Parfocal

A parfocal lens maintains its focus point when the focal length is adjusted. If a lens is focused on a subject at 100mm and then zoomed to 200mm, a parfocal lens keeps the subject in focus at the new focal length. This makes in-shot zooming practical for video work — the cameraperson can push in or pull out during a live recording without losing focus. True parfocal lenses are expensive to manufacture because they require the optical design to maintain focus throughout the zoom range, which adds elements and complexity.

Most photographic zoom lenses — including the majority of M43 zooms — are varifocal rather than parfocal. A varifocal lens shifts focus when the focal length changes. For still photography this is largely irrelevant, because the photographer sets the focal length before acquiring focus. For video, zooming during a shot with a varifocal lens requires the autofocus system to refocus after the zoom, which introduces a visible hunting or refocusing moment in the footage. For professional video work where manual follow focus is standard practice, all lenses behave as parfocal because the focus puller adjusts focus throughout the zoom.

In M43 video applications, parfocal behaviour is most relevant for documentary and event videography where the operator needs to zoom during a shot without a dedicated focus puller. Cine-style lenses and dedicated video zoom lenses are more likely to be designed with parfocal behaviour than photographic zooms. For operators using autofocus on the GH7 or G9 II, the camera's continuous AF can compensate for varifocal focus shift during slower zooms, but may not keep pace with rapid zoom changes.