∞ generated and posted on 2016.01.14 ∞
Similar or nearly identical plane of focus associated with the different objective lenses employed in a compound microscope.
As one replaces one with another objective lens, typically while going from lower to higher magnification, a microscope with parfocal lenses will remain more or less in focus. In practical terms, this may not mean that no adjustment of focus will be necessary, just not an enormous amount of change in focus (ideally) should occur.
Physically, what is happening with parfocal lens is that the focus of the lenses involved occurs in the same plane. That is, the sharpest perception of the magnified image can be found in the same place which in practical terms means that changing magnification by switching objective lenses does not change focus.