A current thread has some debate about whether or not a particular zoom eyepiece is parfocal. In reading threads over the years I've noticed that some people consider eyepieces from a line that only require a small tweak in focus to be parfocal. However, others may consider an eyepiece line or a zoom eyepiece as parfocal if there is zero focus adjustment required when switching eyepieces or zooming the zoom eyepiece.
I'm just curious what "parfocal" means to different people and if you think there can be a slight focus adjustment for a parfocal eyepiece line then how much focus adjustment would you allow?
The industry standard is +/- 1mm. if two eyepieces focus in that range, they are considered parfocal.
For me personally, EXACT parfocality is impossible because the eye focuses slightly differently with different exit pupils.
But, if the eyepiece, when inserted, is close enough to the focus of the previous eyepiece that a small planetary nebula is still identifiable as a nebula and not a star,
then it's close enough to parfocal to consider it parfocal.
For a technical answer, see Ernest's.
For a good answer how to use a zoom, see Jon's: focus at the highest power and then move down--the eyepiece will seem to stay in close focus.
Focus at low power and move up and you will need to refocus at the higher powers.
That is, of course, if the zoom is more or less parfocal across its range, and many zooms are not.
But so? The purpose of a zoom is to have an entire kit of eyepieces in one eyepiece so you always find the right magnification.
That doesn't change if a tiny amount of refocusing is necessary.