Parfocal microscopes maintain focus throughout real-time magnification changes. These microscopes also remain focused when the user rotates objectives. This capability is a function of the lens and is a particularly popular feature on field microscopes. Parfocal lenses also appear on high quality digital cameras.
The focus of a parfocal microscope remains crisp during magnification changes in either direction. Celestron explains that the quickest way to determine if a microscope has a parfocal lens is to load an objective, position a specimen slide and set the microscope to its most powerful magnification setting. Focus the image and reduce the magnification. If the specimen image stays focused, the microscope is parfocal.
Celestron states that many parfocal microscopes are also parcentered, which means that rotated and swapped objectives remain at or near the center of the vision field. This convenient feature reduces time spent positioning and tweaking objectives and gives the user more time to view specimens.
Parfocal microscopes reduce wear on mechanical parts and are valuable in classrooms because they make it easier for multiple users to view the same sample in rapid succession. Parfocal and parcentered microscopes also reduce the risk of accidentally viewing irrelevant structures due to magnification errors. This is particularly important in medical laboratories and other environments in which lives depend on scientific accuracy.