Parfocal lens

A parfocal lens is a lens that stays in focus when magnification/focal length is changed. There is inevitably some amount of focus error, but too small to be considered significant.

Microscopy

Parfocal microscope objectives stay in focus when magnification is changed; i.e., if the microscope is switched from a lower power objective to a higher power objective, the object stays in focus. Most modern bright-field microscopes are parfocal.

Photography

Zoom lenses are ideally parfocal, in that focus is maintained as the lens is zoomed, which is convenient and has the advantage of allowing more accurate focusing at maximal focal length then zooming back to a shorter focal length to compose the image.
Many zoom lenses, particularly in the case of fixed-lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs than parfocal zoom, which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length, making operation practically indistinguishable from a parfocal zoom.

Cinematography

Zoom lenses used for moviemaking applications must have the parfocal ability in order to be of practical use. It is almost impossible to stay in correct focus while zooming. This could be possible theoretically, but if the subject and/or camera also move, there are too many variables to correct consistently.

Telescopy

Parfocal telescope eyepieces stay in focus when magnification is changed; i.e., if the telescope is switched from a lower-power eyepiece to a higher-power eyepiece, or vice versa, the object stays in focus.