you turn the knob.
The questions that keep arriving
Are any eyepieces parfocal?
Eyepieces within a single designed range often are, or close to it, and some makers state so. Mixing families is where the difference becomes noticeable.
Can I make mine parfocal?
Parfocal rings exist for exactly that: a collar that sets how deep each barrel sits in the focuser. It is a small purchase that removes a small irritation repeated all night.
Last reviewed 17 September 2026