parfocal eyepieces

parfocal eyepieces

[pär′fō·kəl ′ī‚pēs·əz]
(optics)
Eyepieces whose lower focal points lie in the same plane, so that they can be interchanged without changing the focus of the instrument with which they are used.
References in periodicals archive ?
Parfocal eyepieces are needed to keep the arm positioned in its nominally ideal spot.
The focus travel is almost 1 1/2 inches, which, while adequate for so-called parfocal eyepieces, could be a problem for anyone with a set of eye-pieces that span a wide range of focal positions.