parfocal

Parfocal is an optical term describing a property of certain lens systems whereby the focal plane remains in roughly the same position when the magnification or focal length is changed. In practice, an image that is in focus at one setting should require little refocusing when another setting is selected, making transitions smoother and faster.

In microscopy, parfocal objectives are designed so that switching between objective lenses or changing magnification keeps the specimen near the same focal plane. When a parfocal objective is focused at one objective, changing to another objective typically only requires minor fine adjustment or a quick coarse focus adjustment. This relies on calibration against a reference focal plane and precise tolerances in the turret mechanism.

In astronomy and ocular practice, parfocal eyepieces are chosen so that switching eyepieces or adjusting magnification does not demand extensive refocusing. In telescopes, parfocality is achieved by matching focal planes across eyepieces and accessories, aiding efficient observation.

In photography, the term is used for parfocal zoom lenses, which maintain focus when the focal length changes. Historically many consumer zooms were varifocal, requiring refocus during zooming; modern parfocal designs aim to keep focus constant, though manufacturing tolerances mean perfection is not always guaranteed.

Limitations and usage: Parfocality is an idealized criterion and depends on mechanical precision, sensor distance in cameras, sample height in microscopes, and accessory configuration. Users should verify parfocal behavior for their specific instrument.