Zoom lenses used for moviemaking applications must have the parfocal ability in order to be of practical use.

The lenses are typically designed to be parfocal, which means that when one changes from one to another the sample stays in focus.

A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes.

Note, many of today's superzooms are not "parfocal"; that is, not true zooms.

A suggestion by Köhler led to the development of parfocal lenses which allow the specimen to remain in focus when changing objectives on a microscope.

A parfocal lens is a lens that stays in focus when magnification/focal length is changed.