parfokal

Parfokal, often written parfocal, is an optical design property in which the focal plane remains aligned when the effective magnification changes, so an image remains in focus after switching to a different optical element or zoom setting. The concept is used in several fields of optics, including microscopy, telescopes, and camera lenses.

In light microscopy, a parfokal objective set means that the objectives share a common focal plane. After focusing with one objective, switching to another typically requires only a small refocus. This enables rapid switching between magnifications when scanning a specimen, improving workflow and efficiency in observation and imaging.

In astronomy and photography, parfocality can apply to eyepieces and zoom lenses. Parfocal eyepieces are designed so that the image remains in focus when swapping to a different eyepiece. Many modern zoom lenses are marketed as parfocal because they maintain focus across the zoom range, though real-world performance can vary by design and manufacturing tolerances.

Origin and usage of the term trace to the idea of maintaining the same focal plane across different optical configurations. The term is widely used in English as parfocal and is also found in German as parfokal. While the concept is straightforward, achieving perfect parfocality is subject to manufacturing tolerances and system alignment, and minor refocusing may still be needed in precise work or with complex optical assemblies.