parfokale

Parfokale (often translated as parfocal in English) refers to a property of optical systems in which the focal plane remains aligned when changing magnification or switching between optical elements. The concept means that once the image is brought into focus at a given magnification, other magnifications or objectives can be used with little or no additional focusing required, because the focal planes of the lenses are matched within a small tolerance. The term is used in several fields of optics, including microscopy, astronomy, and photography.

In microscopy, parfokal objectives are designed so that a specimen stays nearly in focus when the objective is rotated to a different magnification. When switching from one objective to another, the observer typically only needs a small adjustment using the coarse or fine focus controls. This capability speeds up observations and reduces disruption during examination of slides or specimens.

In astronomy and telescope use, parfocality is often discussed with eyepieces. Parfocal eyepieces maintain focus when switching between eyepieces of different focal lengths on the same telescope, allowing observers to change magnification with minimal refocusing. Some telescope and camera setups aim for parfocal alignment between objectives, eyepieces, and sensors to simplify switching components.

In photography and cinematography, parfocal lenses are designed so that focus is maintained as the focal length changes (such as when zooming). Not all lenses are truly parfocal, and performing a quick focus check is still common, especially with non-parfocal zooms.

Overall, parfokale describes an alignment of focal planes across optical elements to reduce the need for refocusing during changes in magnification or component configuration.