|
Luke,
What matters is the distance between the focal plane of the eyepiece, and the top of the sleeve on the eyepiece (the tube that fits into the eyepiece holder).
If this dimension is the same for your eyepieces, then no, you do not need to refocus when swapping them. This condition is also called "Parfocal", and some sets of eyepieces (generally expensive ones) are par-focal.
For example, if I insert a 5mm Vixen LVW eyepiece and focus my scope for this eyepiece, then yes, it will also be correctly focussed for the other Vixen LVW eyepieces (8, 13, 17, 22 and 42mm). But not for any other type of eyepiece.
Cheaper generic eyepieces are usually NOT par focal, even within the same set from the same manufacturer.
Sadly, eyepiece manufacturers have never agreed on standardising on this dimension (a great pity) and unfortunately this means that if you "mix & match" eyepieces from different sets and/or different manufacturers, you will be refocussing each time you swap from one to another.
|