parfocal

Definition: Adjective: ~ lenses: set up or arranged so that when one lens is changed for another, there is no change in focus. * Applied usually to turret mounted lenses e.g. on microscope; also in telescopes: ~ eyepieces: changing one for another does not require refocusing. * When the distance from the object to the rear principal plane of each lens is equal, they are said to be parfocal.